Publications

We demonstrate super-resolved coherent vibrational imaging based on single-pulse Coherent Anti-Stokes Raman Scattering (CARS) combined with Image Scanning Microscopy (ISM), enabling label-free vibrational imaging beyond the diffraction limit. As required for coherent imaging, and unlike conventional ISM applied to fluorescence or spontaneous Raman signals, we perform pixel reassignment on the complex optical field rather than the intensity, accounting for the interference-based nature of coherent imaging. In contrast with previous implementations of coherent ISM, here we do not rely on the use of interferometry with an external reference, but rather utilize the intrinsic nonresonant background as reference, while a programmable pulse shaper enables spectral-phase interferometry between the resonant and nonresonant components of the CARS field. After retrieving both the amplitude and the phase of the resonant CARS signal, we apply coherent ISM to achieve resolution enhancement in chemically specific imaging. This technique opens new avenues for high-speed, label-free super-resolved vibrational microscopy and especially for its extension to the low-frequency spectral range and to an implementation in a backscattering geometry.

Recent years have seen significant interest in photovoltaic conversion systems tailored not for solar insolation but for particular laser frequencies operating in transmission windows as a means to deliver energy remotely to a consumer. The recent advent of lead halide perovskite-based photovoltaics, calls for the exploration of their use also in such applications. Here, we present perovskite laser cells (PLCs) with significantly enhanced performance by hybridizing the CsPbBr₃ with CoOx nanocrystals for optimizing the active layer and its interface with the cathode. The CoO_x composite can accelerate solute consumption and guarantee low-density nucleation in the precursor for improving the crystalline grains size. We present a laser-electric photovoltaic generator with an active absorber layer of CoO_x-CsPbBr₃ under conditions of atmospheric (420 nm) and underwater (505 nm) transmission windows, whose performance significantly exceeds that of the reference CsPbBr₃ one. The champion device obtained an ultrahigh PCE of 62.77 % under laser irradiation with a wavelength of 505 nm and power intensity of 80 mW·cm⁻². This study not only provides a hybridization engineering strategy technique with the synergy of gradient engineering and surface optimization for improving the absorber film and its interface with the cathode, but also provides opportunities for multiple scenarios of remote power transmission applications.

How molecular-level understanding of the crystal growth mechanisms and their relation to lattice bonds informs the rational design of crystals with desired shapes and properties has remained elusive. Here we employ theophylline crystals and drive them into classical growth mode, in which the crystals grow molecule-by-molecule and new layers are generated by two-dimensional nucleation. We demonstrate that classical growth allows for controlling the crystal's shape and dimensions. We correlate the anisotropic responses to the supersaturation of the growth rates of crystal layers and crystal faces to the hydrogen and π−π stacking bond chains in the crystal lattice. The obtained insights suggest strategies to direct the crystal shape to either one-dimensional needles or flat sheets. Moreover, we show that crystals that grow by the classical mode of direct monomer incorporation have the potential to regrow and heal once a defect is introduced by mechanical cut or local thermal subliming of crystalline sections.

Self-healing materials can become game changers for developing sustainable (opto)electronics. APbX₃ halide (=X^-) perovskites, HaPs, have shown a remarkable ability to self-heal damage. While we demonstrated self-healing in pure HaP compounds, in single crystals, and in polycrystalline thin films (as used in most devices), HaP compositions with multiple A⁺ (and X^-) constituents are preferred for solar cells. We now show self-healing in mixed A⁺ HaPs. Specifically, if at least 15 atom % of the methylammonium (MA⁺) A cation is substituted for by guanidinium (Gua⁺) or acetamidinium (AA⁺), then the self-healing rate after damage is enhanced. In contrast, replacing MA⁺ with dimethylammonium (DMA⁺), comparable in size to Gua⁺ or AA⁺, does not alter this rate. Based on the times for self-healing, we infer that the rate-determining step involves short-range diffusion of A⁺ and/or Pb²⁺ cations and that the self-healing rate correlates with the strain in the material, the A⁺ cation dipole moment, and H-bonding between A⁺ and I^-. These insights may offer clues for developing a detailed self-healing mechanism and understanding the kinetics to guide the design of self-healing materials. Fast recovery kinetics are important from the device perspective, as they allow complete recovery in devices during operation or when switched off (LEDs)/in the dark (photovoltaics).

Modern imaging technologies are widely based on classical principles of light or electromagnetic wave propagation. They can be remarkably sophisticated, with recent successes ranging from single-molecule microscopy to imaging far-distant galaxies. However, new imaging technologies based on quantum principles are gradually emerging. They can either surpass classical approaches or provide novel imaging capabilities that would not otherwise be possible. Here we provide an overview of the most recently developed quantum imaging systems, highlighting the nonclassical properties of sources, such as bright squeezed light, entangled photons and single-photon emitters that enable their functionality. We outline potential upcoming trends and the associated challenges, all driven by a central enquiry, which is to understand whether quantum light can make visible the invisible.

Organic crystals are widely used by animals to manipulate light for producing structural colors and for improving vision. To date only seven crystal types are known to be used, and among them β-guanine crystals are by far the most widespread. The fact that almost all these crystals have unusually high refractive indices (RIs) is consistent with their light manipulation function. Here, the physical, structural, and optical principles of how light interacts with the polarizable free-electron-rich environment of these quasiaromatic molecules are addressed. How the organization of these molecules into crystalline arrays introduces optical anisotropy and finally how organisms control crystal morphology and superstructural organization to optimize functions in light reflection and scattering are also discussed. Many open questions remain in this fascinating field, some of which arise out of this in-depth analysis of the interaction of light with crystal arrays. More types of organic crystals will probably be discovered, as well as other organisms that use these crystals to manipulate light. The insights gained from biological systems can also be harnessed for improving synthetic light-manipulating materials.

Spherical particles with diameters within the wavelength of visible light, known as spherulites, manipulate light uniquely due to their spatial organization and their structural birefringence. Most of the known crystalline spherulites are branched, and composed of metals, alloys, and semi-crystalline polymers. Recently, a different spherulite architecture is discovered in the vision systems of decapod crustaceans - core-shell spherulites composed of highly birefringent ((Formula presented.)) organic single-crystal platelets, with exceptional optical properties. These metastructures, which efficiently scatter light even in dense aqueous environments, have no synthetic equivalence and serve as a natural proof-of-concept as well as synthetic inspiration for thin scattering media. Here, the synthesis of core-shell spherulites composed of guanine crystal platelets (((Formula presented.)) is presented in a two-step emulsification process in which a water/oil/water emulsion and induced pH changes are used to promote interfacial crystallization. Carboxylic acids neutralize the dissolved guanine salts to form spherulites composed of single, radially stacked, β-guanine platelets, which are oriented tangentially to the spherulite surface. Using Mie theory calculations and forward scattering measurements from single spherulites, it is found that due to the single-crystal properties and orientation, the synthetic spherulites possess a high tangential refractive index, similarly to biogenic particles.

Regulating strain in perovskite films via utilizing the crystal structure relationship between solid-phase materials (SPMs) and perovskite is an effective method to achieve high-performance perovskite solar cells. It is crucial to investigate and manipulate the heterointerfaces between perovskites and SPMs since the mismatched crystal structure and energy band structure of SPMs will bring recombination sites to the heterointerfaces. In this work, CdS-modified PbS nanosheets (CPS) were prepared through cation exchange with (200)-preferred PbS nanosheets. The wide-band gap CdS charge-blocking layer of CPS nanosheets distributed at the grain boundaries effectively passivates the heterointerfaces in FAPbI₃-CPS heterostructures (FAPI-CPS). Further, it potentially blocks carrier transportation and suppresses carrier recombination at grain boundaries and in FAPbI₃-PbS nanosheet heterointerfaces. Attributed to the CdS layer, the FAPI-CPS devices achieve an enhanced power conversion efficiency. CPS nanosheets with an interplanar spacing slightly smaller than that of α-FAPbI₃ could provide compressive strain to FAPbI₃ at the FAPI-CPS heterointerfaces, leading to significantly improved device stability. The unencapsulated FAPI-CPS solar cells maintained 92% of their initial PCE after being stored at 20 ± 5 °C, 20 ± 5% RH for 2500 h.

Photoisomerization of azobenzenes from their stable E isomer to the metastable Z state is the basis of numerous applications of these molecules. However, this reaction typically requires ultraviolet light, which limits applicability. In this study, we introduce disequilibration by sensitization under confinement (DESC), a supramolecular approach to induce the E-to-Z isomerization by using light of a desired color, including red. DESC relies on a combination of a macrocyclic host and a photosensitizer, which act together to selectively bind and sensitize E-azobenzenes for isomerization. The Z isomer lacks strong affinity for and is expelled from the host, which can then convert additional E-azobenzenes to the Z state. In this way, the host-photosensitizer complex converts photon energy into chemical energy in the form of out-of-equilibrium photostationary states, including ones that cannot be accessed through direct photoexcitation.

Quantum dots are engineered to use dopant states to achieve substantially enhanced impact ionization, which is potentially useful for light-harvesting applications.

A fundamental question regarding light scattering is how whiteness, generated from multiple scattering, can be obtained from thin layers of materials. This challenge arises from the phenomenon of optical crowding, whereby, for scatterers packed with filling fractions higher than ~30%, reflectance is drastically reduced due to near-field coupling between the scatterers. Here we show that the extreme birefringence of isoxanthopterin nanospheres overcomes optical crowding effects, enabling multiple scattering and brilliant whiteness from ultra-thin chromatophore cells in shrimp. Strikingly, numerical simulations reveal that birefringence, originating from the spherulitic arrangement of isoxanthopterin molecules, enables intense broadband scattering almost up to the maximal packing for random spheres. This reduces the thickness of material required to produce brilliant whiteness, resulting in a photonic system that is more efficient than other biogenic or biomimetic white materials which operate in the lower refractive index medium of air. These results highlight the importance of birefringence as a structural variable to enhance the performance of such materials and could contribute to the design of biologically inspired replacements for artificial scatterers like titanium dioxide.

In terms of sustainable use, halide perovskite (HaP) semiconductors have a strong advantage over most other classes of materials for (opto)electronics, as they can self-heal (SH) from photodamage. While there is considerable literature on SH in devices, where it may not be clear exactly where damage and SH occur, there is much less on the HaP material itself. Here we perform \u201cfluorescence recovery after photobleaching\u201d (FRAP) measurements to study SH on polycrystalline thin films for which encapsulation is critical to achieving complete and fast self-healing. We compare SH in three photoactive APbI₃ perovskite films by varying the A-site cation ranging from (relatively) small inorganic Cs through medium-sized MA to large FA (the last two are organic cations). While the A cation is often considered electronically relatively inactive, it significantly affects both SH kinetics and the threshold for photodamage. The SH kinetics are markedly faster for γ-CsPbI₃ and α-FAPbI₃ than for MAPbI₃. Furthermore, γ-CsPbI₃ exhibits an intricate interplay between photoinduced darkening and brightening. We suggest possible explanations for the observed differences in SH behavior. This studys results are essential for identifying absorber materials that can regain intrinsic, insolation-induced photodamage-linked efficiency loss during its rest cycles, thus enabling applications such as autonomously sustainable electronics.

Impulsive stimulated Raman scattering (ISRS) using a single short femtosecond pump pulse to excite molecular vibrations offers an elegant pump-probe approach to perform vibrational imaging below 200cm^-1. One shortcoming of ISRS is its inability to offer vibrational selectivity as all the vibrational bonds whose frequencies lie within the short pump-pulse bandwidth are excited. To date, several coherent control techniques have been explored to address this issue and selectively excite a specific molecular vibration by shaping the pump pulse. There has not been any systematic work that reports an analogous shaping of the probe pulse to implement preferential detection. In this work, we focus on vibrational imaging and report vibrational selective detection by shaping the probe pulse in time. We demonstrate numerically and experimentally two pulse-shaping strategies with one functioning as a vibrational notch filter and the other functioning as a vibrational low-pass filter. This enables fast (25μs/pixel) and selective hyperspectral imaging in the low-frequency regime (

One of the key phenomena that determine the fluorescence of nanocrystals is the nonradiative Auger-Meitner recombination of excitons. This nonradiative rate affects the nanocrystals fluorescence intensity, excited state lifetime, and quantum yield. Whereas most of the above properties can be directly measured, the quantum yield is the most difficult to assess. Here we place semiconductor nanocrystals inside a tunable plasmonic nanocavity with subwavelength spacing and modulate their radiative de-excitation rate by changing the cavity size. This allows us to determine absolute values of their fluorescence quantum yield under specific excitation conditions. Moreover, as expected considering the enhanced Auger-Meitner rate for higher multiple excited states, increasing the excitation rate reduces the quantum yield of the nanocrystals.

The future of halide perovskites (HaPs) is beclouded by limited understanding of their long-term stability. While HaPs can be altered by radiation that induces multiple processes, they can also return to their original state by \u201cself-healing.\u201d Here two-photon (2P) absorption is used to effect light-induced modifications within MAPbI₃ single crystals. Then the changes in the photodamaged region are followed by measuring the photoluminescence, from 2P absorption with 2.5 orders of magnitude lower intensity than that used for photodamaging the MAPbI₃. After photodamage, two brightening and one darkening process are found, all of which recover but on different timescales. The first two are attributed to trap-filling (the fastest) and to proton-amine-related chemistry (the slowest), while photodamage is attributed to the lead-iodide sublattice. Surprisingly, while after 2P-irradiation of crystals that are stored in dry, inert ambient, photobrightening (or \u201clight-soaking\u201d) occurs, mostly photodarkening is seen after photodamage in humid ambient, showing an important connection between the self-healing of a HaP and the presence of H₂O, for long-term steady-state illumination, practically no difference remains between samples kept in dry or humid environments. This result suggests that photobrightening requires a chemical-reservoir that is sensitive to the presence of H₂O, or possibly other proton-related, particularly amine, chemistry.

Well-defined Zn2GeO4/g-C3N4 nanocomposites with a band alignment of type-I were prepared by the ultrasound-assisted solvent method, starting from g-C3N4 nanosheets and incorporating 0, 10, 20, and 40 wt% of Zn2GeO4. In this study, we have investigated in-depth the photoluminescence emission and photocatalytic activity of these nanocomposites. Our experimental results showed that an increased mass ratio of Zn2GeO4 to g-C3N4 can significantly improve their photoluminescence and photocatalytic responses. Additionally, we have noted that the broadband photoluminescence (PL) emission for these nanocomposites reveals three electronic transitions; the first two well-defined transitions (at ca. 450 nm and 488 nm) can be attributed to π*→ lone pair (LP) and π*→π transitions of g-C3N4, while the single shoulder at ca. 532 nm is due to the oxygen vacancy (Vo) as well as the hybridization of 4s and 4p orbital states in the Zn and Ge belonging to Zn2GeO4. These experimental findings are also supported by theoretical calculations performed under periodic conditions based on the density functional theory (DFT) fragment. The theoretical findings for these nanocomposites suggest a possible strain-induced increase in the Zn-O bond length, as well as a shortening of the Ge-O bond of both tetrahedral [ZnO4] and [GeO4] clusters, respectively. Thus, this disordered structure promotes local polarization and a charge gradient in the Zn2GeO4/g-C3N4 interface that enable an efficient separation and transfer of the photoexcited charges. Finally, theoretical results show a good correlation with our experimental data.

We extend image scanning microscopy to second harmonic generation (SHG) by extracting the complex field amplitude of the second-harmonic beam. While the theory behind coherent image scanning microscopy (ISM) is known, an experimental demonstration was not yet established. The main reason is that the naive intensity-reassignment procedure cannot be used for coherent scattering as the point spread function is now defined for the field amplitude rather than for the intensity. We use an inline interferometer to demonstrate super-resolved phase-sensitive SHG microscopy by applying the ISM reassignment machinery on the resolved field. This scheme can be easily extended to third harmonic generation and stimulated Raman microscopy schemes.

Spectrometry of a quantum state of light is a fundamental challenge with practical implications. Here, we demonstrate how such a technique can super-resolve the exciton and biexciton energies in a single quantum dot at room temperature.

Polar materials display a series of interesting and widely exploited properties owing to the inherent coupling between their fixed electric dipole and any action that involves a change in their charge distribution. Among these properties are piezoelectricity, ferroelectricity, pyroelectricity, and the bulk photovoltaic effect. Here we report the observation of a related property in this series, where an external electric field applied parallel or anti-parallel to the polar axis of a crystal leads to an increase or decrease in its second-order nonlinear optical response, respectively. This property of electric-field-modulated second-harmonic generation (EFM-SHG) is observed here in nanowires of the polar crystal ZnO, and is exploited as an analytical tool to directly determine by optical means the absolute direction of their polarity, which in turn provides important information about their epitaxy and growth mechanism. EFM-SHG may be observed in any type of polar nanostructures and used to map the absolute polarity of materials at the nanoscale.

Metal halide perovskites (MHPs) have unique characteristics and hold great potential for next-generation optoelectronic technologies. Recently, the importance of lattice strain in MHPs has been gaining recognition as a significant optimization parameter for device performance. While the effect of strain on the fundamental properties of MHPs has been at the center of interest, its combined effect with an external electric field has been largely overlooked. Here we perform an electric-field-dependent photoluminescence study on heteroepitaxially strained surface-guided CsPbBr₃ nanowires. We reveal an unexpected strong linear dependence of the photoluminescence intensity on the alternating field amplitude, stemming from an induced internal dipole. Using low-frequency polarized-Raman spectroscopy, we reveal structural modifications in the nanowires under an external field, associated with the observed polarity. These results reflect the important interplay between strain and an external field in MHPs and offer opportunities for the design of MHP-based optoelectronic nanodevices.

Upconverting semiconductor quantum dots (QDs) combine the stability of an inorganic crystalline structure with the spectral tunability afforded by quantum confinement. Here, we present upconverting type-II/type-I colloidal double QDs that enable enhancement of the performance of near-infrared to visible photon upconversion in QDs and broadening the range of relevant materials used. The resulting ZnTe/CdSe@CdS@CdSe/ZnSe type-II/type-I double QDs possess a very high photoluminescence quantum yield, monoexponential decay dynamics, and a narrow line width, approaching those of state-of-the-art upconverting QDs. We quantitatively characterize the upconversion cross section by direct comparison with two-photon absorption when varying the pump frequency across the absorption edge. Finally, we show that these upconversion QDs maintain their optical performance in a much more demanding geometry of a dense solid film and can thus be incorporated in devices as upconversion films. Our design provides guidance for fabricating highly efficient upconverting QDs with potential applications such as security coding and bioimaging.

microscope, facilitates up to two-fold improvement in lateral resolution, and has become an indispensable element in the toolbox of the bio-imaging community. Recently, super-resolution optical fluctuation image scanning microscopy (SOFISM) integrated the analysis of intensityfluctuations information into the basic ISM architecture, to enhance its resolving power. Both of these techniques typically rely on pixel-reassignment as a fundamental processing step, in which the parallax of different detector elements to the sample is compensated by laterally shifting the point spread function (PSF). Here, we propose an alternative analysis approach, based on the recent high-performing sparsity-based super-resolution correlation microscopy (SPARCOM) method. Through measurements of DNA origami nano-rulers and fixed cells labeled with organic dye, we experimentally show that confocal SPARCOM (cSPARCOM), which circumvents pixel-reassignment altogether, provides enhanced resolution compared to pixel-reassigned based analysis. Thus, cSPARCOM further promotes the effectiveness of ISM, and particularly that of correlation based ISM implementations such as SOFISM, where the PSF deviates significantly from spatial invariance.

The rediscovery of the halide perovskite class of compounds and, in particular, the organic and inorganic lead halide perovskite (LHP) materials and lead-free derivatives has reached remarkable landmarks in numerous applications. First among these is the field of photovoltaics, which is at the core of todays environmental sustainability efforts. Indeed, these efforts have born fruit, reaching to date a remarkable power conversion efficiency of 25.2% for a double-cation Cs, FA lead halide thin film device. Other applications include light and particle detectors as well as lighting. However, chemical and thermal degradation issues prevent perovskite-based devices and particularly photovoltaic modules from reaching the market. The soft ionic nature of LHPs makes these materials susceptible to delicate changes in the chemical environment. Therefore, control over their interface properties plays a critical role in maintaining their stability. Here we focus on LHP nanocrystals, where surface termination by ligands determines not only the stability of the material but also the crystallographic phase and crystal habit. A surface analysis of nanocrystal interfaces revealed the involvement of Brønsted type acidbase equilibrium in the modification of the ligand moieties present, which in turn can invoke dissolution and recrystallization into the more favorable phase in terms of minimization of the surface energy. A large library of surface ligands has already been developed showing both good chemical stability and good electronic surface passivation, resulting in near-unity emission quantum yields for some materials, particularly CsPbBr3. However, most of those ligands have a large organic tail hampering charge carrier transport and extraction in nanocrystal-based solid films.

A fluorescence correlation contrast is as traightforward a pproach to super-resolution imaging. Combining a SPAD array with a novel detection scheme (ISM), we obtain images with up to x4 times resolution enhancement.

Applied cutting-edge electronic structure and phonon simulations provide a reliable knowledge about the stability of perovskite structures and their electronic properties, which are crucial for design of effective nanomaterials. Gold is one of the exceptional elements, which can exist both as a monovalent and a trivalent ion in the B site of a double perovskite such as A2BIBIIIX6. However, until now, electronic properties of Cs2AuIAuIIIX6 have not been sufficiently explored and this material was never synthesized using Au1+ and Au3+ precursors in the preparation route. Here, computational simulations combined with an experimental study provide new insight into the properties and synthesis route of Cs2AuIAuIIIX6 (X = Cl, Br, and I) perovskites. First-principles calculations reveal that tetragonal Cs2AuIAuIIIX6 (X = I, Br, Cl) molecules present a band gap of 1.10, 1.15, and 1.40 eV, respectively. Application of novel approaches in the simulations of the VB-XPS for Cs2AuIAuIIICl6 allows replication of the observed spectrum and provides strong evidence of the reliability of the obtained results for the other perovskites Cs2AuIAuIIIX6, X = Br, I. Following theoretical findings, a one-step preparation route of the Cs2AuIAuIIICl6 is developed using a combination of monovalent and trivalent gold precursors at a relatively low temperature. It should be emphasized that this is the first synthesis of this material at low temperatures, allowing for obtaining highly crystalline Cs2Au2Cl6 particles with controlled morphology and without gold impurities. The band gap of synthesized Cs2AuIAuIIICl6 is extended into the NIR spectral range, where most other double perovskites are limited to higher energies, limiting their usage in single junction solar cells or in photocatalysis. The as-synthesized Cs2AuIAuIIICl6 exhibits high efficiency in a photocatalytic toluene degradation reaction under visible light irradiation. The developed approach provides information necessary for structure manipulation at the early stage of its synthesis and offers a new and useful guidance for design of novel improved lead-free inorganic halide perovskite with interesting optical and photocatalytic properties.

Super-resolution optical microscopy is a rapidly evolving scientific field dedicated to imaging sub-wavelength-sized objects, leaving its mark in multiple branches of biology and technology. While several super-resolution optical microscopy methods have become a common tool in life science imaging, new methods, supported by cutting-edge technology, continue to emerge. One rather recent addition to the super-resolution toolbox, image scanning microscopy (ISM), achieves up to twofold lateral resolution enhancement in a robust and straightforward manner. To further enhance ISM's resolution in all three dimensions, we present and experimentally demonstrate here super-resolution optical fluctuation ISM (SOFISM). Measuring the fluorescence fluctuation contrast in an ISM architecture, we obtain images with a ×2.5 lateral resolution beyond the diffraction limit along with an enhanced axial resolution for a fixed cell sample labeled with commercially available quantum dots. The inherent temporal averaging of the ISM technique enables image acquisition of the fluctuation correlation contrast within millisecond-scale pixel dwell times. SOFISM can therefore offer a robust path to achieve high-resolution images within a slightly modified confocal microscope, using standard fluorescent labels and reasonable acquisition times.

We demonstrate the formation of uniform and oriented metal-organic frameworks using a combination of anion-effects and surface-chemistry. Subtle but significant morphological changes result from the nature of the coordinative counter-anion of the following metal salts: NiX2 with (X = Br-, Cl-, NO3-, and OAc-). Crystals could be obtained in solution or by template surface growth. The latter resulting in truncated crystals that resemble a half-structure of the solution-grown ones. The oriented surface-bound metal-organic frameworks (sMOFs) are obtained via a one-step solvothermal approach, rather than in a layer-by-layer approach. The MOFs are grown on Si/SiOx substrates modified with an organic monolayer or on glass substrates covered with a transparent conductive oxide (TCO). Regardless of the different morphologies, the crystallographic packing is nearly identical and is not affected by the type of anion, nor by solution versus the surface chemistry. A propeller-type arrangement of the non-chiral ligands around the metal center affords a chiral structure with two geometrically different helical channels in a 2:1 ratio with the same handedness. To demonstrate the accessibility and porosity of the macroscopically-oriented channels, a chromophore (resorufin sodium salt) was successfully embedded into the channels of the crystals by diffusion from solution, resulting in fluorescent crystals. These "colored" crystals displayed polarized emission (red) with a high polarization ratio because of the alignment of these dyes imposed by the crystallographic structure. A second-harmonic generation (SHG) study revealed Kleinman-symmetry forbidden non-linear optical properties. These surface-bound and oriented SHG-active MOFs have the potential for use as single non-linear optical (NLO) devices.

In the past decade, optical imaging methods have significantly improved our understanding of the information processing principles in the brain. Although many promising tools have been designed, sensors of membrane potential are lagging behind the rest. Semiconductor nanoparticles are an attractive alternative to classical voltage indicators, such as voltage-sensitive dyes and proteins. Such nanoparticles exhibit high sensitivity to external electric fields via the quantum-confined Stark effect. Here we report the development of semiconductor voltage-sensitive nanorods (vsNRs) that self-insert into the neuronal membrane. To facilitate interaction of the nanorods with the membrane, we functionalized their surface with the lipid mixture derived from brain extract. We describe a workflow to detect and process the photoluminescent signal of vsNRs after wide-field time-lapse recordings. We also present data indicating that vsNRs are feasible for sensing membrane potential in neurons at a single-particle level. This shows the potential of vsNRs for the detection of neuronal activity with unprecedentedly high spatial and temporal resolution.

Ligand-induced chirality in semiconducting nanocrystals has been the subject of extensive study in the past few years and shows potential applications in optics and biology. Yet, the origin of the chiroptical effect in semiconductor nanoparticles is still not fully understood. Here, we examine the effect of the interaction with amino acids on both the fluorescence and the optical activity of chiral semiconductor quantum dots (QDs). A significant fluorescence enhancement is observed for L/D-Cys-CdTe QDs upon interaction with all the tested amino acids, indicating suppression of non-radiative pathways as well as the passivation of surface trap sites brought via the interaction of the amino group with the CdTe QDs' surface. Hetero-chiral amino acids are shown to weaken the Circular Dichroism (CD) signal, which may be attributed to a different binding configuration of cysteine molecules on the QDs surface. Furthermore, a red shift of both CD and fluorescence signals in L/D-Cys-CdTe QDs is only observed upon adding cysteine, while other tested amino acids do not exhibit such an effect. We speculate that the thiol group induces orbital hybridization of the highest occupied molecular orbital (HOMOs) of cysteine and the valance band of CdTe QDs, leading to the decrease of the energy band-gap and a concomitant red shift of CD and fluorescence spectra. This is further verified by density functional theory (DFT) calculations. Both the experimental and theoretical findings indicate that the addition of ligands which do not "directly" interact with the VB of the QD (non-cysteine moieties) changes the QD photophysical properties as it probably modifies the way cysteine is bound to the surface. Hence, we conclude that it is not only the chemistry of the amino acid ligand which affects both CD and PL, rather, it is also the exact geometry of binding which modifies these properties. Understanding the relationship between QD's surface and chiral amino acid thus provides an additional perspective on the fundamental origin of induced chiroptical effects in semiconductor nanoparticles, potentially enabling us to optimize the design of chiral semiconductor QDs for chiroptic applications.

Colloidal semiconductor nanoplatelets, in which carriers are strongly confined only along one dimension, present fundamentally different excitonic properties than quantum dots, which support strong confinement in all three dimensions. In particular, multiple excitons strongly confined in just one dimension are free to rearrange in the lateral plane, reducing the probability for multibody collisions. Thus, while simultaneous multiple photon emission is typically quenched in quantum dots, in nanoplatelets its probability can be tuned according to size and shape. Here, we focus on analyzing multiexciton dynamics in individual CdSe/CdS nanoplatelets of various sizes through the measurement of second-, third-, and fourth-order photon correlations. For the first time, we can directly probe the dynamics of the two, three, and four exciton states at the single nanocrystal level. Remarkably, although higher orders of correlation vary substantially among the synthesis products, they strongly correlate with the value of second order antibunching. The scaling of the higher-order moments with the degree of antibunching presents a small yet clear deviation from the accepted model of Auger recombination through binary collisions. Such a deviation suggests that many-body contributions are present already at the level of triexcitons. These findings highlight the benefit of high-order photon correlation spectroscopy as a technique to study multiexciton dynamics in colloidal semiconductor nanocrystals.

Real-time vibrational microscopy has been recently demonstrated by various techniques, most of them utilizing the well-known schemes of coherent anti-stokes Raman scattering and stimulated Raman scattering. These techniques readily provide valuable chemical information mostly in the higher vibrational frequency regime (>400 cm(-1)). Addressing the low vibrational frequency regime (

Coherent anti-Stokes Raman scattering (CARS) microscopy is becoming a more
common tool in biomedical research. High-speed CARS microscopy has important applications
in live cell imaging and in label-free pathology. However, only a few realizations exist of CARS
imaging applied in the few terahertz spectral range (

The evolution of experimental superresolution microscopy has been accompanied by the development of advanced computational imaging capabilities. Recently introduced, quantum image scanning microscopy (Q-ISM) has successfully harnessed quantum correlations of light to establish an improved viable imaging modality that builds upon the preceding image scanning microscopy (ISM) superresolution method. While offering improved resolution, at present the inherently weak signal demands exhaustively long acquisition periods. Here we exploit the fact that the correlation measurement in Q-ISM is complementary to the standard ISM data, acquired simultaneously, and demonstrate joint sparse recovery from Q-ISM and ISM images. Reconstructions from images of fluorescent quantum dots are validated through correlative electron microscope measurements, and exhibit superior resolution enhancement as compared to Q-ISM images. In addition, the algorithmic fusion facilitates a drastic reduction in the requisite measurement duration, since low signal-to-noise-ratio Q-ISM measurements suffice for augmenting ISM images. Finally, we obtain enhanced superresolved reconstructions from short scans of a biological sample labeled with quantum dots, demonstrating the potential of our method for quantum imaging in life science microscopy.

We demonstrate an approach for super-resolution imaging by combining flu-orophore fluctuations analysis with a confocal detector array setup. This combination facilitates obtaining high resolution images without a complex experimental setup and prohibitively long data acquisition.

Coherent anti-Stokes Raman scattering (CARS) has found wide applications in biomedical research. Compared with alternatives, single-beam CARS is especially attractive at low frequencies. Yet, currently existing schemes necessitate a relatively complicated setup to perform high-resolution spectroscopy. Here we show that the spectral sharp edge formed by an ultra-steep long-pass filter is sufficient for performing CARS spectroscopy, simplifying the system significantly. We compare the sensitivity of the presented methodology with available counterparts both theoretically and experimentally. Importantly, we show that this method, to the best of our knowledge, is the simplest and most suitable for vibrational imaging and spectroscopy in the very low-frequency regime (

Quasi-two-dimensional (2D) CdSe nanoplatelets (NPLs) are distinguished by their unique optical properties in comparison to classical semiconductor nano crystals, such as extremely narrow emission line widths, reduced Auger recombination, and relatively high absorption cross sections. Inherent to their anisotropic 2D structure, however, is the loss of continuous tunability of their photoluminescence (PL) properties due to stepwise growth. On top of that, limited experimental availability of NPLs of different thicknesses and ultimately the bulk band gap of CdSe constrain the achievable PL wavelengths. Here, we report on the doping of CdSe NPLs with mercury, which gives rise to additional PL in the red region of the visible spectrum and in the near-infrared region. We employ a seeded-growth method with injection solutions containing cadmium, selenium, and mercury. The resulting NPLs retain their anisotropic structure, are uniform in size and shape, and present significantly altered spectroscopic characteristics due to the existence of additional energetic states. We conclude that doping takes place by employing elemental analysis in combination with PL excitation spectroscopy, X-ray photoelectron spectroscopy, and single particle fluorescence spectroscopy, confirming single emitters being responsible for multiple distinct emission signals.

Solution-processed core/multishell semiconductor quantum dots (QDs) could be tailored to facilitate the carrier separation, promotion, and recombination mechanisms necessary to implement photon upconversion. In contrast to other upconversion schemes, upconverting QDs combine the stability of an inorganic crystalline structure with the spectral tunability afforded by quantum confinement. Nevertheless, their upconversion quantum yield (UCQY) is fairly low. Here, design rules are uncovered that enable to significantly enhance the performance of double QD upconversion systems, and these findings are leveraged to fabricate upconverting QDs with increased photon upconversion efficiency and reduced saturation intensities under pulsed excitation. The role of the intra-QD band alignment is exemplified by comparing the upconversion process in PbS/CdS/ZnSe QDs with that of PbS/CdS/CdSe ones with variable CdSe shell thicknesses. It is shown that electron delocalization into the shell leads to a longer-lived intermediate state in the QDs, facilitating further absorption of photons, and enhancing the upconversion process. The performance of these upconversion QDs under pulsed excitation versus continuous pumping is also compared; the reasons for the significant differences between these two regimes are discussed. The results show how one can overcome some of the limitations of previous upconverting QDs, with potential applications in biophotonics and infrared detection.

The mechanisms of exciton generation and recombination in semiconductor nanocrystals are crucial to the understanding of their photophysics and for their application in nearly all fields. While many studies have been focused on type-I heterojunction nanocrystals, the photophysics of type-II nanorods, where the hole is located in the core and the electron is located in the shell of the nanorod, remain largely unexplored. In this work, by scanning single nanorods through the focal spot of radially and azimuthally polarized laser beams and by comparing the measured excitation patterns with a theoretical model, we determine the dimensionality of the excitation transition dipole of single type-II nanorods. Additionally, by recording defocused patterns of the emission of the same particles, we measure their emission transition dipoles. The combination of these techniques allows us to unambiguously deduce the dimensionality and orientation of both excitation and emission transition dipoles of single type-II semiconductor nanorods. The results show that in contrast to previously studied quantum emitters, the particles possess a 3D degenerate excitation and a fixed linear emission transition dipole.

The principles of quantum optics have yielded a plethora of ideas to surpass the classical limitations of sensitivity and resolution in optical microscopy. While some ideas have been applied in proof-of-principle experiments, imaging a biological sample has remained challenging, mainly due to the inherently weak signal measured and the fragility of quantum states of light. In principle, however, these quantum protocols can add new information without sacrificing the classical information and can therefore enhance the capabilities of existing super-resolution techniques. Image scanning microscopy, a recent addition to the family of super-resolution methods, generates a robust resolution enhancement without reducing the signal level. Here, we introduce quantum image scanning microscopy: combining image scanning microscopy with the measurement of quantum photon correlation allows increasing the resolution of image scanning microscopy up to twofold, four times beyond the diffraction limit. We introduce the Q-ISM principle and obtain super-resolved optical images of a biological sample stained with fluores-cent quantum dots using photon antibunching, a quantum effect, as a resolution-enhancing contrast mechanism.

Vibrational spectroscopic imaging is useful and important in biological and medical studies. Yet, vibrational imaging in the terahertz region (

Vision mechanisms in animals, especially those living in water, are diverse. Many eyes have reflective elements that consist of multilayers of nanometer-sized crystalline plates, composed of organic molecules. The crystal multilayer assemblies owe their enhanced reflectivity to the high refractive indices of the crystals in preferred crystallographic directions. The high refractive indices are due to the molecular arrangements in their crystal structures. Herein, data regarding these difficult-to-characterize crystals are reviewed. This is followed by a discussion on the function of these crystalline assemblies, especially in visual systems whose anatomy has been well characterized under close to in vivo conditions. Three test cases are presented, and then the relations between the reflecting crystalline components and their functions, including the relations between molecular structure, crystal structure, and reflecting properties are discussed. Some of the underlying mechanisms are also discussed, and finally open questions in the field are identified.

In recent years, wavefront shaping has been utilized to control and correct distorted light for enhancing a bright spot, generation of a Bessel beam or darkening a complete area at the output of a scattering system. All these outcomes can be thought of as enhancing a particular mode of the output field. In this letter, we study the relation between the attainable enhancement factor, corresponding to the efficiency of mode conversion, and the field distribution of the target mode. Working in the limit of a thin diffuser enables not only a comparison between experimental and simulated results, but also allows for derivation of an analytic formula. These results shed light on the ability to use a scattering medium as a mode converter and on the relationship between the desired shape and the efficiency. (C) 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

In this study, a facile and effective approach to synthesize high-quality perovskite-quantum dots (QDs) hybrid film is demonstrated, which dramatically improves the photovoltaic performance of a perovskite solar cell (PSC). Adding PbS QDs into CH3NH3PbI3 (MAPbI(3)) precursor to form a QD-in-perovskite structure is found to be beneficial for the crystallization of perovskite, revealed by enlarged grain size, reduced fragmentized grains, enhanced characteristic peak intensity, and large percentage of (220) plane in X-ray diffraction patterns. The hybrid film also shows higher carrier mobility, as evidenced by Hall Effect measurement. By taking all these advantages, the PSC based on MAPbI(3)-PbS hybrid film leads to an improvement in power conversion efficiency by 14% compared to that based on pure perovskite, primarily ascribed to higher current density and fill factor (FF). Ultimately, an efficiency reaching up to 18.6% and a FF of over approximate to 0.77 are achieved based on the PSC with hybrid film. Such a simple hybridizing technique opens up a promising method to improve the performance of PSCs, and has strong potential to be applied to prepare other hybrid composite materials.

Induced chirality in colloidal semiconductor nanoparticles has raised significant attention in the past few years as an extremely sensitive spectroscopic tool and due to the promising applications of chiral quantum dots in sensing, quantum optics, and spintronics. Yet, the origin of the induced chiroptical effects in semiconductor nanoparticles is still not fully understood, partly because almost all the theoretical and experimental studies to date are based on the simple model system of a spherical nanocrystal. Here, the realization of induced chirality in atomically flat 2D colloidal quantum wells is shown. A strong circular dichroism (CD) response as well as an absorptive-like CD line shape is observed in chiral CdSe nanoplatelets (NPLs), significantly differing from that previously observed in spherical dots. Furthermore, this intense CD signal almost completely disappears after coating with a very thin CdS shell. In contrast, CdSe-CdS core-crown NPLs exhibit a spectral response which seems to originate independently from the core and the crown regions of the NPL. This work on the one hand further advances the understanding of the fundamental origin of induced chiroptical effects in semiconductor nanoparticles, and on the other opens a pathway toward applications using chiroptical materials.

We present a new method for the measurement of the stimulated Raman spectrum based on time-dependent spatial modulation of a laser beam as it passes through a Raman active medium. This effect is similar to the instantaneous Kerr lensing and Kerr deflection yet involves resonant vibrations which result in a time-dependent refractive index change. We use sub-nanojoule pulses together with a sensitive pump-probe measurement apparatus to excite and detect the fine (10(-5)-10(-4)) temporal and spatial variations in intensity resulting from the Raman-induced Kerr effect. We demonstrate the effect by changing the spatial overlap between the pump and probe at the sample and measuring the time-dependent deformation of the probe beam's cross section. This method is particularly useful for detection of low-frequency Raman lines, as we demonstrate by measuring the Raman spectrum of neat liquids in a cuvette.

Creating sub-micron hotspots for applications such as heat-assisted magnetic recording (HAMR) is a challenging task. The most common approach relies on a surface-plasmon resonator (SPR), whose design dictates the size of the hotspot to always be larger than its critical dimension. Here, we present an approach which circumvents known geometrical restrictions by resorting to electric field confinement via excitation of a gap-mode (GM) between a comparatively large Gold (Au) nano-sphere (radius of 100 nm) and the magnetic medium in a grazing-incidence configuration. Operating a lambda = 785 nm laser, sub-200 nm hot spots have been generated and successfully used for GM-assisted magnetic switching on commercial CoCrPt perpendicular magnetic recording media at laser powers and pulse durations comparable to SPR-based HAMR. Lumerical electric field modelling confirmed that operating in the near-infrared regime presents a suitable working point where most of the light's energy is deposited in the magnetic layer, rather than in the nano-particle. Further, modelling is used for predicting the limits of our method which, in theory, can yield sub-30 nm hotspots for Au nano-sphere radii of 25-50 nm for efficient heating of FePt recording media with a gap of 5 nm. Published by AIP Publishing.

The eyes of some aquatic animals form images through reflective optics. Shrimp, lobsters, crayfish, and prawns possess reflecting superposition compound eyes, composed of thousands of square-faceted eye units (ommatidia). Mirrors in the upper part of the eye (the distal mirror) reflect light collected from many ommatidia onto the photosensitive elements of the retina, the rhabdoms. A second reflector, the tapetum, underlying the retina, back-scatters dispersed light onto the rhabdoms. Using microCT and cryo-SEM imaging accompanied by in situ micro-X-ray diffraction and micro-Raman spectroscopy, we investigated the hierarchical organization and materials properties of the reflective systems at high resolution and under close-to-physiological conditions. We show that the distal mirror consists of three or four layers of plate-like nanocrystals. The tapetum is a diffuse reflector composed of hollow nanoparticles constructed from concentric lamellae of crystals. Isoxanthopterin, a pteridine analog of guanine, forms both the reflectors in the distal mirror and in the tapetum. The crystal structure of isoxanthopterin was determined from crystal-structure prediction calculations and verified by comparison with experimental X-ray diffraction. The extended hydrogen-bonded layers of the molecules result in an extremely high calculated refractive index in the H-bonded plane, n = 1.96, which makes isoxanthopterin crystals an ideal reflecting material. The crystal structure of isoxanthopterin, together with a detailed knowledge of the reflector superstructures, provide a rationalization of the reflective optics of the crustacean eye.

Ficus trees are adapted to diverse environments and have some of the highest rates of photosynthesis among trees. Ficus leaves can deposit one or more of the three major mineral types found in leaves: amorphous calcium carbonate cystoliths, calcium oxalates, and silica phytoliths. In order to better understand the functions of these minerals and the control that the leaf exerts over mineral deposition, we investigated leaves from 10 Ficus species from vastly different environments (Rehovot, Israel; Bologna, Italy; Issa Valley, Tanzania; and Ngogo, Uganda). We identified the mineral locations in the soft tissues, the relative distributions of the minerals, and mineral volume contents using microcomputed tomography. Each Ficus species is characterized by a unique 3D mineral distribution that is preserved in different environments. The mineral distribution patterns are generally different on the adaxial and abaxial sides of the leaf. All species examined have abundant calcium oxalate deposits around the veins. We used micromodulated fluorimetry to examine the effect of cystoliths on photosynthetic efficiency in two species having cystoliths abaxially and adaxially (Ficus microcarpa) or only abaxially (Ficus carica). In F. microcarpa, both adaxial and abaxial cystoliths efficiently contributed to light redistribution inside the leaf and, hence, increased photosynthetic efficiency, whereas in F. carica, the abaxial cystoliths did not increase photosynthetic efficiency.

Halide perovskite (HaP) semiconductors are revolutionizing photovoltaic (PV) solar energy conversion by showing remarkable performance of solar cells made with HaPs, especially tetragonal methylammonium lead triiodide (MAPbI₃). In particular, the low voltage loss of these cells implies a remarkably low recombination rate of photogenerated carriers. It was suggested that low recombination can be due to the spatial separation of electrons and holes, a possibility if MAPbI₃ is a semiconducting ferroelectric, which, however, requires clear experimental evidence. As a first step, we show that, in operando, MAPbI₃ (unlike MAPbBr₃) is pyroelectric, which implies it can be ferroelectric. The next step, proving it is (not) ferroelectric, is challenging, because of the material's relatively high electrical conductance (a consequence of an optical band gap suitable for PV conversion) and low stability under high applied bias voltage. This excludes normal measurements of a ferroelectric hysteresis loop, to prove ferroelectricity's hallmark switchable polarization. By adopting an approach suitable for electrically leaky materials as MAPbI₃, we show here ferroelectric hysteresis from well-characterized single crystals at low temperature (still within the tetragonal phase, which is stable at room temperature). By chemical etching, we also can image the structural fingerprint for ferroelectricity, polar domains, periodically stacked along the polar axis of the crystal, which, as predicted by theory, scale with the overall crystal size. We also succeeded in detecting clear second harmonic generation, direct evidence for the material's noncentrosymmetry. We note that thematerial's ferroelectric nature, can, but need not be important in a PV cell at room temperature.

Colloidal CdSe nanoplatelets (NPLs) deposited on TiO₂ and overcoated by ZnS were explored as light absorbers in semiconductor-sensitized solar cells (SSSCs). Significant red shifts of both absorption and steady-state photoluminescence (PL) along with rapid PL quenching suggest a type II band alignment at the interface of the CdSe NPL and the ZnS barrier layer grown on the NPL layer, as confirmed by energy band measurements. The considerable red shift leads to enhanced spectral absorption coverage. Cell characterization shows an increased open-circuit voltage of 664 mV using a polysulfide electrolyte, which can be attributed to a photoinduced dipole effect created by the spatial charge separation across the nanoplatelet sensitizers. The observed short-circuit current density of 11.14 mA cm^-2 approaches the maximal theoretical current density for this choice of absorber, yielding an internal quantum efficiency of close to 100%, a clear signature of excellent charge transport and collection yields. With their steep absorption onset and negligible inhomogeneous broadening, NPL-based SSSCs are intriguing candidates for future high-voltage sensitized cells.

Calcium oxalate and silica minerals are common components of a variety of plant leaves. These minerals are found at different locations within the leaf, and there is little conclusive evidence about the functions they perform. Here tools are used from the fields of biology, optics, and imaging to investigate the distributions of calcium oxalate, silica minerals, and chloroplasts in okra leaves, in relation to their functions. A correlative approach is developed to simultaneously visualize calcium oxalates, silica minerals, chloroplasts, and leaf soft tissue in 3D without affecting the minerals or the organic components. This method shows that in okra leaves silica and calcium oxalates, together with chloroplasts, form a complex system with a highly regulated relative distribution. This distribution points to a significant role of oxalate and silica minerals to synergistically optimize the light regime in the leaf. The authors also show directly that the light scattered by the calcium oxalate crystals is utilized for photosynthesis, and that the ultraviolet component of light passing through silica bodies, is absorbed. This study thus demonstrates that calcium oxalates increase the illumination level into the underlying tissue by scattering the incoming light, and silica reduces the amount of UV radiation entering the tissue.

The recovery of a signal from the magnitude of its Fourier transform, also known as phase retrieval, is of fundamental importance in many scientific fields. It is well known that due to the loss of Fourier phase the problem in one-dimensional (1D) is ill-posed. Without further constraints, there is no unique solution to the problem. In contrast, uniqueness up to trivial ambiguities very often exists in higher dimensions, with mild constraints on the input. In this paper, we focus on the 2D phase retrieval problem and provide insight into this uniqueness property by exploring the connection between the 2D and 1D formulations. In particular, we show that 2D phase retrieval can be cast as a 1D problem with additional constraints, which limit the solution space. We then prove that only one additional constraint is sufficient to reduce the many feasible solutions in the 1D setting to a unique solution for almost all signals. These results allow to obtain an analytical approach (with combinatorial complexity) to solve the 2D phase retrieval problem when it is unique.

The growth of horizontal nanowires (NWs) guided by epitaxial and graphoepitaxial relations with the substrate is becoming increasingly attractive owing to the possibility of controlling their position, direction, and crystallographic orientation. In guided NWs, as opposed to the extensively characterized vertically grown NWs, there is an increasing need for understanding the relation between structure and properties, specifically the role of the epitaxial relation with the substrate. Furthermore, the uniformity of crystallographic orientation along guided NWs and over the substrate has yet to be checked. Here we perform highly sensitive second harmonic generation (SHG) polarimetry of polar and nonpolar guided ZnO NWs grown on R-plane and M-plane sapphire. We optically map large areas on the substrate in a nondestructive way and find that the crystallographic orientations of the guided NWs are highly selective and specific for each growth direction with respect to the substrate lattice. In addition, we perform SHG polarimetry along individual NWs and find that the crystallographic orientation is preserved along the NW in both polar and nonpolar NWs. While polar NWs show highly uniform SHG along their axis, nonpolar NWs show a significant change in the local nonlinear susceptibility along a few micrometers, reflected in a reduction of 40% in the ratio of the SHG along different crystal axes. We suggest that these differences may be related to strain accumulation along the nonpolar wires. We find SHG polarimetry to be a powerful tool to study both selectivity and uniformity of crystallographic orientations of guided NWs with different epitaxial relations.

Heat-assisted magnetic recording (HAMR) is often considered the next major step in the storage industry: it is predicted to increase the storage capacity, the read/write speed and the data lifetime of future hard disk drives. However, despite more than a decade of development work, the reliability is still a prime concern. Featuring an inherently fragile surface-plasmon resonator as a highly localized heat source, as part of a near-field transducer (NFT), the current industry concepts still fail to deliver drives with sufficient lifetime. This study presents a method to aid conventional NFT-designs by additional grazing-incidence laser illumination, which may open an alternative route to high-durability HAMR. Magnetic switching is demonstrated on consumer-grade CoCrPt perpendicular magnetic recording media using a green and a near-infrared diode laser. Sub-500 nm magnetic features are written in the absence of a NFT in a moderate bias field of only μ₀H = 0.3 T with individual laser pulses of 40 mW power and 50 ns duration with a laser spot size of 3 μm (short axis) at the sample surface - six times larger than the magnetic features. Herein, the presence of a nanoscopic object, i.e., the tip of an atomic force microscope in the focus of the laser at the sample surface, has no impact on the recorded magnetic features - thus suggesting full compatibility with NFT-HAMR.

We investigated the effect of cation exchange on the anionic framework of lightly doped CdSe:Te/CdS nanorods. In contrast with previously studied core/shell systems, the Te dopant, located in the center of the CdSe core, provides an extremely sensitive indicator for any structural changes of the anionic framework that may occur as a result of the cation exchange process. We first optimized the cation exchange procedure in order to retain the fluorescence properties of the CdSe:Te/CdS nanorods after exchange of Cd2+ for Cu+ and back to Cd2+. Next, using multiexciton spectroscopy, we were able to probe the magnitude of the exciton exciton repulsion interaction and use that to assess the degree of crystal structure conservation. Our findings provide a much stronger proof that the anion framework is indeed rigid, showing no evidence of significant migration of the anionic dopant.

The synthesis of hybrid nanostructures that have specific properties has become a significant topic for construction of "smart" nanomaterials for various applications. Formation of hybrid nanostructures, particularly those combining metals and semiconductors, often introduces new chemical, optical, and electronic properties. Here, we show a simple solution phase synthesis of multicomponent heterostructures based on the growth of metal and semiconductor onto the tips of semiconductor nanorods, leading to the formation of a hybrid semiconductor/semiconductor/metal structure. The synthesis involves the reduction of Pt-acetylacetonate to achieve selective growth of a Pt metal tip onto one side of the CdS rod, followed by the thermal decomposition of Pb-bis(diethyldithiocarbamate) to grow a PbS nanocrystal onto the other tip of the nanorod. The band alignment between the two semiconductor components as well as the alignment with the Fermi level of the metal could support intraparticle charge transfer, which is often sought after for photocatalysis applications. Yet, we show, using femtosecond transient differential absorption spectroscopy (TDA), that carrier dynamics in such a hybrid system can be more complex than that predicted simply by bulk band alignment considerations. This highlights the importance of the design of band alignment and interface passivation and its role in affecting carrier dynamics within hybrid nanostructures.

Cadmium chalcogenide nanoplatelet (NPL) synthesis has recently witnessed a significant advance in the production of more elaborate structures such as core/shell and core/crown NPLs. However, controlled doping in these structures has proved difficult because of the restrictive synthetic conditions required for 2D anisotropic growth. Here, we explore the incorporation of tellurium (Te) within CdSe NPLs with Te concentrations ranging from doping to alloying. For Te concentrations higher than similar to 30%, the CdSexTe(1-x) NPLs show emission properties characteristic of an alloyed material with a bowing of the band gap for increased concentrations of Te. This behavior is in line with observations in bulk samples and can be put in the context of the transition from a pure material to an alloy. In the dilute doping regime, CdSe: Te NPLs, in comparison to CdSe NPLs, show a distinct photoluminescence (PL) red shift and prolonged emission lifetimes (LTs) associated with Te hole traps which are much deeper than in bulk samples. Furthermore, single particle spectroscopy reveals dramatic modifications in PL properties. In particular, doped NPLs exhibit photon antibunching and emission dynamics significantly modified compared to undoped or alloyed NPLs.

Light-induced tunable photonic systems are rare in nature, and generally beyond the state-of-the-art in artificial systems. Sapphirinid male copepods produce some of the most spectacular colors in nature. The male coloration, used for communication purposes, is structural and is produced from ordered layers of guanine crystals separated by cytoplasm. It is generally accepted that the colors of the males are related to their location in the epipelagic zone. By combining correlative reflectance and cryoelectron microscopy image analyses, together with optical time lapse recording and transfer matrix modeling, it is shown that male sapphirinids have the remarkable ability to change their reflectance spectrum in response to changes in the light conditions. It is also shown that this color change is achieved by a change in the thickness of the cytoplasm layers that separate the guanine crystals. This change is reversible, and is both intensity and wavelength dependent. This capability provides the male with the ability to efficiently reflect light under certain conditions, while remaining transparent and hence camouflaged under other conditions. These copepods can thus provide inspiration for producing synthetic tunable photonic arrays.

In the hybrid structure employing CdS/N719 QD/dye co-sensitized photoelectrode, it is revealed by consistent results from the steady-state and time-resolved photoluminescence (PL) that the QD regeneration was realized by the effective charge transfer from the dye sensitizer, resulting in a significant PL quenching. This is helpful for retarding the recombination in the device. Furthermore, by varying the thickness of the TiO₂ film, the light harvesting efficiency and the charge collection efficiency were balanced, leading to an optimal TiO₂ thickness of 22 μm and a power conversion efficiency (PCE) of about 6.5%. Compared with the dye N719 only sensitized system with the same dye loading(with an efficiency of 5.09%), the co-sensitization of dye N719 and CdS (with an efficiency of 6.44%) enhanced the overall PCE significantly by 26.5% Notably, this is one of the best reported efficiencies for the QD-dye co-sensitized solar cells.

Upconversion is a nonlinear process in which two, or more, long wavelength photons are converted to a shorter wavelength photon. It holds great promise for bioimaging, enabling spatially resolved imaging in a scattering specimen and for photovoltaic devices as a means to surpass the Shockley-Queisser efficiency limit. Here, we present dual near-infrared and visible emitting PbSe/CdSe/CdS nanocrystals able to upconvert a broad range of NIR wavelengths to visible emission at room temperature. The synthesis is a three-step process, which enables versatility and tunability of both the visible emission color and the NIR absorption edge. Using this method, one can achieve a range of desired upconverted emission peak positions with a suitable NIR band gap.

Nonlinear optical processes can be dramatically enhanced via the use of localized surface plasmon modes in metal nanoparticles. Here we show how more elaborate structures, based on shape-controlled Au/Cu₂O core/shell nanostructures, enable further enhancement of the nanoparticle third-harmonic scattering cross-section. The semiconducting component takes a twofold role in this structure, both providing a knob to tune the resonant frequency of the gold plasmon and providing resonant enhancement by virtue of its excitonic states. The advantages and deficiencies of using such core/shell metal/semiconductor structures are discussed.

The fresh water fish neon tetra has the ability to change the structural color of its lateral stripe in response to a change in the light conditions, from blue-green in the light-adapted state to indigo in the dark-adapted state. The colors are produced by constructive interference of light reflected from stacks of intracellular guanine crystals, forming tunable photonic crystal arrays. We have used micro X-ray diffraction to track in time distinct diffraction spots corresponding to individual crystal arrays within a single cell during the color change. We demonstrate that reversible variations in crystal tilt within individual arrays are responsible for the light-induced color variations. These results settle a long-standing debate between the two proposed models, the "Venetian blinds" model and the "accordion" model. The insight gained from this biogenic light-induced photonic tunable system may provide inspiration for the design of artificial optical tunable systems.

(Figure Presented). The performances of hybrid organic-inorganic photovoltaics composed of conjugated polymers and metal oxides are generally limited by poor electronic coupling at hybrid interfaces. In this study, physicochemical interactions and bonding at the organic-inorganic interfaces are promoted by incorporating organoruthenium dye molecules into self-assembled mesostructured conjugated polymer-titania composites. These materials are synthesized from solution in the presence of surfactant structure-directing agents (SDA) that solubilize and direct the nanoscale compositions and structures of the conjugated polymer, dye, and inorganic precursor species. Judicious selection of the SDA and dye species, in particular, exploits interactions that direct the dye species to the inorganic-organic interfaces, leading to significantly enhanced electronic coupling, as well as increased photoabsorption efficiency. This is demonstrated for the hydrophilic organoruthenium dye N3, used in conjunction with alkyleneoxide triblock copolymer SDA, polythiophene conjugated polymer, and titania species, in which the N3 dye species are localized in molecular proximity to and interact strongly with the titania framework, as established by solid-state NMR spectroscopy. In contrast, a closely related but more hydrophobic organoruthenium dye, Z907, is shown to interact more weakly with the titania framework, yielding significantly lower photocurrent generation. The strong SDA-directed N3-TiO_x interactions result in a significant reduction of the lifetime of the photoexcited state and enhanced macroscopic photocurrent generation in photovoltaic devices. This study demonstrates that multicomponent self-assembly can be harnessed for the fabrication of hierarchical materials and devices with nanoscale control of chemical compositions and surface interactions to improve photovoltaic properties.

Phase measurement is a long-standing challenge in a wide range of applications, from X-ray imaging to astrophysics and spectroscopy. While in some scenarios the phase is resolved by an interferometric measurement, in others it is reconstructed via numerical optimization, based on some a-priori knowledge about the signal. The latter commonly use iterative algorithms, and thus have to deal with their convergence, stagnation, and robustness to noise. Here we combine these two approaches and present a new scheme, termed double blind Fourier holography, providing an efficient solution to the phase problem in two dimensions, by solving a system of linear equations. We present and experimentally demonstrate our approach for the case of lens-less imaging.

The organic-inorganic interfacial chemical composition and interaction have a critical influence on the performance of corresponding hybrid photovoltaic devices. Such interfaces have been shown to be controlled by using surfactants to promote contact between the organic electron-donating conjugated polymer species and the inorganic electron-accepting metal oxides. However, the location of the surfactant at the organic-inorganic interface often hinders donor-acceptor charge transfer and limits the device performance. In this study we have replaced the conventional surfactant with an optically and electronically active amphiphilic polythiophene that both compatibilizes the conjugated polymer and metal oxide, and plays an active role in the charge generation process. Specifically, a polythiophene with hydrophilic urethane side-groups allows the formation of a fine continuous ZnO network in P3HT with an organic-inorganic interfacial chemical interaction and a high surface area. A combination of FTIR, absorption and photoluminescence life-time spectroscopies yields insights into the composition and nano-scale interaction of the components, while high-resolution electron microscopy reveals the morphology of the films. The control over morphology and electronic coupling at the hybrid interface is manifested in photovoltaic devices with improved performances.

The optical and electronic properties of suspensions of inorganic fullerene-like nanoparticles of MoS₂ are studied through light absorption and zeta-potential measurements and compared to those of the corresponding microscopic platelets. The total extinction measurements show that, in addition to excitonic peaks and the indirect band gap transition, a new peak is observed at 700-800 nm. This spectral peak has not been reported previously for MoS₂. Comparison of the total extinction and decoupled absorption spectrum indicates that this peak largely originates from scattering. Furthermore, the dependence of this peak on nanoparticle size, shape, and surface charge, as well as solvent refractive index, suggests that this transition arises from a plasmon resonance.

Temporal focusing (TF) allows for axially confined wide-field multi-photon excitation at the temporal focal plane. For temporally focused Gaussian beams, it was shown both theoretically and experimentally that the temporal focus plane can be shifted by applying a quadratic spectral phase to the incident beam. However, the case for more complex wave-fronts is quite different. Here we study the temporal focus plane shift (TFS) for a broader class of excitation profiles, with particular emphasis on the case of temporally focused computer generated holography (CGH) which allows for generation of arbitrary, yet speckled, 2D patterns. We present an analytical, numerical and experimental study of this phenomenon. The TFS is found to depend mainly on the autocorrelation of the CGH pattern in the direction of the beam dispersion after the grating in the TF setup. This provides a pathway for 3D control of multi-photon excitation patterns.

The use of wavefront shaping to generate extended optical excitation patterns which are confined to a predetermined volume has become commonplace on various microscopy applications. For multiphoton excitation, three-dimensional confinement can be achieved by combining the technique of temporal focusing of ultra-short pulses with different approaches for lateral light shaping, including computer generated holography or generalized phase contrast. Here we present a theoretical and experimental study on the effect of scattering on the propagation of holographic beams with and without temporal focusing. Results from fixed and acute cortical slices show that temporally focused spatial patterns are extremely robust against the effects of scattering and this permits their three-dimensionally confined excitation for depths more than 500 μm. Finally we prove the efficiency of using temporally focused holographic beams in two-photon stimulation of neurons expressing the red-shifted optogenetic channel C1V1.

Luminescence upconversion nanocrystals capable of converting two low-energy photons into a single photon at a higher energy are sought-after for a variety of applications, including bioimaging and photovoltaic light harvesting. Currently available systems, based on rare-earth-doped dielectrics, are limited in both tunability and absorption cross-section. Here we present colloidal double quantum dots as an alternative nanocrystalline upconversion system, combining the stability of an inorganic crystalline structure with the spectral tunability afforded by quantum confinement. By tailoring its composition and morphology, we form a semiconducting nanostructure in which excited electrons are delocalized over the entire structure, but a double potential well is formed for holes. Upconversion occurs by excitation of an electron in the lower energy transition, followed by intraband absorption of the hole, allowing it to cross the barrier to a higher energy state. An overall conversion efficiency of 0.1% per double excitation event is achieved.

An all-inorganic compound colloidal quantum dot incorporating a highly emissive CdSe core, which is linked by a CdS tunneling barrier to an engineered charge carrier trap composed of PbS, is designed, and its optical properties are studied in detail at the single-particle level. Study of this structure enables a deeper understanding of the link between photoinduced charging and surface trapping of charge carriers and the phenomenon of quantum dot blinking. In the presence of the hole trap, a "gray" emissive state appears, associated with charging of the core. Rapid switching is observed between the "on" and the "gray" state, although the switching dynamics in and out of the dark "off" state remain unaffected. This result completes the links in the causality chain connecting charge carrier trapping, charging of QDs, and the appearance of a "gray" emission state.

We demonstrate the existence of the spectral phase shift a pulse experiences when it is subjected to spectral focusing. This π2 phase shift is the spectral analog of the Gouy phase shift a 2D beam experiences when it crosses its focal plane. This spectral Gouy phase shift is measured using spectral interference between a reference pulse and a negatively chirped parabolic pulse experiencing spectral focusing in a nonlinear photonic crystal fiber. To avoid inherent phase instability in the measurement, both reference and parabolic pulses are generated with a 4-f pulse shaper and copropagate in the same fiber. We measure a spectral phase shift of π2, reaffirming its generality as a consequence of wave confinement in the spatial, temporal and spectral domains.

α-Ag₂S, with a direct forbidden bandgap of about 1.0 eV, is a non-toxic low bandgap semiconductor which can readily be deposited in the form of a thin film by chemical bath deposition. In a solar cell configuration, it can potentially provide a high short-circuit current due to the infrared absorption, and is compatible with the polysulfide electrolyte. Its practical use in a solar cell depends, however, critically on band alignment between the Ag₂S, the oxide anode and the electrolyte redox potential. Here we examine the conduction band (CB) offsets in the nanostructured α-Ag ₂S sensitized TiO₂ and SnO₂ electrodes by X-ray Photoelectron Spectroscopy, and show that they can significantly differ from the extrapolated bulk values. The much higher CB offset for SnO ₂/Ag₂S interface (∼0.6 eV) compared with that of ∼0.2 eV for TiO₂/Ag₂S, supplied a sufficient injection driving force and was favorable for the electron separation at the heterojunction. When fabricated into solar cells, a dramatically higher current density under AM 1.5 illumination for the SnO₂/Ag₂S heterojunction was obtained, which was contributed by the efficient electron injection.

Plasmonic antennas are key elements to control the luminescence of quantum emitters. However, the antenna's influence is often hidden by quenching losses. Here, the luminescence of a quantum dot coupled to a gold dimer antenna is investigated. Detailed analysis of the multiply excited states quantifies the antenna's influence on the excitation intensity and the luminescence quantum yield separately.

Although colloidal quantum dots (QDs) exhibit excellent photostability under mild excitation, intense illumination makes their emission increasingly intermittent, eventually leading to photobleaching. We study fluorescence of two commonly used types of QDs under pulsed excitation with varying power and repetition rate. The photostability of QDs is found to improve dramatically at low repetition rates, allowing for prolonged optical saturation of QDs without apparent photodamage. This observation suggests that QD blinking is facilitated by absorption of light in a transient state with a microsecond decay time. Enhanced photostability of generic quantum dots under intense illumination opens up new prospects for fluorescence microscopy and spectroscopy.

A highly efficient quantum dot (QD)/inorganic layer/dye molecule sandwich structure was designed and applied in electrochemical QD-sensitized solar cells. The key component TiO ₂/CdS/ZnS/N719 hybrid photoanode with ZnS insertion between the two types of sensitizers was demonstrated not only to efficiently extend the light absorption but also to suppress the charge recombination from either TiO ₂ or CdS QDs to electrolyte redox species, yielding a photocurrent density of 11.04 mA cm ^-2, an open-circuit voltage of 713 mV, a fill factor of 0.559, and an impressive overall energy conversion efficiency of 4.4%. More importantly, the cell exhibited enhanced photostability with the help of the synergistic stabilizing effect of both the organic and the inorganic passivation layers in the presence of a corrosive electrolyte.

Intraband hole relaxation of colloidal Te-doped CdSe quantum dots is studied using state-selective transient absorption spectroscopy. The dots are excited at the band edge, and the defect band bleach caused by state filling of the hole is probed. Close to the defect energy, the hole relaxation is substantially slowed down, indicating a gap separating the defect state from the CdSe band edge. A clear dependence of the relaxation time with the QDs size is presented, implying that the hole relaxation is mediated by longitudinal optical (LO) phonon modes of the CdSe host. In addition, we find that overcoating the quantum dots by two monolayers of a ZnS shell extends the hole relaxation time by a factor of 2, suggesting a combined effect of LO phonons and surface effects governing intraband hole relaxation.

A synthetic route for preparation of inorganic WS ₂ nanotube (INT)-colloidal semiconductor quantum dot (QD) hybrid structures is developed, and transient carrier dynamics on these hybrids are studied via transient photoluminescence spectroscopy utilizing several different types of QDs. Measurements reveal efficient resonant energy transfer from the QDs to the INT upon photoexcitation, provided that the QD emission is at a higher energy than the INT direct gap. Charge transfer in the hybrid system, characterized using QDs with band gaps below the INT direct gap, is found to be absent. This is attributed to the presence of an organic barrier layer due to the relatively long-chain organic ligands of the QDs under study. This system, analogous to carbon nanotube-QD hybrids, holds potential for a variety of applications, including photovoltaics, luminescence tagging and optoelectronics. This journal is

The use of optogenetics, the technology that combines genetic and optical methods to monitor and control the activity of specific cell populations, is now widely adopted in neuroscience. The development of optogenetic tools, such as natural photosensitive ion channels and pumps or calcium- and voltage-sensitive proteins, has been growing tremendously during the past 10 years, thanks to the improvement of their performances in terms of facilitating light stimulation. To this aim, efficient illumination methods are also needed. The most common way to photostimulate optogenetic tools has been, so far, widefield illumination with visible light. However, the necessity of addressing the complexity of brain architecture has recently imposed switching to the use of two-photon excitation, which provides a better spatial specificity and deeper penetration in scattering tissue. Two-photon excitation is still challenging, due to intrinsic characteristics of optogenetic tools (e.g., the low conductivity of light-sensitive channels), and efficient illumination methods are therefore essential for advancing in this domain. Here, we present a review on the existing two-photon optical approaches for photoactivation of optogenetic tools, and future perspectives for the widespread implementation of these techniques.

The use of Förster resonant energy transfer (FRET) has recently shown promise for significant improvement in various aspects of photoelectrochemical cells. Considering the particular case of semiconductor quantum dot donors, we show that they can enable broadening of the spectral response and increased optical density of the cell, thus increasing the current while potentially decreasing the electrode thickness. Moreover, the use of FRET and the separation of optical and electrical function within the cell provide flexibility in the choice of materials for both the antenna and sensitizer, opening new paths for performance optimization and achievement of long-term cell stability.

Understanding how quantum dot (QD)-sensitized solar cells operate requires accurate determination of the offset between the lowest-unoccupied molecular orbital (LUMO) of the sensitizer quantum dot and the conduction band of the metal oxide electrode. We present detailed optical spectroscopy, low-energy photoelectron spectroscopy, and two-photon photoemission studies of the energetics of size-selected CdSe colloidal QDs deposited on TiO₂ electrodes. Our experimental findings show that in contrast to the prediction of simplified models based on bulk band offsets and effective mass considerations, band alignment in this system is strongly modified by the interaction between the QDs and the electrode. In particular, we find relatively small conduction band- LUMO offsets, and near "pinning" of the QD LUMO relative to the conduction band of the TiO₂ electrode, which is explained by the strongQD-electrode interaction. That interaction is the origin for the highly efficientQDto electrode charge transfer, and it also bears on the possibility of hot carrier injection in these types of cells.

We study second-harmonic generation from single CdTe/CdS core/shell rod-on-dot nanocrystals with different geometrical parameters, which allow to fine tune the nonlinear properties of the nanostructure. These hybrid semiconductor-semiconductor nanoparticles exhibit extremely strong and stable second-harmonic emission, although the size of CdTe core is still within the strong quantum confinement regime. The orientation sensitive polarization response is analyzed by means of a pointwise additive model of the third-order tensors associated to the nanoparticle components. These findings prove that engineering of semiconducting complex heterostructures at the single nanoparticle scale can lead to extremely bright nanometric nonlinear light sources.

Spectral compression by self-phase modulation of amplitude-and phase-shaped pulses is demonstrated as superior compared to pulses that have only been phase shaped. We synthesize linearly negatively chirped parabolic pulses, which we send through a nonlinear photonic crystal fiber, in which self-phase modulation compresses the spectrum of the pulses to within 20% of the Fourier transform limit.

We investigated how isolated are the electronic states of the core in a core-shell (c/s) nanoparticles (NPs) from the surface, when the particles are self-assembled on Au substrates via a dithiol (DT) organic linker. Applying photoemission spectroscopy the electronic states of CdSe core only and CdSe/ZnS c/s NPs were compared. The results indicate that in the c/s NPs the HOMO interacts strongly with electronic states in the Au substrate and is pinned at the same energies, relative to the Fermi level, as the core only NPs. When the capping molecules of the NPs were replaced with thiolated molecules, an interaction between the thiol groups and the electronic states of the NPs was observed that depends on the properties of the NPs studied. Thiols binding to the NPs induce the formation of surface trap states. However, while for the core only CdSe NPs the LUMO states are strongly coupled to the surface traps, independent of their size, this coupling is size dependent in the case of the CdSe/ZnS c/s NPs. For a large core, the LUMO is decoupled from the surface trap states. When the core is small enough, the LUMO is delocalized and interacts with these states.

We propose a new linear and all-optical method for attosecond pulses characteri-zation. Our scheme is based only on spectral and polarization measurements. We demonstrate this method numerically on attosecond pulses generated from aligned CO₂ molecules.

Optical antennas are essential devices to interface light to nanoscale volumes and locally enhance the electromagnetic intensity. Various experimental methods can be used to quantify the antenna amplification on the emission process, yet characterizing the antenna amplification at the excitation frequency solely is a challenging task. Such experimental characterization is highly needed to fully understand and optimize the antenna response. Here, we describe a novel experimental tool to directly measure the antenna amplification on the excitation field independently of the emission process. We monitor the transient emission dynamics of colloidal quantum dots and show that the ratio of doubly to singly excited state photoluminescence decay amplitudes is an accurate tool to quantify the local excitation intensity amplification. This effect is demonstrated on optical antennas made of polystyrene microspheres and gold nanoapertures, and supported by numerical computations. The increased doubly excited state formation on nanoantennas realizes a new demonstration of enhanced light-matter interaction at the nanoscale.

The observed intermittent light emission from colloidal semiconductor nanocrystals has long been associated with Auger recombination assisted quenching. We test this view by observing transient emission dynamics of CdSe/CdS/ZnS semiconductor nanocrystals using time-resolved photon counting. The size and intensity dependence of the observed decay dynamics seem inconsistent with those expected from Auger processes. Rather, the data suggest that in the "off" state the quantum dot cycles in a three-step process: photoexcitation, rapid trapping, and subsequent slow nonradiative decay.

Biological photonic systems composed of anhydrous guanine crystals evolved separately in several taxonomie groups. Here, two such systems found in fish and spiders, both of which make use of anhydrous guanine crystal plates to produce structural colors, are examined. Measurements of the photoniccrystal structures using cryo-SEM show that the crystal plates in both fish skin and spider integument are ∼20-nm thick. The reflective unit in the fish comprises stacks of single plates alternating with ∼ 230-nm-thick cytoplasm layers. In the spiders the plates are formed as doublet crystals, cemented by 30-nm layers of amorphous guanine, and are stacked with ∼200nm of cytoplasm between crystal doublets. They achieve light reflective properties through the control of crystal morphology and stack dimensions, reaching similar efficiencies of light reflectivity in both fish skin and spider integument. The structure of guanine plates in spiders are compared with the more common situation in which guanine occurs in the form of relatively unorganized prismatic crystals, yielding a matt white coloration.

Blinking in colloidal nanocrystals is studied through photon counting from single nanocrystals. Size independent 'off' state dynamics are observed in contrast to predictions by prevailing models which attribute 'dark' states to Auger recombination assisted quenching.

We demonstrate an optical nonlinearity free ultrashort pulse characterization technique relying on spectral component localization in time domain. Ultraweak pulses in NIR to XUV range can be characterized with resolution depending only on integration time.

Nanoparticles emitting two-photon luminescence are broadly used as photostable emitters for nonlinear microscopy. Second-harmonic generation (SHG) as another two-photon mechanism offers complementary optical properties but the reported sizes of nanoparticles are still large, of a few tens of nanometers. Herein, coherent SHG from single core/shell CdTe/CdS nanocrystals with a diameter of 10 to 15nm is reported. The nanocrystal excitation spectrum reveals resonances in the nonlinear efficiency with an overall maximum at about 970 nm. Polarization analysis of the second-harmonic emission confirms the expected zinc blende symmetry, and allows extraction of the three-dimensional nanocrystal orientation. The small size of these nonlinearly active quantum dots, together with the intrinsic coherence and orientation sensitivity of the SHG process, are well adapted for ultrafast probing of optical near-fields with high resolution as well as for orientation tracking for bioimaging applications.

Assemblies of CdSe nanoparticles (NPs) on a dithiol-coated Au electrode were created, and their electronic energetics were quantified. This report describes the energy level alignment of the filled and unfilled electronic states of CdSe nanoparticles with respect to the Au Fermi level. Using cyclic voltammetry, it was possible to measure the energy of the filled states of the CdSe NPs with respect to the Au substrate relative to a Ag/AgNO₃ electrode, and by using photoemission spectroscopy, it was possible to independently measure both the filled state energies (via single photon photoemission) and those of the unfilled states (via two photon photoemission) with respect to the vacuum level. Comparison of these two different measures shows good agreement with the IUPAC-accepted value of the absolute electrode potential. In contrast to the common model of energy level alignment, the experimental findings show that the CdSe filled states become 'pinned' to the Fermi level of the Au electrode, even for moderately small NP sizes.

Depth resolved multiphoton microscopy is performed by collecting the fluorescent emission of two-exciton states in colloidal quantum dots. This process involves two consecutive resonant absorption events, thus requiring unprecedented low excitation energy and peak power.

Temporal focusing of ultrashort pulses has been shown to enable wide-field depth-resolved two-photon fluorescence microscopy. In this process, an entire plane in the sample is selectively excited by introduction of geometrical dispersion to an ultrashort pulse. Many applications, such as multiphoton lithography, uncaging or region-of-interest imaging, require, however, illumination patterns which significantly differ from homogeneous excitation of an entire plane in the sample. Here we consider the effects of such spatial modulation of a temporally focused excitation pattern on both the generated excitation pattern and on its axial confinement. The transition in the axial response between line illumination and wide-field illumination is characterized both theoretically and experimentally. For 2D patterning, we show that in the case of amplitude-only modulation the axial response is generally similar to that of wide-field illumination, while for phase-and-amplitude modulation the axial response slightly deteriorates when the phase variation is rapid, a regime which is shown to be relevant to excitation by beams shaped using spatial light modulators. Finally, general guidelines for the use of spatially modulated temporally focused excitation are presented.

We present a simplified single-beam scheme for depletion-based sub-diffraction-limited imaging which allows for less restrictive illumination conditions. This is done by introducing the concept of fluorophores exhibiting emission depletion upon saturation. We discuss the circumstances under which such a depletion based process is possible, and derive the scaling of the spatial resolution utilizing this scheme. Next, we analyze the proper illumination conditions both in space and time required for sub diffraction limited imaging, and show that it is applicable only under pulsed excitation. Finally, our scheme's advantages and shortcomings relative to alternative realizations of depletion-based sub-diffraction-limited microscopy are discussed.

Depth resolved multiphoton microscopy is performed by collecting the fluorescent emission of two-exciton states in colloidal quantum dots. This process involves two consecutive resonant absorption events, thus requiring unprecedented low excitation energy and peak power.

Depth resolved multiphoton microscopy is performed by collecting the fluorescent emission of two-exciton states in colloidal quantum dots. The biexciton is formed via two sequential resonant absorption events. Due to the large absorption cross-section and the long lifetime of the intermediate (singly excited) state, unprecedented low excitation energy and peak powers (down to 105W/cm2) are required to generate this nonlinear response.Depending on the quantum dot parameters, the effective two-photon cross section can be as large as 1010 GM,orders of magnitude higher than for nonresonant excitation. The biexciton emission can be differentiated from that of the singly excited state by utilizing its different transient dynamics. Alternate methods for discrimination are also discussed. This system is ideal for performing three-dimensional microscopy using low excitation power. Moreover, it enables to perform multiphoton imaging even with near-infrared emitting quantum dots, which are highly compatible with imaging deep into a scattering tissue. The depth resolution of our microscope is shown to be equivalent to a standard two-photon microscope. The system also shows slow saturation due to the contribution of higher (triply and above) excited states to the emitted signal.

We present a novel technique to achieve both high efficiency and broad bandwidth in SFG process using adiabatic conversion scheme, adapted from NMR and light-matter interaction. The robustness and tunability of the scheme are discussed.

Multiphoton excitation by temporally focused pulses can be combined with spatial Fourier-transform pulse shaping techniques to enhance spatial control of the excitation volume. Here we propose and demonstrate an optical system for the generation of such spatiotemporally engineered light pulses using a combination of spatial control by a two-dimensional reconfigurable light modulator, with a dispersive optical setup for temporal focusing. We show that although the properties of a holographic beam significantly differ from those of plane-wave illumination used in previous temporal focusing realizations, this leads only to a slightly reduced axial resolution. We show that the system can provide scanning-less, arbitrarily shaped, depth resolved excitation patterns that offer new perspectives for multiphoton photoactivation and optical lithography applications.

Optical sectioning is performed by collecting the fluorescent emission of two-exciton states in colloidal quantum dots. The two-exciton state is created by two consecutive resonant absorption events, thus requiring unprecedented low excitation energy and peak powers as low as 10⁶ W/cm². The depth resolution is shown to be equivalent to that of standard multiphoton microscopy, and it was found to deteriorate only slowly as saturation of the two-exciton state is approached, owing to signal contribution from higher excitonic states.

Carrier (exciton) multiplication in colloidal InAs/CdSe/ZnSe core-shell quantum dots (QDs) is investigated using terahertz time-domain spectroscopy, time-resolved transient absorption, and quasi-continuous wave excitation spectroscopy. For excitation by high-energy photons (∼2.7 times the band gap energy), highly efficient carrier multiplication (CM) results in the appearance of multi-excitons, amounting to ∼ 1.6 excitons per absorbed photon. Multi-exciton recombination occurs within tens of picoseconds via Auger-type processes. Photodoping (i.e., photoinjection of an exciton) of the QDs prior to excitation results in a reduction of the CM efficiency to ~1.3. This exciton-induced reduction of CM efficiency can be explained by the twofold degeneracy of the lowest conduction band energy level. We discuss the implications of our findings for the potential application of InAs QDs as light absorbers in solar cells.

The effect of a self-assembled organized organic monolayer on the two-photon photoemission from semiconductor substrates was investigated. It has been found that the monolayer affects the relative yield of photoelectrons emitted by p -polarized versus s -polarized light. In addition, the monolayer affects the angular distribution of the ejected electrons. The effect on the photoelectron yield is attributed to the monolayer "smoothing" the electronic potential on the surface by eliminating surface states and dangling bonds. The effect on the angular distribution is attributed to a post-ejection interaction between the photoelectrons and the adsorbed molecules.

The transition probability of a two-photon absorption (TPA) process in atomic Cesium, excited by phase-controlled temporally focused ultrashort pulses is shown to be spatially modulated in a controlled manner. In particular, we demonstrate the generation of a dark nonlinear focus. By controlling the excitation pulse shape along the propagation coordinate we create a region in space where the TPA rate vanishes which is flanked by bright regions.

Polarization gating of high-order harmonic generation takes advantage of the significant reduction of harmonic generation efficiency for elliptically polarized excitation fields, in order to generate short bursts of harmonic radiation from relatively long pulses. We show that the currently used method for generation of polarization gated pulses using wave-plate combinations is inefficient, and propose an alternative method based on polarization pulse shaping techniques. This method is shown to be significantly more efficient and to enable significant shortening of the gate duration. Using this scheme, isolated attosecond pulses should be achievable with excitation pulses of a duration as long as 20 fs.

In most coherent control experiments with femtosecond pulses, the temporal shape of the pulses is maintained throughout the interaction region. Here we show how pulses can be controlled such that their shapes vary rapidly even when propagating very short distances of a few micrometers. This changing pulse shape has a significant effect on coherent nonlinear optical processes. Here we study third-harmonic generation induced by a coherently controlled excitation pulse whose temporal profile changes along the axial coordinate. We show how such manipulations can be used to improve the axial resolution in a multiphoton optical microscope.

The ability to perform optical sectioning is one of the great advantages of laser-scanning microscopy. This introduces, however, a number of difficulties due to the scanning process, such as lower frame rates due to the serial acquisition process. Here we show that by introducing spatiotemporal pulse shaping techniques to multiphoton microscopy it is possible to obtain full-frame depth resolved imaging completely without scanning. Our method relies on temporal focusing of the illumination pulse. The pulsed excitation field is compressed as it propagates through the sample, reaching its shortest duration at the focal plane, before stretching again beyond it. This method is applied to obtain depth-resolved two-photon excitation fluorescence (TPEF) images of drosophila egg-chambers with nearly 10⁵ effective pixels using a standard Ti:Sapphire laser oscillator.

All-optical processing of the entire vibrational spectrum was investigated by using a more complex and carefully designed probe pulse. The probe pulse was constructed containing contributions from several narrow spectral bands with controllable intensities and phases. Strong interference effcts were observed in the coherent anti-Stokes Raman spectroscopy (CARS) which resulted from the presence of several spectrally separated probe frequencies. A scheme for nonlinear spectroscopy was also suggested, where the information from the entire spectrum could be collected into a single coherent entity through coherent control of the nonlinear process.

We demonstrate a new scanning femtosecond pulse-shaping technique that allows pulse shapes to be modulated at kilohertz rates. This technique is particularly useful for lock-in measurements in which the signal is synchronized with the alternating pulse shapes. The pulse-shape lock-in technique is demonstrated in resonant coherent anti-Stokes Raman scattering, where it is shown to significantly improve the ratio of the resonant signal to both the nonresonant background and to noise.

The late-time growth rate of the Richtmyer-Meshkov instability was experimentally studied at different Atwood numbers with two-dimensional (2D) and three-dimensional (3D) single-mode initial perturbations. The results of these experiments were found to be in good agreement with the results of the theoretical model and numerical simulations. In another set of experiments a bubble-competition phenomenon, which was observed in previous work for 2D initial perturbation (Sadot et al., 1998), was shown to exist also when the initial perturbation is of a 3D nature.

The present article describes an experimental study that is a part of an integrated theoretical (Rikanati et al. 2003) and experiential investigation of the Richtmyer-Meshkov (RM) hydrodynamic instability that develops on a perturbed contact surface by a shock wave. The Mach number and the high initial-amplitude effects on the evolution of the single-mode shock-wave-induced instability were studied. To distinguish between the above-mentioned effects, two sets of shock-tube experiments were conducted: high initial amplitudes with a low-Mach incident shock and small amplitude initial conditions with a moderate-Mach incident shock. In the high-amplitude experiments a reduction of the initial velocity with respect to the linear prediction was measured. The results were compared to those predicted by a vorticity deposition model and to previous experiments with moderate and high Mach numbers done by others and good agreement was found. The result suggested that the high initial-amplitude effect is the dominant one rather than the high Mach number effect as suggested by others. In the small amplitude-moderate Mach numbers experiments, a reduction from the impulsive theory was noted at late stages. It is concluded that while high Mach number effect can dramatically change the behavior of the flow at all stages, the high initial-amplitude effect is of minor importance at the late stages. That result is supported by a two-dimensional numerical simulation.

The phase-and-polarization coherent control, which was applied to control the nonlinear response of a quantum system was discussed. It was used to obtain high-resolution background-free single-pulse coherent anti-Stokes Raman spectra. Analysis showed that this form of single-pulse Raman spectroscopy was a demonstration of the capabilities and promise of this new pulse shaping technique.

We demonstrate third-harmonic microscopy of thin biological objects using a Ti:sapphire laser source. A very simple modification to the collection and detection systems of the microscope allows for the detection of the third-harmonic signal near 270 nm. We show that, using a Ti:sapphire laser, we can combine third-harmonic imaging with two-photon excitation fluorescence simultaneously. Compared to other possible laser sources for third-harmonic microscopy, the Ti:sapphire laser provides better optical resolution, better power availability, and is less absorbed in water. We find that the Ti:sapphire laser is the most suitable laser source for third-harmonic microscopy for thin biological specimens.

The coherent transient enhancement of optically induced resonant transitions was discussed. Using pulse shaping techniques, the total transient population was enhanced by inducing constructive interference, and for 100 fs pulses, an enhancement by a factor of about 2.5 was demonstrated. The results showed that the population transient peak remains nearly constant, even when the absorption coefficient was increased by over three order of magnitude.

The creation of a plasma atmosphere in laser-target interactions increases the distance between the regions of laser absorption and hydrodynamic instability (ablation front), thus allowing thermal smoothing and a reduction of laser-imprinted modulations that reach the unstable ablation region. The total laser imprinting is reduced with pulse shapes that produce a plasma atmosphere more rapidly and by the implementation of temporal beam smoothing. These effects are measured and found to be consistent with models for the hydrodynamics and optical smoothing by spectral dispersion (SSD). Imprinting is reduced as the laser bandwidth is increased from 0.2 to 1.0 THz.

The nonlinear growth of the multimode Rayleigh-Taylor (RT), Richtmyer-Meshkov (RM), and Kelvin-Helmholtz (KH) instabilities is treated by a similar statistical mechanics merger model, using bubbles as the elementary particle in the RM and RT instabilities and eddies in the KH instability. Two particle interaction is demonstrated and merger rates are calculated. Using a statistical merger model, the mixing front evolution scaling law is derived. For the RT bubble front height a scaling law of αAgt2, with α ≅ 0.05, is derived. For the RM bubble front, a power law of t0.4 is obtained for all Atwood numbers. For the KH case the mixing zone grows linearly with time through a mechanism of eddy merger. Good agreement with simulations and experiments is achieved.

A theoretical model for the ablatively driven RayleighTaylor (RT) instability single-mode and multimode mixing fronts is presented. The effect of ablation is approximately included in a Layzer-type potential flow model, yielding the description of both the single-mode evolution and the two-bubble nonlinear competition. The reduction factor of the linear growth rate due to ablative stabilization obtained by the model is similar to the Takabe formula. The single-bubble terminal velocity is found to be similarly reduced by ablation, in good agreement with numerical simulations. Two-bubble competition is calculated, and a statistical mechanics model for multimode fronts is presented. The asymptotic ablation correction to the classical RT [formula omitted] mixing zone growth law is derived. The effect of ablative stabilization on the allowed in-flight aspect ratio of inertial confinement fusion pellets is estimated using the results of the statistical mechanics model.