We show that recently discovered rotational echoes of molecules provide an efficient tool for studying collisional molecular dynamics in high-pressure gases. Our study demonstrates that rotational echoes enable the observation of extremely fast collisional dissipation, at timescales of the order of a few picoseconds, and possibly shorter. The decay of the rotational alignment echoes in CO2 gas and CO2-He mixture up to 50 bar was studied experimentally, delivering collision rates that are in good agreement with the theoretical expectations. The suggested measurement protocol may be used in other high-density media, and potentially in liquids.
Controlling the nonlinear optical response of nano scale metamaterials opens new exciting applications such as frequency conversion or flat metal optical elements. To utilize the already well-developed fabrication methods, a systematic design methodology for obtaining high nonlinearities is required. In this paper we consider an optimization-based approach, combining a multiparameter genetic algorithm with three-dimensional finite-difference time domain (FDTD) simulations. We investigate two choices of the optimization function: one which looks for plasmonic resonance enhancements at the frequencies of the process using linear FDTD, and another one, based on nonlinear FDTD, which directly computes the predicted nonlinear response. We optimize a four-wave-mixing process with specific predefined input frequencies in an array of rectangular nanocavities milled in a thin free-standing gold film. Both approaches yield a significant enhancement of the nonlinear signal. Although the direct calculation gives rise to the maximum possible signal, the linear optimization provides the expected triply resonant configuration with almost the same enhancement, while being much easier to implement in practice.
Orientation and alignment of molecules by ultrashort laser pulses is crucial for a variety of applications and has long been of interest in physics and chemistry, with the special emphasis on stereodynamics in chemical reactions and molecular orbitals imaging. As compared to the laser-induced molecular alignment, which has been extensively studied and demonstrated, achieving molecular orientation is a much more challenging task, especially in the case of asymmetric-top molecules. Here, we report the experimental demonstration of all-optical field-free three-dimensional orientation of asymmetric-top molecules by means of phase-locked cross-polarized two-color laser pulse. This approach is based on nonlinear optical mixing process caused by the off-diagonal elements of the molecular hyperpolarizability tensor. It is demonstrated on SO2 molecules and is applicable to a variety of complex nonlinear molecules.
We report measurements of the optical transmission through a plasmonic flat surface interferometer. The transmission spectrum shows Fabry-Perot-like modes, where for each mode order, the maximal transmission occurs at a gap that grows linearly with wavelength, giving the appearance of diagonal dependence on gap and wavelength. The experimental results are supported by numerical solutions of the wave equations and by a simplified theoretical model that is based on the coupling between localized and propagating surface plasmon. This work explains not only the appearance of the modes but also their sharp dependence on the gap, taking into consideration the refractive indices of the surrounding media. The transmission spectra provide information about the phase difference between the light impinging on the two cavities, enabling interferometric measurement of the light phase by transmission through the coupled - plasmonic cavities. The 1 degrees phase-difference resolution is obtained without any propagation distance, thus making this interferometer suitable for on-chip operation.
Directional emission of electromagnetic radiation can be achieved using a properly shaped single antenna or a phased array of individual antennas. Control of the individual phases within an array enables scanning or other manipulations of the emission, and it is this property of phased arrays that makes them attractive in modern systems. Likewise, the propagation of surface plasmons at the interface between metal films and dielectric materials can be determined by shaping the individual surface nanostructures or via the phase control of individual elements in an array of such structures. Here, we demonstrate control of the propagation of surface plasmons within a linear array of nanostructures. The generic situation of plasmonic surface propagation that is different on both sides of a metal film provides a unique opportunity for such control: plasmons propagating on the slower side feed into the side with the faster propagation, creating a phased array of interfering antennas and thus controlling the directionality of the wake fields. We further show that by shaping the individual nanoantennas, we can generate an asymmetric propagation geometry.
We report experimental observations of rotated echoes of alignment induced by a pair of time-delayed and polarization-skewed femtosecond laser pulses interacting with an ensemble of molecular rotors. Rotated fractional echoes, rotated high order echoes and rotated imaginary echoes are directly visualized by using the technique of coincident Coulomb explosion imaging. We show that the echo phenomenon not only exhibits temporal recurrences but also spatial rotations determined by the polarization of the time-delayed second pulse. The dynamics of echo formation is well described by the laser-induced filamentation in rotational phase space. The quantum-mechanical simulation shows good agreements with the experimental results. (C) 2017 Optical Society of America
Nanostructured metasurfaces offer unique capabilities for subwavelength control of optical waves. Based on this potential, a large number of metasurfaces have been proposed recently as alternatives to standard optical elements. In most cases, however, these elements suffer from large chromatic aberrations, thus limiting their usefulness for multiwavelength or broadband applications. Here, in order to alleviate the chromatic aberrations of individual diffractive elements, we introduce dense vertical stacking of independent metasurfaces, where each layer is made from a different material, and is optimally designed for a different spectral band. Using this approach, we demonstrate a triply red, green and blue achromatic metalens in the visible range. We further demonstrate functional beam shaping by a self-aligned integrated element for stimulated emission depletion microscopy and a lens that provides anomalous dispersive focusing. These demonstrations lead the way to the realization of ultra-thin superachromatic optical elements showing multiple functionalities- all in a single nanostructured ultra-thin element.
Echo in mountains is a well-known phenomenon, where an acoustic pulse is mirrored by the rocks, often with reverberating recurrences. For spin echoes in magnetic resonance and photon echoes in atomic and molecular systems, the role of the mirror is played by a second, time-delayed pulse that is able to reverse the flow of time and recreate the original impulsive event. Recently, alignment and orientation echoes were discussed in terms of rotational-phase-space filamentation, and they were optically observed in laser-excited molecular gases. Here, we observe hitherto unreported fractional echoes of high order, spatially rotated echoes, and the counterintuitive imaginary echoes at negative times. Coincidence Coulomb explosion imaging is used for a direct spatiotemporal analysis of various molecular alignment echoes, and the implications to echo phenomena in other fields of physics are discussed.
We report the observation of fractional echoes in a double-pulse excited nonlinear system. Unlike standard echoes, which appear periodically at delays which are integer multiples of the delay between the two exciting pulses, the fractional echoes appear at rational fractions of this delay. We discuss the mechanism leading to this phenomenon, and provide experimental demonstration of fractional echoes by measuring third harmonic generation in a thermal gas of CO2 molecules excited by a pair of femtosecond laser pulses.
The development of antibacterial drugs to overcome various pathogenic species, which inhabit the oral cavity, faces several challenges, such as salivary flow and enzymatic activity that restrict dosage retention. Owing to their amphipathic nature, antimicrobial peptides (AMPs) serve as the first line of defense of the innate immune system. The ability to synthesize different types of AMPs enables exploitation of their advantages as alternatives to antibiotics. Sustained release of AMPs incorporated in biodegradable polymers can be advantageous in maintaining high levels of the peptides. In this study, four potent ultra-short lipopeptides, conjugated to an aliphatic acid chain (16C) were incorporated in two different biodegradable polymers: poly (lactic acid co castor oil) (PLACO) and ricinoleic acid-based poly (ester-anhydride) (P(SA-RA)) for sustained release. The lipopeptide and polymer formulations were tested for antibacterial activity during one week, by turbidometric measurements of bacterial outgrowth, anti-biofilm activity by live/dead staining, biocompatibility by hemolysis and XTT colorimetric assays, mode of action by fluorescence-activated cell sorting (FACS) and release profile by a fluorometric assay. The results show that an antibacterial and anti-biofilm effect, as well as membrane disruption, can be achieved by the use of a formulation of lipopeptide incorporated in biodegradable polymer.
A hologram is an optical element storing phase and possibly amplitude information enabling the reconstruction of a three-dimensional image of an object by illumination and scattering of a coherent beam of light, and the image is generated at the same wavelength as the input laser beam. In recent years, it was shown that information can be stored in nanometric antennas giving rise to ultrathin components. Here we demonstrate nonlinear multilayer metamaterial holograms. A background free image is formed at a new frequency-the third harmonic of the illuminating beam. Using e-beam lithography of multilayer plasmonic nanoantennas, we fabricate polarization-sensitive nonlinear elements such as blazed gratings, lenses and other computer-generated holograms. These holograms are analysed and prospects for future device applications are discussed.
Metasurfaces, and in particular those containing plasmonic-based metallic elements, constitute an attractive set of materials with a potential for replacing standard bulky optical elements. In recent years, increasing attention has been focused on their nonlinear optical properties, particularly in the context of second and third harmonic generation and beam steering by phase gratings. Here, we harness the full phase control enabled by subwavelength plasmonic elements to demonstrate a unique metasurface phase matching that is required for efficient nonlinear processes. We discuss the difference between scattering by a grating and by subwavelength phase-gradient elements. We show that for such interfaces an anomalous phase-matching condition prevails, which is the nonlinear analogue of the generalized Snell's law. The subwavelength phase control of optical nonlinearities paves the way for the design of ultrathin, flat nonlinear optical elements. We demonstrate nonlinear metasurface lenses, which act both as generators and as manipulators of the frequency-converted signal.
Optimizing the shape of nanostructures and nano-antennas for specific optical properties has evolved to be a very fruitful activity. With modern fabrication tools a large variety of possibilities is available for shaping both nanoparticles and nanocavities; in particular nanocavities in thin metal films have emerged as attractive candidates for new metamaterials and strong linear and nonlinear optical systems. Here we rationally design metallic nanocavities to boost their Four-Wave Mixing response by resonating the optical plasmonic resonances with the incoming and generated beams. The linear and nonlinear optical responses as well as the propagation of the electric fields inside the cavities are derived from the solution of Maxwell's equations by using the 3D finite-differences time domain method. The observed conversion-efficiency of near-infrared to visible light equals or surpasses that of BBO of equivalent thickness. Implications to further optimization for efficient and broadband ultrathin nonlinear optical materials are discussed.
We present one of the simplest classical systems featuring the echo phenomenon-a collection of randomly oriented free rotors with dispersed rotational velocities. Following excitation by a pair of time-delayed impulsive kicks, the mean orientation or alignment of the ensemble exhibits multiple echoes and fractional echoes. We elucidate the mechanism of the echo formation by the kick-induced filamentation of phase space, and provide the first experimental demonstration of classical alignment echoes in a thermal gas of CO2 molecules excited by a pair of femtosecond laser pulses.
We demonstrate strong coupling of nanocavities in metal films, sparked by propagating surface plasmons. Unlike the coupling of metallic nanoparticles which decays over distances of tens of nanometers, the metallic nanocavities display long range coupling at distances of hundreds of nanometers for the properly selected metal/wavelength combinations. Such strong coupling drastically changes the symmetry of the charge distribution around the nanocavities as is evidenced by the nonlinear optical response of the medium. We show that when strongly coupled, equilateral triangular nanocavities lose their individual symmetry to adopt the lower symmetry of the coupled system and respond like a single dipolar entity. A quantitative model is suggested for the transition from individual to strongly coupled nanocavities.
The laser-induced deformation of a typical commercial cantilever commonly used for scanning near-field optical microscopes was investigated by means of a software package based on the finite element method. The thermo-mechanical behaviour of such a cantilever whose tip was irradiated by a laser beam was calculated in the temperature regime between room temperature and 850 K. The spatial tip displacement was simulated at timescales
We consider the optical properties of a gas of molecules that are brought to fast unidirectional spinning by a pulsed laser field. It is shown that a circularly polarized probe light passing through the medium inverts its polarization handedness and experiences a frequency shift controllable by the sense and the rate of molecular rotation. Our analysis is supported by two recent experiments on the laser-induced rotational Doppler effect in molecular gases and provides a good qualitative and quantitative description of the experimental observations.
The nonlinear optical dynamics of nanomaterials comprised of plasmons interacting with quantum emitters is investigated by a self-consistent model based on the coupled Maxwell-Liouville-von Neumann equations. It is shown that ultrashort resonant laser pulses significantly modify the optical properties of such hybrid systems. It is further demonstrated that the energy transfer between interacting molecules and plasmons occurs on a femtosecond time scale and can be controlled with both material and laser parameters.
A systematic study of the influence of the excitation angle, the light polarization and the coating thickness of commercial SPM tips on the field enhancement in an apertureless scanning near-field optical microscope is presented. A new method to optimize the alignment of the electric field vector along the major tip axis by measuring the resonance frequency was developed. The simulations were performed with a MNPBEM toolbox based on the Boundary Element Method (BEM). The influence of the coating thickness was investigated for the first time. Coatings below 40 nm showed a drastic influence both on the resonance wavelength and the enhancement. A shift to higher angles of incidence for the maximum enhancement could be observed for greater tip radii.
The nonlinear response of subwavelength nanocavities in thin silver films are investigated. We report on significant enhancements of the second harmonic generation (SHG) when the fundamental wavelength matches dimensional resonances within the nanocavities. The nonlinear polarization properties of the nanocavities are studied as well and found to be correlated with the cavity shape and symmetry. In some nanocavities with internal nanocorrugations, giant field enhancements are observed, making them excellent candidates for high sensitivity spectroscopy.
When a wave is reflected from a moving object, its frequency is Doppler shifted(1). Similarly, when circularly polarized light is scattered from a rotating object, a rotational Doppler frequency shift may be observed(2,3), with manifestations ranging from the quantum world (fluorescence spectroscopy, rotational Raman scattering and so on(3,4)) to satellite-based global positioning systems(5). Here, we observe for the first time the Doppler frequency shift phenomenon for a circularly polarized light wave propagating through a gas of synchronously spinning molecules. An ensemble of such spinning molecules was produced by double-pulse laser excitation, with the first pulse aligning the molecules and the second (linearly polarized at a 45 degrees angle) causing a concerted unidirectional rotation of the 'molecular propellers'(6,7). We observed the resulting rotating birefringence of the gas by detecting a Doppler-shifted wave that is circularly polarized in a sense opposite to that of the incident probe.
A pair of linearly polarized pump pulses induce field-free unidirectional molecular rotation, which is detected by a delayed circularly polarized probe. The polarization and spectrum of the probe are modified by the interaction with the molecules, in accordance with the Rotational Doppler Effect.
We demonstrate strong coupling between molecular excited states and surface plasmon modes of a slit array in a thin metal film. The coupling manifests itself as an anticrossing behavior of the two newly formed polaritons. As the coupling strength grows, a new mode emerges, which is attributed to long-range molecular interactions mediated by the plasmonic field. The new, molecular-like mode repels the polariton states, and leads to an opening of energy gaps both below and above the asymptotic free molecule energy.
Recently, several femtosecond-laser techniques have demonstrated molecular excitation to high rotational states with a preferred sense of rotation. We consider collisional relaxation in a dense gas of such unidirectionally rotating molecules, and suggest that due to angular momentum conservation, collisions lead to the generation of macroscopic vortex gas flows. This argument is supported using the Direct Simulation Monte Carlo method, followed by a computational gas-dynamic analysis.
Spectroscopy aims at extracting information about matter through its interaction with light. However, when performed on gas and liquid phases as well as solid phases lacking long-range order, the extracted spectroscopic features are in fact averaged over the molecular isotropic angular distributions. The reason is that lightmatter processes depend on the angle between the transitional molecular dipole and the polarization of the light interacting with it. This understanding gave birth to the constantly expanding field of laser-induced molecular alignment. In this paper, we attempt to guide the readers through our involvement (both experimental and theoretical) in this field in the last few years. We start with the basic phenomenon of molecular alignment induced by a single pulse, continue with selective alignment of close molecular species and unidirectional molecular rotation induced by two time-delayed pulses, and lead up to novel schemes for manipulating the spatial distributions of molecular samples through rotationally controlled scattering off inhomogeneous fields and surfaces.
The incorporation of spectral resolution in time resolved Four Wave Mixing spectroscopy provides important information for the interpretation of the observed spectra. We demonstrate experimentally a new method whereby the combined time and frequency resolved information may be obtained within a single laser pulse. The method is based on Phase Matching Spectral Filtering for the tuning of the generated nonlinear signal, which in turn utilizes the constraints imposed by strict phase matching of parallel input beams in our geometrical arrangement. The measurements were performed on a simple molecule (CH2Br2), and are used for identification of molecular degrees of freedom not otherwise possible in pure time resolved methods. (C) 2011 Elsevier B.V. All rights reserved.
Single shot time resolved four wave mixing is a powerful tool for the acquisition of dynamic and spectroscopic data from molecules susceptible to bleaching or other photo-induced damage. We add polarization dependence to single shot methods, and demonstrate how magic angle measurements are made simpler by this methodology. We propose a new approach to single shot combined time/polarization measurements which can be generalized to other two dimensional combinations.
Numerous examples of closed-cage nanostructures, such as nested fullerene-like nanoparticles and nanotubes, formed by the folding of materials with layered structure are known. These compounds include WS2, NiCl2, CdCl2, Cs2O, and recently V2O5. Layered materials, whose chemical bonds are highly ionic in character, possess relatively stiff layers, which cannot be evenly folded. Thus, stress-relief generally results in faceted nanostructures seamed by edge-defects. V2O5, is a metal oxide compound with a layered structure. The study of the seams in nearly perfect inorganic "fullerene-like" hollow V2O5 nanoparticles (NIF-V2O5) synthesized by pulsed laser ablation (PLA), is discussed in the present work. The relation between the formation mechanism and the seams between facets is examined. The formation mechanism of the NIF-V2O5 is discussed in comparison to fullerene-like structures of other layered materials, like IF structures of MoS2, CdCl2, and Cs2O. The criteria for the perfect seaming of such hollow closed structures are highlighted.
Nanoparticles of materials with layered structure are able to spontaneously form closed-cage nanostructures such as nested fullerene-like nanoparticles and nanotubes. This propensity has been demonstrated in a large number of compounds such as WS2, NiCl2, and others. Layered metal oxides possess a higher ionic character and consequently are stiffer and cannot be evenly folded. Vanadium pentoxide (V2O5), a layered metal oxide, has received much attention due to its attractive qualities in numerous applications such as catalysis and electronic and optical devices and as an electrode material for lithium rechargeable batteries. The synthesis by pulsed laser ablation (PLA) of V2O5 hollow nanoparticles, which are closely (nearly) associated with inorganic "fullerene-like" (NIF-V2O5) nanoparticles, but not quite as perfect, is reported in the present work. The relation between the PLA conditions and the NIF-V2O5 morphology is elucidated. A new mechanism leading to hollow nanostructure via crystallization of lower density amorphous nanoparticles is proposed. Transmission electron microscopy (TEM) is used extensively in conjunction with structural modeling of the NIF-V2O5 in order to study the complex 3-D structure of the NIF-V2O5 nanoparticles. This structure was shown to be composed of facets with their low-energy surfaces pointing outward and seamed by defective domains. These understandings are used to formulate a formation mechanism and may improve the function of V2O5 in its many uses through additional morphological control. Furthermore, this study outlines which properties are required from layered compounds to fold into perfectly closed-cage IF nanoparticles.
We demonstrate a new approach to Four Wave Mixing spectroscopy involving simultaneous measurements at time and frequency domains, where spectral selectivity is achieved by phase matching filtering, and the time resolution is obtained within a single ultra-short pulse. We analyze the Four Wave Mixing signal. and show that our method is capable for discrimination between different spectroscopic pathways of vibrational coherences modulating the scattered signal. (C) 2010 Elsevier B.V. All rights reserved.
For various applications of nanoscale surface modification by an Atomic Force Microscope, one would like to maintain the AFM tip near the surface and at an accurately controlled elevated temperature. We study the laser heating of an ordinary AFM silicon tip under ambient conditions, and show that a tightly focused laser beam can heat the tip apex to the desired temperature, while affecting the cantilever quite moderately. We demonstrate that the observation of the shift of the silicon Raman line scattered from the tip is an efficient and accurate way to determine the tip temperature, and we substantiate our observations by theoretically modeling the dynamics of heat accumulation in the tip-cantilever system. For situations where Raman measurements are not feasible, we introduce a new method for estimating the tip temperature by monitoring the mechanical resonance frequency shift of the probe.
We introduce a new scheme for controlling the sense of molecular rotation. By varying the polarization and the delay between two ultrashort laser pulses, we induce unidirectional molecular rotation, thereby forcing the molecules to rotate clockwise/counterclockwise under field-free conditions. We show that unidirectionally rotating molecules are confined to the plane defined by the two polarization vectors of the pulses, which leads to a permanent anisotropy in the molecular angular distribution. The latter may be useful for controlling collisional cross-sections and optical and kinetic processes in molecular gases. We discuss the application of this control scheme to individual components within a molecular mixture in a selective manner.
We present a new approach to nonresonant laser deceleration and cooling of atoms based on their interaction with a bistable optical cavity. The cooling mechanism presents a photonic version of Sisyphus cooling, in which the conservative motion of atoms is interrupted by sudden transitions between two stable states of the cavity mode. The mechanical energy is extracted due to the hysteretic nature of those transitions. The bistable character of the cavity may be achieved by an external feedback loop, or by means of nonlinear intracavity optical elements. In contrast to the conventional cavity cooling, in which atoms experience a viscoustype force, bistable cavity cooling imitates "dry friction" and stops atoms much faster. Based on this novel approach, we explore the prospects of using optical bistability for efficient radiation pressure cooling of micromechanical devices that are modeled as a Fabry-Perot resonator with one fixed and one oscillating mirror. In all cases, analytical results are presented, supported by realistic numerical examples.
By varying the polarization and delay between two ultrashort laser pulses, we control the plane, speed, and sense of molecular rotation. This control may be implemented to individual components within a molecular mixture.
We demonstrate noncontact, high quality surface modification of soft and hard materials with spatial resolution of similar to 20 nm. The nanowriting is based on the interaction between the surface and the tip of a standard atomic force microscope illuminated by a focused femtosecond laser beam and hovering (at ambient conditions) 1-4 nanometers above the surface without touching it. Field enhancement at the tip-sample gap or high tip temperature are identified as the causes of material ablation.
We demonstrate selective control over rotational motion of small, linear molecules. By means of sequential excitation of the rotational motion by ultrashort pulses, we first prepare transiently aligned molecules with periodically revived angular distribution. Upon further, properly timed excitation, the rotational energy can be increased or decreased, depending on the exact timing of the second pulse. We show how this approach can be applied for selective rotational control of a single component in a molecular mixture. We discuss this selectivity in the context of molecular isotopes ( (14)N(2), (15)N(2)), where the difference in isotopic mass gives rise to different rotational revival times. We further apply the method to the selective addressing of molecular spin isomers ( para, ortho (15)N(2)) in a mixture, where wavefunction symmetry differences replace the mass differences as the origin of the selectivity. In both cases the method is demonstrated experimentally and the results are analysed theoretically.
A novel heterodyne detection technique is introduced for femtosecond time resolved four wave mixing (TRFWM). A 'local oscillator' field is generated in situ either by the self alignment of the molecules in the ultrashort field, or by the rotational alignment signal from a small amount of anisotropic molecules added to the sample. Like other heterodyne detection schemes, the method enables linearization of the third-order nonlinear signal and clear identification of fundamental vibrational modes and their separation from vibrational beat frequencies. However, unlike others, the method is easy to implement and does not require interferometric stability of the optical setup. (c) 2007 Elsevier B.V. All rights reserved.
Novel mode of AFM operation is proposed providing the small, few nanometers tip to sample gap, appropriate for the ANSOM experiments. A set-up open for the run-time adjustments, working at ambient conditions is considered. Efficiency of a method is demonstrated by applying it to the laser nano-lithography on different materials with a regular AFM tip.
We propose a generic approach to nonresonant laser cooling of atoms and molecules in a bistable optical cavity. The method exemplifies a photonic version of Sisyphus cooling, in which the matter-dressed cavity extracts energy from the particles and discharges it to the external field as a result of sudden transitions between two stable states.
We experimentally demonstrate field-free, spin-selective alignment of ortho- and para molecular spin isomers at room temperature. By means of two nonresonant, strong, properly delayed femtosecond pulses within a four wave mixing arrangement, we observed selective alignment for homonuclear diatomics composed of spin 1/2 (N-15) or spin 1 (N-14) atoms. The achieved selective control of the isomers' angular distribution and rotational excitation may find applications to analysis, enrichment, and actual physical separation of molecular spin modifications.
Single-shot time resolved Coherent Anti-Stokes Raman Scattering ( CARS) is presented as a viable method for fast measurements of molecular spectra. The method is based on the short spatial extension of femtosecond pulses and maps time delays between pulses onto the region of intersection between broad beams. The image of the emitted CARS signal contains full temporal information on the field-free molecular dynamics, from which spectral information is extracted. The method is demonstrated on liquid samples of CHBr3 and CHCl3 and the Raman spectrum of the low-lying vibrational states of these molecules is measured. (c) 2007 Optical Society of America.
We demonstrate single shot retrieval of coherent molecular field-free evolution by geometric space-time mapping combined with non-linear signal imaging. The method was experimentally tested to yield accurate spectrum of CHBr3 and CHCl3 molecules.
Following excitation by a strong ultra-short laser pulse, molecules develop coordinated rotational motion, exhibiting transient alignment along the direction of the laser electric field, followed by periodic full and fractional revivals that depend on the molecular rotational constants. In mixtures, the different species undergo similar rotational dynamics, all starting together but evolving differently with each demonstrating its own periodic revival cycles. For a bimolecular mixture of linear molecules, at predetermined times, one species may attain a maximally aligned state while the other is anti-aligned (i.e. molecular axes are confined in a plane perpendicular to the laser electric field direction). By a properly timed second laser pulse, the rotational excitation of the undesired species may be almost completely removed leaving only the desired species to rotate and periodically realign, thus facilitating further selective manipulations by polarized light. In this paper, such double excitation schemes are demonstrated for mixtures of molecular isotopes (isotopologues) and for nuclear spin isomers.
MoS2 nanooctahedra are believed to be the smallest stable closed-cage structures of MoS2, i.e., the genuine inorganic fullerenes. Here a combination of experiments and density functional tight binding calculations with molecular dynamics annealing are used to elucidate the structures and electronic properties of octahedral MoS2 fullerenes. Through the use of these calculations MoS2 octahedra were found to be stable beyond n(Mo) > 100 but with the loss of 12 sulfur atoms in the six corners. In contrast to bulk and nanotubular MoS2, which are semiconductors, the Fermi level of the nanooctahedra is situated within the band, thus making them metallic-like. A model is used for extending the calculations to much larger sizes. These model calculations show that, in agreement with experiment, the multiwall nanooctahedra are stable over a limited size range of 10(4)-10(5) atoms, whereupon they are converted into multiwall MoS2 nanoparticles with a quasi-spherical shape. On the experimental side, targets of MoS2 and MoSe2 were laser-ablated and analyzed mostly through transmission electron microscopy. This analysis shows that, in qualitative agreement with the theoretical analysis, multilayer nanooctahedra of MoS2 with 1000-25 000 atoms (Mo + S) are stable. Furthermore, this and previous work show that beyond similar to 10(5) atoms fullerene-like structures with quasi-spherical forms and 30-100 layers become stable. Laser-ablated WS2 samples yielded much less faceted and sometimes spherically symmetric nanocages.
It is well accepted by now that nanoparticles of inorganic layered compounds form closed-cage structures (IF). In particular closed-cage nanoparticles of metal dihalides, like NiCl2, CdCl2 and CdI2, were shown to produce such structures in the past. In the present report IF-NiBr2 polyhedra and quasi-spherical structures were obtained by the evaporation/recrystallization technique as well as by laser ablation. When the nanoclusters were formed in humid atmosphere, nickel perbromate hydrate [Ni(BrO4)(2)(H2O)(6)] polyhedra and short tubules were produced, as a result of a reaction with water. Nanooctahedra of NiBr2 were found occasionally in the irradiated soot. The reoccurrence of this structure in the IF family suggests that it is a generic one. Consistent with previous observations, this study showed that formation of the IF materials stabilized the material under the electron-beam irradiation. The growth mechanism of these nanostructures is briefly discussed. (c) 2006 Elsevier Ltd. All rights reserved.
We experimentally demonstrate isotope-selective alignment in a mixture of N-14(2), N-15(2) isotopes. Following a strong ultrashort laser pulse rotational excitation, the angular distributions of the isotopes gradually become different due to the mismatch in their moments of inertia. At predetermined times, the desired isotope attains an aligned state while the other component is antialigned, facilitating further selective manipulations by polarized light. By a properly timed second laser pulse, the rotational excitation of the undesired isotope is almost completely removed.
We explore the prospects of optical shaking, a recently suggested generic approach to laser cooling of neutral atoms and molecules. Optical shaking combines elements of Sisyphus cooling and of stochastic cooling techniques and is based on feedback-controlled interaction of particles with strong nonresonant laser fields. The feedback loop guarantees a monotonous energy decrease without a loss of particles. We discuss two types of feedback algorithms and provide an analytical estimation of their cooling rate. We study the robustness of optical shaking against noise and establish minimal stability requirements for the lasers. The analytical predictions are in a good agreement with the results of detailed numerical simulations.
We propose a novel generic approach to laser cooling based on the nonresonant interactions of atoms and molecules with optical standing waves experiencing sudden phase jumps. The technique, termed "optical shaking," combines the elements of stochastic cooling and Sisyphus cooling. An optical signal that measures the instantaneous force applied by the standing wave on the ensemble of particles is used as feedback to determine the phase jumps. This guarantees a drift towards lower energies and higher phase-space density without the loss of particles typical of evaporative cooling.
Ultrashort pulses are routinely used for material processing. Since the ablation cannot be faster than the electron phonon equilibration times, temporal pulse shaping, and proper selection of pulse duration may offer advantages. We find that the shortest pulse is not always the best in terms of ablation efficiency and quality, and develop tools for using adaptive pulse shaping for the optimization process.
Femtosecond laser ablation occurs on timescales faster than the thermalization of the excited electrons and the lattice in solid materials. The ultrafast deposition of energy competes with the slower electron-phonon energy redistribution, raising the question of what is the optimal pulse duration for efficient deposition of energy while minimizing peripheral damage, and whether the shortest pulse is always the most efficient. We studied femtosecond laser ablation of silicon and several metals, varied the pulse duration while keeping all other parameters equal, and looked for optimal conditions. The main findings in our study are that at low fluences, not too high above the ablation threshold, the shortest pulses are the most efficient, whereas under high fluence conditions, well above the ablation threshold, longer pulses ablate more efficiently. In order to facilitate eventual direct, real time optimization, we developed a diagnostics tool for the monitoring of the ablation efficiency over a wide range of pulse durations. The intensity of the emission at atomic lines (i.e. the 289 nm line in Silicon, calibrated by plasma emission at other wavelengths) provides such information, while optical and AFM microscopy provide reliable information about the quality of ablated structures.
Tin disulfide pellets were laser ablated in an inert gas atmosphere, and closed cage fullerene-like (IF) nanoparticles were produced. The nanoparticles had various polyhedra and short tubular structures. Some of these forms contained a periodic pattern of fringes resulting in a superstructure. These patterns could be assigned to a superlattice created by periodic stacking of layered SnS2 and SnS. Such superlattices are reminiscent of misfit layer compounds, which are known to form tubular morphologies. This mechanism adds up to the established mechanism for IF formation, namely, the annihilation of reactive dangling bonds at the periphery of the nanoparticles. Additionally, it suggests that one of the driving forces to form tubules in misfit compounds is the annihilation of dangling bonds at the rim of the layered structure.
A new method of laser-induced lithography for direct writing of carbon on a glass surface is described, in which deposition occurs from a transparent precursor solution. At the glass-solution interface where the laser spot is focused, a micro-explosion process takes place, leading to the deposition of pure carbon on the glass surface. Transmission electron microscopy (TEM) analysis shows two distinct co-existing phases. The dominant one shows a mottled morphology with diffraction typical of cubic (sp(3)) diamond. The other region shows an ordered array of graphene sheets with diffraction pattern typical of sp(2)-bonded carbon. The sp(3) crystallites range in size from 9 to 30 Angstrom and are scattered randomly throughout the sample. A UV Raman spectrum shows a broad band at the location of the expected diamond peak, together with a peak corresponding to the graphite region. We conclude that the patterned carbon is composed of a mixture of nanocrystalline sp and sp(2) carbon forms.
Laser ablation has been extensively used for the synthesis of nanoparticles of various sorts, and in particular single wall carbon nanotubes and C-60 molecules. NiCl2 nanotubes were recently also produced using this technique. While fullerene-like NiCl2 structures can be obtained through regular ablation, vapor phase enriched with CCl4 gas (reactive ablation) is necessary for the synthesis of the nanotubes. The experimental results indicate that the synthesis of such nanotubes is much more difficult than the synthesis of say MoS2 or WS2 nanotubes. Moreover, the NiCl2 nanotubes are of larger diameter and consist on the average of more layers than their MoS2 predecessors. First principle calculations show that single layer NiCl2 nanotubes of diameter smaller than 54 nm are unstable and lose their outer chlorine atoms. In contrast, MoS2 nanotubes with diameter of 2 nm and larger are found to be stable using the same kind of calculations. To gain better understanding of the differences between the materials, a review of the mechanical properties of layered metal dihalide and metal dichalcogenide compounds is undertaken. First principle calculations show that the Young's and bending moduli of NiCl2 are almost twice larger than those of MoS2. The large ionicity of NiCl2 entails much larger shear and stacking fault energies for this compound as compared to MoS2, which explains its smaller propensity to bend and fold. These observations are supported by analysis of the corresponding Raman modes. Furthermore, metal dihalide compounds are very hygroscopic making their handling, and especially their analysis more difficult. This analysis explains the greater difficulties to grow NiCl2 nanotubes or fullerene-like nanoparticles, as compared to their MoS2 analogues.
Active laser ablation has been used for the synthesis of NiCl2 nanotubes and fullerene-like nanoparticles (see Figure). Spectroscopic measurements on the structures showed that the NiCl2 layers in the nanotubes were quite perfectly crystalline. The growth is thought to occur via a vapor-liquid-solid mechanism through which many different, surprising shapes have been obtained.
Knopp [J. Raman Spectrosc. 31, 51 (2000)] have recently used resonant femtosecond coherent anti-Stokes Raman spectroscopy (CARS) to prepare and probe highly excited vibrational wave packets on the ground electronic potential surface of molecular iodine. The experiment uses a sequence of three resonant femtosecond pulses with two independently variable time delays. The first two pulses act as a pump and dump sequence to create a predefined, highly excited wave packet on the ground electronic state, whose amplitude is optimized by selecting the proper pump-dump (Raman) frequency difference and varying the time delay. The third pulse promotes the pump-dump wave packet to an excited electronic state, resulting in subsequent coherent emission of light at the anti-Stokes frequency. This fully-resonant CARS signal, measured as a function of time delay between the second and third pulses, oscillates at a frequency characteristic of the pump-dump wave packet. Due to anharmonicity, this frequency is a sensitive measure of the amount of vibrational excitation. Knopp observed that under certain conditions the signal exhibits pronounced beating between the pump-dump wave packet frequency and the frequency characteristic of the bottom of the ground state well. In this paper we show that these beats arise only when the final pump-dump-pump wave packet is above the excited state dissociation threshold of the molecule. We derive analytical expressions showing that under these conditions, where the polarization is short-lived, there may be strong interferences between the contributions from molecules originally in different vibrational states of the thermal ensemble. In contrast, the CARS polarization in the below threshold case is long-lived, and these interferences cancel. Numerical evaluation of the CARS signal through vibrational wave packet propagation confirms the predictions of the analytical theory and reproduces the distinctive beating pattern observed in the experiments.
Femtosecond time-delayed coherent anti-Stokes Raman scattering is presented not only as a tool for monitoring but also as a viable method for the preparation of vibrational wavepackets with very high quantum numbers in the ground electronic state of molecules. We experimentally demonstrate a particularly useful approach of using two separate time delays between the pulses for preparing vibrational wavepackets as high as v"=38 [DeltaE(v)=7000 cm(-1)] in bulk gas- phase molecular iodine. By means of an ultrashort laser pulse, we prepare a wavepacket in an electronic excited state, optimize the frequency and timing of a second pulse to efficiently generate the targeted ground-state vibrational wavepacket, and monitor the wavepacket by coherent scattering from a third pulse. The method is further used to probe interference effects in femtosecond four-wave-mixing signals generated by molecular wavepackets. (C) 2001 American Institute of Physics.
The method of coherence observation by interference noise (COIN) [Kinrot , Phys. Rev. Lett. 75, 3822 (1995)] has been shown to be a useful tool for measurements of wave packet motion at the quantum-classical border. We present the first systematic interferometric study of fractional vibrational revivals in the B state of thermal iodine (I-2) vapor. Experimental COIN interferograms ranging from 200 fs to 40 ps are presented for various excitation wavelengths. The complex temporal structure of the observed fluorescence includes rapid initial damping in the short-time regime and the appearance of quarter- and half-revivals on the quantum-mechanical long-time scale. These features arise from a delicate balance between rotational and vibrational molecular coherences. The clear observation of the wave packets on the long time scale is possible due to the long-time stability of the COIN interferometer. Lowest-order perturbative solutions nicely recover the experimental results, and closed-form analytical expressions based upon the factorization approach and the Poisson summation give insights into the nature of dephasing and rephasing of vibrational wave packets subject to rotational inhomogeneous broadening. (C) 2001 American Institute of Physics.
We investigate the interaction of two molecules or nanosized particles with a nearly resonant laser field under the tip of an apertureless near-held microscope. We show that interference of several scattering channels provides means for enhanced spatial resolution. The visibility of two separate nano objects is considered, and a natural definition emerges for the resolution of the apertureless microscope operating under conditions of nearly resonant illumination. The probe tip creates an additional coupling channel between the two molecules, and thus affects the energy transfer between them. We demonstrate that the tip can either enhance or suppress this transfer. Two models fur the tip geometry are considered: a simplified pointlike dipole, and a more realistic elongated spheroid. Quantitative results are obtained for the dependence on irradiation frequency and tip position for dielectric as well as metallic tips. In particular, specific results are obtained for a silver tip under conditions of plasmon resonance, and we show that under fully resonant conditions the tip may enhance the intermolecular energy transfer by nearly two orders of magnitude.
The principle of coherence observation by interference noise [COIN, Kinrot , Phys. Rev. Lett. 75, 3822 (1995)] has been applied as a new approach to measuring wavepacket motion. In the COIN experiment pairs of phase-randomized femtosecond pulses with relative delay time tau prepare interference fluctuations in the excited state population, so the correlated noise of fluorescence intensity-the variance varF(tau)-directly mimics the dynamics of the propagating wavepacket. The scheme is demonstrated by measuring the vibrational coherence of wavepacket motion in the B-state of gaseous iodine. The COIN interferograms obtained recover propagation, recurrences and spreading as the typical signature of wavepackets. The COIN measurements were performed with precisely tuned excitation pulses which cover the bound part of the B-state surface up to the dissociative limit. In combination with preliminary numerical calculations, comparison has been made with results from previous phase-locked wavepacket interferometry and pump-probe experiments, and conclusions drawn about the limitations of the method and its applicability to quantum dynamical research. (C) 2000 American Institute of Physics. [S0021-9606(00)01011-4].
Isolated diamond crystals are grown by hot filament chemical vapor deposition (CVD) on Si-substrates without nucleation enhancement. We investigate the development of isolated diamond crystals under changing growth conditions. The growth parameter alpha = root 3v(100)/v(111) is determined from isolated diamond crystals and the same crystal is regrown several times and detected repeatedly by a scanning electron microscope. The change in morphology can be followed as the growth conditions are changing. Multiply twinned particles are also investigated and observed to change their morphology with the growth parameter alpha. The dependence of the idiomorphic shape on the growth parameter alpha is measured experimentally and compared with model calculations. It is shown that the basic determination of the morphology of a multiply twinned particle occurs at the early part of the nucleation phase. (C) 2000 Published by Elsevier Science B.V. All rights reserved.
The light scattering from a single resonant molecule, or nano-sized particle located near the tip of an apertureless scanning near-field microscope is studied, and different regimes of scattering are analyzed. The tip enhances the external field, and serves as an efficient transmission 'antenna' for the molecular dipole oscillations. The light scattering occurs via two channels: direct scattering from the rip, and tip-mediated molecular scattering. The total detected intensity of the scattered light shows interference of the channels, which we suggest to use for efficient near-field microscopy. At certain detunings from resonances the scanning signal experiences spatial narrowing similar to that one observed in two-photon microscopy, thus allowing for sub-nanometer resolution. (C) 2000 Elsevier Science B.V. All rights reserved.
The interaction of laser light of arbitrary polarization with systems of high angular momentum is considered. We show that elliptically polarized light creates an anisotropic spatial distribution of atomic and molecular angular momentum which is qualitatively different from the alignment and orientation induced by light of circular or linear polarization. Multilevel coherent population trapping within a manifold of ground-stare magnetic sublevels results in a nonclassical behavior of a high-J molecular rotor. The classical approximation for the angular momentum distribution is compared with the exact quantum calculations, and is shown to fail in cases of long interaction times and high intensities of the exciting light. In these limits, the quantum uncertainty defines the spatial width of the angular distribution. The applicability of the classical treatment is analyzed and found to be different in the cases of J-->J-1 and J-->J transitions. A biaxial spatial orientation with two preferential axes of rotation is experimentally created in sodium atoms via coherent population trapping by elliptically polarized light. A method for producing an arbitrary orientation of atomic angular momentum by magnetic field assisted coherent population trapping is proposed. [S1050-2947(99)01708-4].
We describe a method for interferometric distance measurements in the presence of phase noise. The method is based on the beating between white light and a reference beam that travel along the same path through the interferometer. Since both the reference and the white light suffer the same phase noise, the envelope of the high frequency fringes is not affected by the noise. By measuring the signal variance, we recover the envelope while averaging out the high frequency fringes. We demonstrate the usefulness of the method for surface profilometers. (C) 1998 American Institute of Physics. [S0003-6951(98)03431-7].
We experimentally create and theoretically explain a different type of highly anisotropic atomic angular-momentum distribution with two preferential axes. Such a biaxial spatial orientation is created by optical pumping with elliptically polarized light, and measured by the observation of coherent population trapping in a weak external magnetic field. We probe the angular-momentum distribution by the Hanle configuration, and observe two dark resonances at nonzero magnetic fields-the expected signature of the biaxial spatial geometry. A direct method for preparing arbitrarily oriented atomic ensembles is discussed. [S1050-2947(99)50203-5].
Raman spectroscopy has developed as a major technique to determine the quality of chemical vapor deposited (CVD)-grown diamond films. However, the use of spontaneous Raman spectroscopy for in-situ measurements is difficult due to the high luminosity of the CVD reactor. We demonstrate the technique of back-scattering coherent anti-stokes Raman spectroscopy (CARS) as a new way to measure the Raman spectrum of polycrystalline diamond films. After further optimization, back-scattering CARS should prove useful as a tool for in-situ diagnostics during diamond film growth. (C) 1999 Elsevier Science S.A. All rights reserved.
We present a new procedure for pretreatment seeding by ultrasonic agitation of silicon substrates in diamond nano-powder suspensions to which HF and KOH were added X-ray photoelectron spectroscopy (XPS) was used to measure the surface coverage by diamond nuclei immediately after the pretreatment. Coverage percentages of 70, 40 and 55% were obtained for the HF, KOH and the original diamond slurry, respectively. The seeding density (S-D) was calculated from the known nano-particles size, determined independently from X-ray diffraction of the powder. For nano-particle size of similar to 6 nm, we obtain nominal seeding densities of the order similar to 10(12) cm(-2). The advantage of the high coverage was most evident for films deposited at low substrate temperature (570 degrees C). The potential of the new seeding procedure and the XPS characterization method are discussed. (C) 1999 Elsevier Science S.A. All rights reserved.
We describe a method for interferometric distance measurements in the presence of phase noise. The method is based on the beating between white light and a reference beam that travel along the same path through the interferometer. Since both the reference and the white light suffer the same phase noise, the envelope of the high frequency fringes is not affected by the noise. By measuring the signal variance? we recover the envelope while averaging out the high frequency fringes. We demonstrate the usefulness of the method for surface profilometers. (C) 1998 American Institute of Physics.
We present the first experimental proof for the existence of elliptical dark states, the multilevel analog of the well known three-level dark states. An ensemble of multilevel atoms, when prepared by elliptically polarized light, becomes transparent to probe light of the same polarization. The effect stems from laser-induced coherences between many ground-state magnetic sublevels which reflect the polarization of the light that had created the dark state. The novelty and essential character of the dark states are elucidated by the experimental demonstration of their creation by incoherent light. It is anticipated that elliptical dark slates will play an important role in the laser cooling and manipulation of molecules. [S0031-9007(97)05201-0].
Coherent-population trapping, heretofore realized in closed systems, is also possible in highly degenerate open molecular systems. We consider a rovibrational molecular transition, where the ground magnetic m-sublevels are coupled to the corresponding upper states, and are therefore expected to be emptied after a few lifetimes. We show that upon excitation by elliptically polarized light, the population remains trapped in a coherent superposition of ground-state sublevels which does not interact further with the exciting light. Unlike the simple cases of linearly and circularly polarized light and of a closed three-level system (i.e. atoms), here the trapping level is not the one with \m\ = 0, J, but rather a combination of several states, which depends on the ellipticity of the exciting light.
We present a new approach to the measurement of coherence. By monitoring the quantum interference fluctuations in the population excited by a pair of time-delayed, randomly phased pulses, it is possible to extract information on internal dynamics, energy level splittings, and characteristic coherence decay times of the medium. As a proof of concept, we demonstrate the measurement of phase relaxation and doublet separation in atomic potassium. The principle of coherence observation by interference noise is very general, is shown to be robust and with inherent time resolution of a few optical cycles, and is proposed as an alternative to many;interferometric applications.
We show that four-wave mixing in optically dense media is very different than in an optically thin medium. In time-resolved degenerate four-wave mixing we experimentally observe ''negative'' time delay response, fast decay rates at short delay times, and broad shoulders at long delays. The observations are explained very well in terms of pulse propagation effects, and a theoretical framework is presented for the analysis of the nonlinear interaction. We treat both the incident and generated fields self-consistently, and solve for the interaction of short pulses with a resonantly absorbing medium. The implications for the extraction of relaxation rates from four-wave-mixing experiments involving ultrashort pulses in optically dense matter are discussed.
An experimental observation of field-induced resonances in four-wave mixing is reported. In experiments on Na vapor near the D lines, when the input fields are strong, new resonances appear, confirming recent theoretical predictions based on a nonperturbative approach to wave-mixing processes. The analogy to and differences from the pressure-induced extra resonances are
We report the experimental results of two-pulse transient four-wave mixing near the first heavy-hole quantum-well exciton. Input pulses with similar polarizations (circular or linear) measure the exciton dephasing rate; opposite circular polarized pulses produce no four-wave mixing signal, while crossed linear polarized pulses generate a weaker signal with a faster dephasing rate. This signal is attributed to the biexcitonic transition, which was directly observed in a separate nondegenerate four-wave mixing experiment. The selection rules for these transitions are discussed and confirm the treatment of the excitonic transition as a three-level system.