Publications

We consider the orientation of linear and symmetric-top molecules induced by laser and delayed terahertz (THz) pulses at high rotational temperatures (up to room temperature). We introduce an echo-assisted approach in which the achieved transient molecular orientation is an order of magnitude higher than the orientation produced by a single THz pulse. The laser pulse first dissects the wide molecular phase-space distribution into multiple narrow strips (filaments), each being cold and evolving separately. A subsequent THz pulse causes a substantial transient orientation of the individual filaments, which leads to an enhanced orientation of the whole molecular ensemble at later times via the echo mechanism. This enhanced degree of orientation is important in attosecond science, chemical reaction control, ultrafast molecular imaging, and other domains of physics.

We study the excitation of asymmetric-top (including chiral) molecules by two-color femtosecond laser pulses. In the cases of non-chiral asymmetric-top molecules excited by an orthogonally polarized two-color pulse, we demonstrate, classically and quantum mechanically, three-dimensional orientation. For chiral molecules, we show that the orientation induced by a cross-polarized two-color pulse is enantioselective along the laser propagation direction, namely, the two enantiomers are oriented in opposite directions. The classical and quantum simulations are in excellent agreement on the short time scale, whereas on the longer time scale, the enantioselective orientation exhibits quantum beats. These observations are qualitatively explained by analyzing the interaction potential between the two-color pulse and molecular (hyper-)polarizability. The prospects for using the enantioselective orientation for enantiomers' separation is discussed.

We report the experimental observation of molecular unidirectional rotation (UDR) echoes and analyze their origin and behavior both classically and quantum mechanically. The molecules are excited by two time-delayed polarization-twisted ultrashort laser pulses and the echoes are measured by exploding the molecules and reconstructing their spatial orientation from the detected recoil ions momenta. Unlike alignment echoes which are induced by linearly polarized pulses, here the axial symmetry is broken by the twisted polarization, giving rise to molecular unidirectional rotation. We find that the rotation sense of the echo is governed by the twisting sense of the second pulse even when its intensity is much weaker than the intensity of the first pulse. In our theoretical study, we rely on classical phase-space analysis and on three-dimensional quantum simulations of the laser-driven molecular dynamics. Both approaches nicely reproduce the experimental results. Echoes in general and the unique UDR echoes in particular provide powerful tools for studies of relaxation processes in dense molecular gases.

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.

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.

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.

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.

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 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 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.

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.

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.

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 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 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.

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.

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.

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.

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.

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].

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.

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.

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