Publications
2024
-
(2024) Physical Review Research. 6, 1, 013201. Abstract
We investigate the persistent orientation of asymmetric-top molecules induced by time-delayed THz pulses that are either collinearly or cross polarized. Our theoretical and numerical results demonstrate that the orthogonal configuration outperforms the collinear one, and a significant degree of persistent orientation - approximately 10% at 5 K and nearly 3% at room temperature - may be achieved through parameter optimization. The dependence of the persistent orientation factor on temperature and field parameters is studied in detail. The proposed application of two orthogonally polarized THz pulses is both practical and efficient. Its applicability under standard laboratory conditions lays a solid foundation for future experimental realization of THz-induced persistent molecular orientation.
2023
-
(2023) Physical Review A. 107, 2, 023111. Abstract
We theoretically consider the phenomenon of field-free long-lasting orientation of symmetric-top molecules ionized by two-color laser pulses. The anisotropic ionization produces a significant long-lasting orientation of the surviving neutral molecules. The degree of orientation increases with both the pulse intensity and counterintuitively with the rotational temperature. The orientation may be enhanced even further by using multiple-delayed two-color pulses. The long-lasting orientation may be probed by even harmonic generation or by Coulomb-explosion-based methods. The effect may enable the study of relaxation processes in dense molecular gases and may be useful for molecular guiding and trapping by inhomogeneous fields.
2022
-
(2022) Physical review. A.. 106, 6, L061101 . Abstract
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.
-
(2022) The Journal of chemical physics. 157, 3, 034304. Abstract
We present a comprehensive study of enantioselective orientation of chiral molecules excited by a pair of delayed cross-polarized femtosecond laser pulses. We show that by optimizing the pulses parameters, a significant degree (∼10%) of enantioselective orientation can be achieved at 0 and 5 K rotational temperatures. This study suggests a set of reasonable experimental conditions for inducing and measuring strong enantioselective orientation. The strong enantioselective orientation and the wide availability of the femtosecond laser systems required for the proposed experiments may open new avenues for discriminating and separating molecular enantiomers.
-
(2022) Physical review letters. 128, 24, 243201. Abstract
Rotational dynamics of D2 molecules inside helium nanodroplets is induced by a moderately intense femtosecond pump pulse and measured as a function of time by recording the yield of HeD+ ions, created through strong-field dissociative ionization with a delayed femtosecond probe pulse. The yield oscillates with a period of 185 fs, reflecting field-free rotational wave packet dynamics, and the oscillation persists for more than 500 periods. Within the experimental uncertainty, the rotational constant BHe of the in-droplet D2 molecule, determined by Fourier analysis, is the same as Bgas for an isolated D2 molecule. Our observations show that the D2 molecules inside helium nanodroplets essentially rotate as free D2 molecules.
-
(2022) Physical Review A. 105, 6, L060802. Abstract
We consider a quantum system with a time-independent Hamiltonian parametrized by a set of unknown parameters α. The system is prepared in a general quantum state by an evolution operator that depends on a set of unknown parameters P. After the preparation, the system evolves in time, and it is characterized by a time-dependent observable O(t). We show that it is possible to obtain closed-form expressions for the gradients of the distance between O(t) and a calculated observable with respect to α, P, and all elements of the system density matrix, whether for pure or mixed states. These gradients can be used in projected gradient descent to infer α, P, and the relevant density matrix from dynamical observables. We combine this approach with random phase wave function approximation to obtain closed-form expressions for gradients that can be used to infer population distributions from averaged time-dependent observables in problems with a large number of quantum states participating in dynamics. The approach is illustrated by determining the temperature of molecular gas (initially, in thermal equilibrium at room temperature) from laser-induced time-dependent molecular alignment.
-
(2022) ArXiv.org.. Abstract
We present a comprehensive study of enantioselective orientation of chiral molecules excited by a pair of delayed cross-polarized femtosecond laser pulses. We show that by optimizing the pulses' parameters, a significant (~ 10%) degree of enantioselective orientation can be achieved at zero and at five kelvin rotational temperatures. This study suggests a set of reasonable experimental conditions for inducing and measuring strong enantioselective orientation. The strong enantioselective orientation and the wide availability of the femtosecond laser systems required for the proposed experiments may open new avenues for discriminating and separating molecular enantiomers.
-
(2022) Physical Review Research. 4, 1, 013212. Abstract
We present a detailed theoretical and experimental study of the rotation of the plane of polarization of light traveling through a gas of fast-spinning molecules. This effect is similar to the polarization drag phenomenon predicted by Fermi a century ago and it is a mechanical analog of the Faraday effect. In our experiments, molecules were spun up by an optical centrifuge and brought to the super-rotor state that retains its rotation for a relatively long time. Polarizability properties of fast-rotating molecules were analyzed considering the rotational Doppler effect and Coriolis forces. We used molecular dynamics simulations to account for intermolecular collisions. We found, both experimentally and theoretically, a nontrivial nonmonotonic time dependence of the polarization rotation angle. This time dependence reflects the transfer of the angular momentum from rotating molecules to the macroscopic gas flow, which may lead to the birth of gas vortices. Moreover, we show that the long-term behavior of the polarization rotation is sensitive to the details of the intermolecular potential. Thus, the polarization drag effect appears as a novel diagnostic tool for the characterization of intermolecular interaction potentials and studies of collisional processes in gases.
-
Rotation of Polarization of Light Propagating Through a Gas of Molecular Super-rotors [1](2022) International Conference on Ultrafast Phenomena, UP 2022. Abstract
We present a theoretical-experimental study of polarization rotation of light traveling through a gas of fast-spinning molecules. The long-term behavior of the polarization angle can be used for probing relaxation dynamics in molecular gases.
-
(2022) International Conference on Ultrafast Phenomena, UP 2022. Abstract
We present a study of a non-intrusive optical scheme for visualizing the rotational dynamics in an anisotropic molecular gas. The proposed optical method is promising for visualizing the rotations of symmetric- and asymmetric-top molecules.
-
Femtosecond rotational dynamics of D$_2$ molecules in superfluid helium nanodroplets(2022) arxiv.org. Abstract
Rotational dynamics of D2 molecules inside helium nanodroplets is induced by a moderately intense femtosecond (fs) pump pulse and measured as a function of time by recording the yield of HeD+ ions, created through strong-field dissociative ionization with a delayed fs probe pulse. The yield oscillates with a period of 185 fs, reflecting field-free rotational wave packet dynamics, and the oscillation persists for more than 500 periods. Within the experimental uncertainty, the rotational constant BHe of the in-droplet D2 molecule, determined by Fourier analysis, is the same as Bgas for an isolated D2 molecule. Our observations show that the D2 molecules inside helium nanodroplets essentially rotate as free D2 molecules.
-
(2022) International Conference on Ultrafast Phenomena, UP 2022. Abstract
We theoretically demonstrate the long-lasting orientation of symmetric- and asymmetric-top molecules induced by a two-color laser pulse.
2021
-
(2021) Physical review. A. 104, 5, 053113. Abstract
Inducing and controlling the ultrafast molecular rotational dynamics using shaped laser fields is essential in numerous applications. Several approaches exist that allow following the coherent molecular motion in real time, including Coulomb explosion-based techniques and recovering molecular orientation from the angular distribution of high harmonics. We theoretically consider a nonintrusive optical scheme for visualizing the rotational dynamics in an anisotropic molecular gas. The proposed method allows determining the instantaneous orientation of the principal optical axes of the gas. The method is based on probing the sample using ultrashort circularly polarized laser pulses and recording the transmission image through a vortex wave plate. We consider two example excitations: molecular alignment induced by an intense linearly polarized laser pulse and unidirectional molecular rotation induced by a polarization-shaped pulse. The proposed optical method is promising for visualizing the dynamics of complex symmetric- and asymmetric-top molecules.
-
(2021) Journal of physics. B, Atomic, molecular, and optical physics. 54, 16, 164003. Abstract
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.
-
(2021) Physical Review Letters. 127, 7, 073901. Abstract
We report the experimental observation of the rotation of the linear polarization of light propagating in a gas of fast-spinning molecules (molecular superrotors). In the observed effect, related to Fermi's prediction of "polarization drag"by a rotating medium, the vector of linear polarization tilts in the direction of molecular rotation. We use an optical centrifuge to bring the molecules in a gas sample to ultrafast unidirectional rotation and measure the polarization drag angles of the order of 10-4 rad (with an experimental uncertainty about 10-6 rad) over the propagation distance of the order of 1 mm in a number of gases under ambient conditions. We demonstrate an all-optical control of the drag magnitude and direction and investigate the robustness of the mechanical Faraday effect with respect to molecular collisions.
-
(2021) Frontiers in Physics. 9, Abstract
Impulsive orientation of symmetric-top molecules excited by two-color femtosecond pulses is considered. In addition to the well-known transient orientation appearing immediately after the pulse and then reemerging periodically due to quantum revivals, we report the phenomenon of field-free long-lasting orientation. Long-lasting means that the time averaged orientation remains non-zero until destroyed by other physical effects, e.g., intermolecular collisions. The effect is caused by the combined action of the field-polarizability and field-hyperpolarizability interactions. The dependence of degree of long-lasting orientation on temperature and pulse parameters is considered. The effect can be measured by means of second (or higher-order) harmonic generation, and may be used to control the deflection of molecules traveling through inhomogeneous electrostatic fields.
-
(2021) Phys.Rev.Lett. 126, 17, 170403. Abstract
We theoretically study an impulsively excited quantum bouncer (QB)a particle bouncing off a surface in the presence of gravity. A pair of time-delayed pulsed excitations is shown to induce a wave-packet echo effecta partial rephasing of the QB wave function appearing at twice the delay between pulses. In addition, an appropriately chosen observable [here, the population of the ground gravitational quantum state (GQS)] recorded as a function of the delay is shown to contain the transition frequencies between the GQSs, their populations, and partial phase information about the wave-packet quantum amplitudes. The wave-packet echo effect is a promising candidate method for precision studies of GQSs of ultracold neutrons, atoms, and antiatoms confined in closed gravitational traps.
-
(2021) Physical Review Research. 3, 1, 013249. Abstract
Chirality and chiral molecules are key elements in modern chemical and biochemical industries. Individual addressing, and the eventual separation of chiral enantiomers has been and still is an important elusive task in molecular physics and chemistry, and a variety of methods has been introduced over the years to achieve this goal. Here, we theoretically demonstrate that a pair of cross-polarized THz pulses interacting with chiral molecules through their permanent dipole moments induces an enantioselective orientation of these molecules. This orientation persists for a long time, exceeding the duration of the THz pulses by several orders of magnitude, and its dependency on temperature and pulses' parameters is investigated. The persistent orientation may enhance the deflection of the molecules in inhomogeneous electromagnetic fields, potentially leading to viable separation techniques.
-
Echoes in a single quantum Kerr-nonlinear oscillator(2021) Optics InfoBase Conference Papers. JTu3A.36. Abstract
We theoretically study the echo phenomenon in a single impulsively excited (\u201ckicked\u201d) Kerr-nonlinear oscillator. These echoes may be useful for studying decoherence processes in a number of systems related to quantum information processing.
-
(2021) Physical Review A. 103, 1, 013717. Abstract
A quantum Kerr-nonlinear oscillator is a paradigmatic model in cavity and circuit quantum electrodynamics, and quantum optomechanics. We theoretically study the echo phenomenon in a single impulsively excited ("kicked") Kerr-nonlinear oscillator. We reveal two types of echoes, "quantum"and "classical"ones, emerging on the long and short time scales, respectively. The mechanisms of the echoes are discussed, and their sensitivity to dissipation is considered. These echoes may be useful for studying decoherence processes in a number of systems related to quantum information processing.
2020
-
(2020) Physical Review A. 102, 4, 043116. Abstract
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.
-
(2020) Journal of Chemical Physics. 153, 16, 164111. Abstract
We demonstrate an efficient algorithm for inverse problems in time-dependent quantum dynamics based on feedback loops between Hamiltonian parameters and the solutions of the Schrödinger equation. Our approach formulates the inverse problem as a target vector estimation problem and uses Bayesian surrogate models of the Schrödinger equation solutions to direct the optimization of feedback loops. For the surrogate models, we use Gaussian processes with vector outputs and composite kernels built by an iterative algorithm with the Bayesian information criterion (BIC) as a kernel selection metric. The outputs of the Gaussian processes are designed to model an observable simultaneously at different time instances. We show that the use of Gaussian processes with vector outputs and the BIC-directed kernel construction reduces the number of iterations in the feedback loops by, at least, a factor of 3. We also demonstrate an application of Bayesian optimization for inverse problems with noisy data. To demonstrate the algorithm, we consider the orientation and alignment of polyatomic molecules SO2 and propylene oxide (PPO) induced by strong laser pulses. We use simulated time evolutions of the orientation or alignment signals to determine the relevant components of the molecular polarizability tensors. We show that, for the five independent components of the polarizability tensor of PPO, this can be achieved with as few as 30 quantum dynamics calculations.
-
(2020) Physical Review Letters. 125, 1, 013201. Abstract
We present a novel, previously unreported phenomenon appearing in a thermal gas of nonlinear polar molecules excited by a single THz pulse. We find that the induced orientation lasts long after the excitation pulse is over. In the case of symmetric-top molecules, the time-averaged orientation remains indefinitely constant, whereas in the case of asymmetric-top molecules the orientation persists for a long time after the end of the pulse. We discuss the underlying mechanism, study its nonmonotonous temperature and amplitude dependencies, and show that there exist optimal parameters for maximal residual orientation. The persistent orientation implies a long-lasting macroscopic dipole moment, which may be probed by even harmonic generation and may enable deflection by inhomogeneous electrostatic fields.
-
(2020) Laser and Photonics Reviews. 14, 5, 1900344. Abstract
Short laser pulses are widely used for controlling molecular rotational degrees of freedom and inducing molecular alignment, orientation, unidirectional rotation, and other types of coherent rotational motion. To follow the ultrafast rotational dynamics in real time, several techniques for producing molecular movies have been proposed based on the Coulomb explosion of rotating molecules, or recovering molecular orientation from the angular distribution of high harmonics. The present work offers and demonstrates a novel nondestructive optical method for direct visualization and recording of movies of coherent rotational dynamics in a molecular gas. The technique is based on imaging the time-dependent polarization dynamics of a probe light propagating through a gas of coherently rotating molecules. The probe pulse continues through a radial polarizer, and is then recorded by a camera. The technique is illustrated by implementing it with two examples of time-resolved rotational dynamics: alignmentantialignment cycles in a molecular gas excited by a single linearly polarized laser pulse, and unidirectional molecular rotation induced by a pulse with twisted polarization. This method may open new avenues in studies on fast chemical transformation phenomena and ultrafast molecular dynamics caused by strong laser fields of various complexities.
-
(2020) Advanced Photonics. 2, 2, 024002. Abstract
Molecular alignment and orientation by laser fields has attracted significant attention in recent years, mostly due to new capabilities to manipulate the molecular spatial arrangement. Molecules can now be efficiently prepared for ionization, structural imaging, orbital tomography, and more, enabling, for example, shooting of dynamic molecular movies. Furthermore, molecular alignment and orientation processes give rise to fundamental quantum and classical phenomena like quantum revivals, Anderson localization, and rotational echoes, just to mention a few. We review recent progress on the visualization, coherent control, and applications of the rich dynamics of molecular rotational wave packets driven by laser pulses of various intensities, durations, and polarizations. In particular, we focus on the molecular unidirectional rotation and its visualization, the orientation of chiral molecules, and the three-dimensional orientation of asymmetric-top molecules. Rotational echoes are discussed as an example of nontrivial dynamics and detection of prepared molecular states.
-
(2020) Nature Physics. 16, 3, p. 328-333 Abstract
Echoes occur in many physical systems, typically in inhomogeneously broadened ensembles of nonlinear objects. They are often used to eliminate the effects of dephasing caused by interactions with the environment as well as to enable the observation of proper, inherent object properties. Here, we report the experimental observation of quantum wave-packet echoes in a single, isolated molecule. The entire dephasing-rephasing cycle occurs without any inhomogeneous spread of molecular properties, or any interaction with the environment, and offers a way to probe the internal coherent dynamics of single molecules. In our experiments, we impulsively excite a vibrational wave packet in an anharmonic molecular potential and observe its oscillations and eventual dispersion with time. A second, delayed pulse gives rise to an echo-a partial recovery of the initial coherent oscillations. The vibrational dynamics of single molecules is visualized by a time-delayed probe pulse dissociating them, one at a time. Two mechanisms for the echo formation are discussed: a.c. Stark-induced molecular potential shaking and creation of a depletion-induced 'hole' in the nuclear spatial distribution. The single-molecule wave-packet echoes may lead to the development of new tools for probing ultrafast intramolecular processes in various molecules.Following an impulsive laser excitation of a single molecule, a dispersed vibrational wave-packet is partially rephased by a second pulse, and a wave-packet echo is observed. This wave-packet echo probes ultrafast intramolecular processes in the isolated molecule.
-
Observation of persistent orientation of chiral molecules by a laser field with twisted polarization(2020) Physical Review A. 101, 2, 021403(R). Abstract
Molecular chirality is an omnipresent phenomenon of fundamental significance in physics, chemistry, and biology. For this reason, the search for various techniques for enantioselective control, detection, and separation of chiral molecules is of particular importance. It has been recently predicted that laser fields with twisted polarization may induce a persistent enantioselective field-free orientation of chiral molecules. Here, we report an experimental observation of this phenomenon using propylene oxide molecules (CH3CHCH2O, or PPO) spun by an optical centrifuge-a laser pulse-whose linear polarization undergoes an accelerated rotation around its propagation direction. We show that PPO molecules remain oriented on a timescale exceeding the duration of the centrifuge pulse by several orders of magnitude. The demonstrated long-time field-free enantioselective orientation may open new avenues for optical manipulation, discrimination, and, potentially, the separation of molecular enantiomers.
-
(2020) Physical Review A. 101, 2, 021404(R). Abstract
Experiments on light dragging in a moving medium laid the cornerstones of modern physics more than a century ago, and they still are in the focus of current research. When linearly polarized light is transmitted through a rotating dielectric, the polarization plane is slightly rotated-a phenomenon first studied by Fermi in 1923. For typical nonresonant dielectric materials, the measured polarization drag angle does not surpass several microradians. Here, we show that this effect may be dramatically enhanced if the light is sent to a gas of fast unidirectionally spinning molecular super-rotors. Several femtosecond-laser labs have already succeeded in optically creating such a medium. We show that the specific rotation power of the super-rotor medium exceeds the values previously observed in mechanically rotated bulk optical specimens by many orders of magnitude. This nonreciprocal optomechanical phenomenon may open other avenues for ultrafast control of the polarization state of light.
2019
-
(2019) Physical Review A. 100, 4, 043406. Abstract
We study, both classically and quantum mechanically, enantioselective orientation of gas-phase chiral molecules excited by laser fields with twisted polarization. Counterintuitively, the induced orientation, whose direction is laser controllable, does not disappear after the excitation but stays long after the end of the laser pulses. We computationally demonstrate this long-lasting orientation, using propylene oxide molecules (CH3CHCH2O, or PPO) as an example, and consider two kinds of fields with twisted polarization: a pair of delayed cross-polarized pulses and an optical centrifuge. This chiral effect opens avenues for detecting molecular chirality, measuring enantiomeric excess, and separating enantiomers with the help of inhomogeneous external fields.
-
(2019) Physical Review Letters. 122, 22, 223201. Abstract
We report on the first experimental demonstration of enantioselective rotational control of chiral molecules with a laser field. In our experiments, two enantiomers of propylene oxide are brought to accelerated unidirectional rotation by means of an optical centrifuge. Using Coulomb explosion imaging, we show that the centrifuged molecules acquire preferential orientation perpendicular to the plane of rotation, and that the direction of this orientation depends on the relative handedness of the enantiomer and the rotating centrifuge field. The observed effect is in agreement with theoretical predictions and is reproduced in numerical simulations of the centrifuge excitation followed by Coulomb explosion of the centrifuged molecules. The demonstrated technique opens new avenues in optical enantioselective control of chiral molecules with a plethora of potential applications in differentiation, separation, and purification of chiral mixtures.
-
(2019) Physical Review Letters. 122, 19, 193401. Abstract
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.
2018
-
(2018) Nature Communications. 9, 5134. Abstract
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.
-
(2018) Journal of Physical Chemistry Letters. 9, 5, p. 1105-1111 Abstract
We explore a pure optical method for enantioselective orientation of chiral molecules by means of laser fields with twisted polarization. Several field implementations are considered, including a pair of delayed, cross-polarized laser pulses, an optical centrifuge, and polarization-shaped pulses. We show that these schemes lead to out-of-phase time-dependent dipole signals for different enantiomers, and we also predict a substantial permanent molecular orientation persisting long after the laser fields are over. The underlying classical orientation mechanism common to all of these fields is discussed, and its operation is demonstrated for a range of chiral molecules of various complexity: hydrogen thioperoxide (HSOH), propylene oxide (CH3CHCH2O), and ethyl oxirane (CH3CH2CHCH2O). The presented results demonstrate generality, versatility, and robustness of this optical method for manipulating molecular enantiomers in the gas phase.
-
(2018) Physical Review Letters. 120, 8, 083204. Abstract
We study interaction of generic asymmetric molecules with laser fields having twisted polarization, using a pair of strong time-delayed short laser pulses with crossed linear polarizations as an example. We show that such an excitation not only provides unidirectional rotation of the most polarizable molecular axis, but also induces a directed torque along this axis, which results in a transient orientation of the molecules. The asymmetric molecules are chiral in nature and different molecular enantiomers experience the orienting action in opposite directions causing out-of-phase oscillations of their dipole moments. The resulting microwave radiation was recently suggested to be used for analysis or discrimination of chiral molecular mixtures. We reveal the mechanism behind this laser-induced orientation effect, show that it is classical in nature, and envision further applications of light with twisted polarization.
2017
-
(2017) Physical Review A. 96, 4, 043418. Abstract
The rotational Doppler effect occurs when circularly polarized light interacts with a rotating anisotropic material. It is manifested by the appearance of a spectral shift ensuing from the transfer of angular momentum and energy between radiation and matter. Recently, we reported terahertz-range rotational Doppler shifts produced in third-order nonlinear optical conversion [O. Faucher et al., Phys. Rev. A 94, 051402(R) (2016)]. The experiment was performed in an ensemble of coherently spinning molecules prepared by a short laser pulse exhibiting a twisted linear polarization. The present work provides an extensive analysis of the rotational Doppler effect in third-order-harmonic generation from spinning linear molecules. The underlying physics is investigated both experimentally and theoretically. The implication of the rotational Doppler effect in higher-order processes like high-order-harmonic generation is discussed.
-
(2017) Optics Express. 25, 21, p. 24917-24926 Abstract
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.
2016
-
(2016) Physical Review A. 94, 5, 051402(R). Abstract
We present an observation of the rotational Doppler shift in the frequency of optical harmonic generated in fast rotating molecules. Conservation of energy and angular momentum in the light-molecule interaction suggests four different kinds of shifts depending on the mutual handedness of the circularly polarized fundamental and harmonic fields, as well as the handedness of the molecular rotation. All four types of the frequency shifts were observed in our experiments on third-harmonic generation in a gas of fast spinning O-2 molecules.
-
(2016) ChemPhysChem. 17, 22, p. 3795-3810 Abstract
In recent years, several femtosecond laser techniques have been developed that can make gas molecules rotate extremely fast, whereas the gas stays translationally cold. Herein we use molecular-dynamics simulations to investigate the collisional dynamics of gases of such molecules (superrotors). We found that the common route of superrotors to equilibrium is rather generic. It starts with a long-lasting gyroscopic stage, during which the molecules keep their fast rotation and the orientation of their angular momentum despite the many collisions they undergo. The inhibited rotational relaxation is characterized by a persistent anisotropy in the molecular angular distribution, manifested in long-lasting optical birefringence and in anisotropic diffusion of the gas. Later, the gyroscopic stage is abruptly terminated by a self-accelerating explosive rotational-translational energy exchange that generates sound and macroscopic vortices with a hot rotating core.
-
(2016) Physical Review A. 94, 3, 033404. Abstract
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.
-
-
(2016) Journal of Physical Chemistry A. 120, 19, p. 3206-3217 Abstract
We describe a universal behavior of linear molecules excited by a periodic train of short laser pulses under conditions close to the quantum resonance. The quantum resonance effect causes an unlimited ballistic growth of the angular momentum. We show that a disturbance of the quantum resonance, either by the centrifugal distortion of the rotating molecules or a controlled detuning of the pulse train period from the so-called rotational revival time, eventually halts the growth by causing Anderson localization beyond a critical value of the angular momentum, the Anderson wall. Below the wall, the rotational excitation oscillates with the number of pulses due to a mechanism similar to Bloch oscillations in crystalline solids. We suggest optical experiments capable of observing the rotational Anderson wall and Bloch oscillations at near-ambient conditions with the help of existing laser technology.
-
(2016) Physical Review X. 6, 4, 041056. Abstract
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 laserexcited 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.
2015
-
(2015) Physical review letters. 115, 20, 203002. Abstract
We report the observation of rotational Bloch oscillations in a gas of nitrogen molecules kicked by a periodic train of femtosecond laser pulses. A controllable detuning from the quantum resonance creates an effective accelerating potential in angular momentum space, inducing Bloch-like oscillations of the rotational excitation. These oscillations are measured via the temporal modulation of the refractive index of the gas. Our results introduce room-temperature laser-kicked molecules as a new laboratory for studies of localization phenomena in quantum transport.
-
(2015) Physical review letters. 115, 3, 033005. Abstract
We study the dynamics of paramagnetic molecular superrotors in an external magnetic field. An optical centrifuge is used to create dense ensembles of oxygen molecules in ultrahigh rotational states. In is shown, for the first time, that the gas of rotating molecules becomes optically birefringent in the presence of a magnetic field. The discovered effect of "magneto-rotational birefringence" indicates the preferential alignment of molecular axes along the field direction. We provide an intuitive qualitative model, in which the influence of the applied magnetic field on the molecular orientation is mediated by the spin-rotation coupling. This model is supported by the direct imaging of the distribution of molecular axes, the demonstration of the magnetic reversal of the rotational Raman signal, and by numerical calculations.
-
(2015) Nature Communications. 6, 7791. Abstract
Recently, femtosecond laser techniques have been developed that are capable of bringing gas molecules to extremely fast rotation in a very short time, while keeping their translational motion relatively slow. Here we study collisional equilibration dynamics of this new state of molecular gases. We show that the route to equilibrium starts with a metastable 'gyroscopic stage' in the course of which the molecules maintain their fast rotation and orientation of the angular momentum through many collisions. The inhibited rotational-translational relaxation is characterized by a persistent anisotropy in the molecular angular distribution, and is manifested in the optical birefringence and anisotropic diffusion in the gas. After a certain induction time, the 'gyroscopic stage' is abruptly terminated by an explosive rotational-translational energy exchange, leading the gas towards the final equilibrium. We illustrate our conclusions by direct molecular dynamics simulation of several gases of linear molecules.
-
(2015) Physical Review E. 91, 5, 052911. Abstract
We present a quantum localization phenomenon that exists in periodically kicked three-dimensional rotors, but is absent in the commonly studied two-dimensional ones: edge localization. We show that under the condition of a fractional quantum resonance there are states of the kicked rotor that are strongly localized near the edge of the angular momentum space at J=0. These states are analogs of surface states in crystalline solids, and they significantly affect resonant excitation of molecular rotation by laser pulse trains.
-
(2015) Physical review letters. 114, 15, 153601. Abstract
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.
-
(2015) Proceedings of Frontiers in Optics 2015, FIO 2015. Abstract
Nitrogen gas is rotationally excited by a train of eight impulsive kicks. The resulting population alignment evolution exhibits two phenomena explained by analogy to the famous condensed matter effects of dynamical localization and Bloch oscillations.
-
(2015) Journal of Chemical Physics. 142, 2, 024311. Abstract
We present full quantum mechanical scattering calculations using spinning molecules as target states for nuclear spin selective atom-diatom scattering of reactive D+H2 and F+H2 collisions. Molecules can be forced to rotate uni-directionally by chiral trains of short, non-resonant laser pulses, with different nuclear spin isomers rotating in opposite directions. The calculations we present are based on rotational wavepackets that can be created in this manner. As our simulations show, target molecules with opposite sense of rotation are predominantly scattered in opposite directions, opening routes for spatially and quantum state selective scattering of close chemical species. Moreover, two-dimensional state resolved differential cross sections reveal detailed information about the scattering mechanisms, which can be explained to a large degree by a classical vector model for scattering with spinning molecules.
2014
-
(2014) Physical review letters. 113, 4, 043002. Abstract
We describe a universal behavior of linear molecules excited by a periodic train of short laser pulses under quantum resonance conditions. In a rigid rotor, the resonance causes an unlimited ballistic growth of the angular momentum. We show that the centrifugal distortion of rotating molecules eventually halts the growth, by causing Anderson localization beyond a critical value of the angular momentum - the Anderson wall. Its position solely depends on the molecular rotational constants and lies in the range of a few tens of. Below the wall, rotational excitation oscillates with the number of pulses due to a mechanism similar to Bloch oscillations in crystalline solids. We suggest optical experiments capable of observing the rotational Anderson wall and Bloch oscillations at near-ambient conditions with the help of existing laser technology.
-
(2014) Physical review letters. 112, 1, 013004. Abstract
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.
2013
-
(2013) Nature Photonics. 7, 9, p. 711-714 Abstract
When a wave is reflected from a moving object, its frequency is Doppler shifted. Similarly, when circularly polarized light is scattered from a rotating object, a rotational Doppler frequency shift may be observed, with manifestations ranging from the quantum world (fluorescence spectroscopy, rotational Raman scattering and so on) to satellite-based global positioning systems. 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 45angle) causing a concerted unidirectional rotation of the 'molecular propellers'. 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.
-
(2013) Physical Review A. 88, 2, 023426. Abstract
The paper explores the prospects of observing the phenomenon of dynamical Anderson localization via nonresonant Raman-type rotational excitation of molecules by periodic trains of short laser pulses. We define conditions for such an experiment and show that current femtosecond technology used for nonadiabatic laser alignment of linear molecules is sufficient for this task. Several observables which can serve as indicators for Anderson localization are suggested for measurement, and the influence of experimental limitations imposed by the laser intensity noise, finite pulse duration, limited number of pulses in a train, and thermal effects is analyzed.
-
(2013) Molecular Physics. 111, 12-13, p. 1716-1730 Abstract
We consider solid surface scattering of molecules that were subject to strong non-resonant ultrashort laser pulses just before hitting the surface. The pulses modify the rotational state of the molecules, causing their field-free alignment, or a rotation with a preferred sense. We show that field-free laser-induced molecular alignment leads to correlation between the scattering angle and the sense of rotation of the scattered molecules. Moreover, by controlling the sense of laser-induced unidirectional molecular rotation, one may affect the scattering angle of the molecules. This provides a new means for separation of mixtures of molecules (such as isotopes and nuclear-spin isomers) by laser-controlled surface scattering.
-
(2013) Xviiith International Conference On Ultrafast Phenomena. 41, Abstract
We show that molecules kicked periodically by laser pulses currently used in molecular alignment experiments allow to observe effects of the periodically kicked quantum rotor in a real rotational system. Among these effects are Anderson localisation in angular momentum and the scaling of the quantum resonance. Based on this, we propose a new scheme for selective molecular rotational excitation.
-
(2013) Xviiith International Conference On Ultrafast Phenomena. 41, Abstract
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.
2012
-
(2012) Physical Review A. 86, 6, 063414. Abstract
We provide a detailed theoretical analysis of molecular rotational excitation by a chiral pulse train, a sequence of linearly polarized pulses with the polarization direction rotating from pulse to pulse by a controllable angle. Molecular rotation with a preferential rotational sense (clockwise or counterclockwise) can be excited by this scheme. We show that the directionality of the rotation is caused by quantum interference of different excitation pathways. The chiral pulse train is capable of selective excitation of molecular isotopologs and nuclear spin isomers in a mixture. We demonstrate this using 14N2 and 15N2 as examples for isotopologs and para- and ortho-nitrogen as examples for nuclear-spin isomers.
-
(2012) Physical Review A. 86, 2, 021401. Abstract
We show that the current laser technology used for field-free molecular alignment via impulsive Raman rotational excitation allows for observing long-discussed nonlinear quantum phenomena in the dynamics of the periodically kicked rotor. This includes the scaling of the absorbed energy near the conditions of quantum resonance and Anderson-like localization in the angular momentum. Based on this, we show that periodic trains of short laser pulses provide an efficient tool for selective rotational excitation and alignment in a molecular mixture. We demonstrate the efficiency of this approach by applying it to a mixture of two nitrogen isotopologues (14N 2 and 15N 2), and show that strong selectivity is possible even at room temperature.
-
(2012) Physical review letters. 109, 4, 043003. Abstract
We experimentally investigate the effect of quantum resonance in the rotational excitation of the simplest quantum rotor-a diatomic molecule. Using the techniques of high-resolution femtosecond pulse shaping and rotational state-resolved detection, we measure directly the amount of energy absorbed by molecules interacting with a periodic train of laser pulses, and study their dependence on the train period. We show that the energy transfer is significantly enhanced at quantum resonance, and use this effect to demonstrate selective rotational excitation of two nitrogen isotopologs, N214 and N215. Moreover, by tuning the period of the pulse train in the vicinity of a fractional quantum resonance, we achieve selective rotational excitation of para- and ortho-isomers of N215.
-
(2012) Physical review letters. 109, 3, 033001. Abstract
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 MonteCarlo method, followed by a computational gas-dynamic analysis.
-
(2012) Israel Journal of Chemistry. 52, 5, p. 414-437 Abstract
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 light-matter 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.
2011
-
(2011) Physical review letters. 107, 24, 243004. Abstract
Trains of ultrashort laser pulses separated by the time of rotational revival (typically, tens of picoseconds) have been exploited for creating ensembles of aligned molecules. In this work we introduce a chiral pulse train-a sequence of linearly polarized pulses with the polarization direction rotating from pulse to pulse by a controllable angle. The chirality of such a train, expressed through the period and direction of its polarization rotation, is used as a new control parameter for achieving selectivity and directionality of laser-induced rotational excitation. The method employs chiral trains with a large number of pulses separated on the time scale much shorter than the rotational revival (a few hundred femtosecond), enabling the use of conventional pulse shapers.
-
(2011) Physical Review A. 84, 5, 053420. Abstract
In recent years it has become possible to align molecules in free space using ultrashort laser pulses. Here we explore two schemes for controlling molecule-surface scattering processes and which are based on laser-induced molecular alignment. In the first scheme, a single ultrashort nonresonant laser pulse is applied to a molecular beam hitting the surface. This pulse modifies the angular distribution of the incident molecules and causes the scattered molecules to rotate with a preferred sense of rotation (clockwise or counterclockwise). In the second scheme, two properly delayed laser pulses are applied to a molecular beam composed of two chemically close molecular species (isotopes, or nuclear-spin isomers). As the result of the double-pulse excitation, these species are selectively scattered to different angles after the collision with the surface. These effects may provide new means for the analysis and separation of molecular mixtures.
-
(2011) Applied Physics B-Lasers And Optics. 105, 2, p. 203-211 Abstract
Molecular alignment of linear molecules (O 2, N 2, CO 2 and CO) is measured photoacoustically in the gas phase. The rotational excitation is accomplished using a simple femtosecond stimulated Raman excitation scheme, employing two femtosecond pulses with variable delay between the pulses. Molecular alignment is determined directly by measuring the energy dumped into the gas by quartz-enhanced photoacoustic spectroscopy (QEPAS), utilizing a quartz tuning fork as a sensitive photoacoustic transducer. The experimental results demonstrate for the first time the use of a tuning fork for resonant photoacoustic detection of Raman spectra excited by femtosecond double pulses and match both simulation and literature values.
-
(2011) Journal of Chemical Physics. 135, 19, 194310. Abstract
The effects of laser-induced pre-alignment on the deflection of paramagnetic molecules by inhomogeneous static magnetic field are studied. Depending on the relevant Hunds coupling case of the molecule, two different effects were identified: either suppression of the deflection by laser pulses (Hunds coupling case (a) molecules, such as ClO), or a dramatic reconstruction of the broad distribution of the scattering angles into several narrow peaks (for Hunds coupling case (b) molecules, such as O2 or NH). These findings are important for various applications using molecular guiding, focusing and trapping with the help of magnetic fields.
-
(2011) New Journal of Physics. 13, 103008. Abstract
We propose three implementations of the Gauss sum factorization schemes discussed in part I of this series (Wölk et al 2011 New J. Phys. 13 103007): (i) A two-photon transition in a multi-level ladder system induced by a chirped laser pulse, (ii) A chirped one-photon transition in a two-level atom with a periodically modulated excited state and (iii) A linearly chirped onephoton transition driven by a sequence of ultrashort pulses. For each of these quantum systems, we show that the excitation probability amplitude is given by an appropriate Gauss sum. We provide rules on how to encode the number N to be factored in our system and how to identify the factors of N in the fluorescence signal of the excited state.
-
(2011) New Journal of Physics. 13, 103007. Abstract
We use the periodicity properties of generalized Gauss sums to factor numbers. Moreover, we derive rules for finding the factors and illustrate this factorization scheme for various examples. This algorithm relies solely on interference and scales exponentially.
-
(2011) Journal of Chemical Physics. 135, 8, 084307. Abstract
We consider deflection of rotating symmetric top molecules by inhomogeneous optical and static electric fields, compare results with the case of linear molecules, and find new singularities in the distribution of the scattering angle. Scattering of the prolate/oblate molecules is analyzed in detail, and it is shown that the process can be efficiently controlled by means of short and strong femtosecond laser pulses. In particular, the angular dispersion of the deflected molecules may be dramatically reduced by laser-induced molecular prealignment. We first study the problem by using a simple classical model, and then find similar results by means of more sophisticated methods, including the formalism of adiabatic invariants and direct numerical simulation of the Euler-Lagrange equations of motion. The suggested control scheme opens new ways for many applications involving molecular focusing, guiding, and trapping by optical and static fields.
-
(2011) Journal of Chemical Physics. 134, 5, 054304. Abstract
We provide a theory of the deflection of polar and nonpolar rotating molecules by inhomogeneous static electric field. Rainbowlike features in the angular distribution of the scattered molecules are analyzed in detail. Furthermore, we demonstrate that one may efficiently control the deflection process with the help of short and strong femtosecond laser pulses. In particular, the deflection process may be turned off by a proper excitation, and the angular dispersion of the deflected molecules can be substantially reduced. We study the problem both classically and quantum mechanically, taking into account the effects of strong deflecting field on the molecular rotations. In both treatments we arrive at the same conclusions. The suggested control scheme paves the way for many applications involving molecular focusing, guiding, and trapping by inhomogeneous fields. (C) 2011 American Institute of Physics. [doi:10.1063/1.3535600]
-
(2011) Physical Review A. 83, 2, 025401. Abstract
Several laser techniques have been suggested and demonstrated recently for preparing polarizable molecules in rapidly spinning states with a disk-like angular distribution. We consider motion of these spinning disks in inhomogeneous fields and show that the molecular trajectories may be precisely controlled by the tilt of the plane of the laser-induced rotation. The feasibility of the scheme is illustrated by optical deflection of linear molecules twirled by two delayed cross-polarized laser pulses. These results open new ways for many applications involving molecular focusing, guiding, and trapping and may be suitable for separating molecular mixtures by optical and static fields.
-
(2011) Physical Review A. 83, 2, 023423. Abstract
Recently, it was predicted theoretically and verified experimentally that a pair of delayed and cross-polarized short laser pulses can create molecular ensembles with a well-defined sense of rotation (clockwise or counterclockwise). Here we provide a comparative study of the classical and quantum aspects of the underlying mechanism for linear molecules and for symmetric tops, like benzene molecules, that were used for the first experimental demonstration of the effect. Very good quantitative agreement is found between the classical description of the process and the rigorous quantum-mechanical analysis at the relevant experimental conditions. Both approaches predict the same optimal values for the delay between pulses and the angle between them, and deliver the same magnitude of the induced oriented angular momentum of the molecular ensemble. As expected, quantum and classical analyses substantially deviate when the delay between pulses is comparable with the period of quantum rotational revivals. However, time-averaged characteristics of the excited molecular ensemble are equally well described by these two approaches. This is illustrated by calculating the anisotropic time-averaged angular distribution of the double-pulse excited molecules, which reflects persistent confinement of the molecular axes to the rotation plane defined by two polarization vectors of the pulses.
2010
-
(2010) Physical Review A. 82, 3, 33401. Abstract
We consider deflection of polarizable molecules by inhomogeneous optical fields and analyze the role of molecular orientation and rotation in the scattering process. It is shown that molecular rotation induces spectacular rainbowlike features in the distribution of the scattering angle. Moreover, by preshaping molecular angular distribution with the help of short and strong femtosecond laser pulses, one may efficiently control the scattering process, manipulate the average deflection angle and its distribution, and reduce substantially the angular dispersion of the deflected molecules. We study the problem both classically and quantum mechanically and arrive at the same conclusions in both treatments. The effects of strong deflecting field on the scattering of rotating molecules are considered by the means of the adiabatic invariants formalism. The suggested control scheme opens new ways for many applications involving molecular focusing, guiding, and trapping by optical and static fields.
-
(2010) Physical Review A. 81, 6, 063809. Abstract
We study the optimal focusing of two-level atoms with a near-resonant standing wave light, using both classical and quantum treatments of the problem in the thin- and thick-lens regimes. It is found that the near-resonant standing wave focuses the atoms with a reduced background in comparison with far-detuned light fields. For some parameters, the quantum atomic distribution shows even better localization than the classical one. Spontaneous emission effects are included via the technique of quantum Monte Carlo wave function simulations. We investigate the extent to which nonadiabatic and spontaneous emission effects limit the achievable minimal size of the deposited structures.
-
(2010) Physical review letters. 104, 15, 153001. Abstract
We consider deflection of polarizable molecules by inhomogeneous optical fields, and analyze the role of molecular orientation and rotation in the scattering process. We show that by preshaping molecular angular distribution with the help of short and strong femtosecond laser pulses, one may efficiently control the scattering process, manipulate the average deflection angle and its distribution, and reduce substantially the angular dispersion of the deflected molecules. This opens new ways for many applications involving molecular focusing, guiding, and trapping by optical and static fields.
-
(2010) Journal Of Physics-Condensed Matter. 22, 30, 304004. Abstract
A classical perturbation theory is developed to study rotational rainbow scattering of molecules from uncorrugated frozen surfaces. Considering the interaction of the rigid rotor with the translational motion towards the surface to be weak allows for a perturbative treatment, in which the known zeroth order motion is that of a freely rotating molecule hitting a surface. Using perturbation theory leads to explicit expressions for the angular momentum deflection function with respect to the initial orientational angle of the rotor that are valid for any magnitude of the initial angular momentum. The rotational rainbows appear as peaks both in the final angular momentum and rotational energy distributions, as well as peaks in the angular distribution, although the surface is assumed to be uncorrugated. The derived analytic expressions are compared with numerical simulation data. Even when the rotational motion is significantly coupled to the translational motion, the predictions of the perturbative treatment remain qualitatively correct.
2009
-
(2009) New Journal of Physics. 11, 105039. Abstract
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.
-
(2009) Laser Physics. 19, 4, p. 752-761 Abstract
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.
-
(2009) International Quantum Electronics Conference, IQEC 2009. p. 2 Abstract
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.
-
Laser Induced Alignment of Water Spin Isomers(2009) Ultrafast Phenomena Xvi. 92, p. 209-211 Abstract
We consider laser alignment of ortho and para spin isomers of water molecules by using strong and short off-resonance laser pulses. A single pulse is found to create distinct transient alignment and antialignment of the isomeric species. We demonstrate selective alignment of one isomeric species (leaving the other species randomly oriented) by a pair of two laser pulses.
-
(2009) Springer Series in Chemical Physics. p. 75-77 Abstract
By varying the polarization and delay between two ultrashort laser pulses, we control the plane, speed, and sense ofmolecular rotation. This control may be implemented to individual components within a molecular mixture.
2008
-
(2008) Physical Review A. 78, 6, 063416. Abstract
We consider laser alignment of ortho and para spin isomers of water molecules by using strong and short off-resonance laser pulses. A single pulse is found to create a distinct transient alignment and antialignment of the isomeric species. We suggest selective alignment of one isomeric species (leaving the other species randomly aligned) by a pair of two laser pulses.
-
(2008) JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS. 41, 7, 074018. Abstract
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 ( 14N2, 15N2), 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, ortho15N2) 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.
2007
-
(2007) Journal of Modern Optics. 54, 16-17, p. 2641-2658 Abstract
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.
-
(2007) Physical review letters. 99, 10, 103002. Abstract
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.
-
(2007) Physical review letters. 99, 9, 093002. Abstract
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 (N15) or spin 1 (N14) 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.
-
(2007) Physical review letters. 98, 12, 120502. Abstract
We factor the number 157573 using an NMR implementation of Gauss sums. Although the current implementation is classical and scales exponentially, we believe that an extension to the quantum regime using entangled states is possible.
-
(2007) Optics Express. 15, 4, p. 1700-1705 Abstract
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 CHCl 3 and the Raman spectrum of the low-lying vibrational states of these molecules is measured.
-
(2007) 2007 Quantum Electronics and Laser Science Conference. Abstract
Double pulse excitation of fractional revivals of rotational wavepackets is demonstrated as an effective tool for spin-selective alignment in a multi-component mixture of molecular spin isomers.
-
(2007) 2007 Conference on Lasers and Electro-Optics (CLEO). Abstract
We demonstrate single-pulse retrieval of coherent vibrational evolution of molecules by geometrical space-time mapping combined with non-linear signal imaging. The method is tested experimentally to yield spectrum of simple liquids.
-
(2007) 2007 Quantum Electronics and Laser Science Conference. Abstract
We propose a generic approach for nonresonant laser cooling of atoms/molecules based on their interaction with a bistable optical cavity. The cooling mechanism is of Sisyphus type, and it does not require high-finesse cavities.
-
(2007) Elements of Quantum Information. Walther P. D. H. & P. Schleich P. D. W.(eds.). p. 339-353 Abstract
-
Molecular orientation via molecular anti-alignment(2007) Ultrafast Phenomena Xv. 88, p. 576-578 Abstract
We show that field-free molecular orientation induced by a half-cycle pulse may be considerably enhanced by an additional laser pulse inducing molecular anti-alignment. Two qualitatively different enhancement mechanisms are identified depending on the pulse order.
2006
-
(2006) Fortschritte Der Physik-Progress Of Physics. 54, 8-10, p. 856-865 Abstract
The periodicity properties of Gauss sums allow us to factor integer numbers. We show that the excitation probability amplitudes of appropriate quantum systems interacting with specific laser fields are determined by Gauss sums. The resulting probabilities are experimentally accessible by measuring the fluorescence from this level. In particular, we discuss a two-photon transition in a ladder system driven by a chirped laser pulse. In addition, we consider two realizations of laser driven one-photon transitions. For each quantum system we demonstrate the power of this factorization scheme using numerical examples.
-
(2006) 2005 Quantum Electronics and Laser Science Conference (QELS). p. 487-489 Abstract
Optical shaking with feedback is proposed as a new method for laser cooling of atoms and molecules to high phase-space density and without the loss of particles typical of evaporative cooling.
-
(2006) International Journal of Modern Physics B. 20, 11-13, p. 1893-1916 Abstract
We present two physical systems which make Gauß sums experimentally accessible. The probability amplitude for a two-photon transition in an appropriate ladder system driven by a chirped laser pulse is determined by a Gauß sum. The autocorrelation function of a quantum rotor is also of the form of a Gauß sum. These examples suggest rules for determining prime factor components on the basis of the properties of Gauß sums. Moreover, we show how Gauß sums are related to the Riemann Zeta function.
-
(2006) Physical Review A. 74, 5, 053414. Abstract
We explore the role of laser induced antialignment in enhancing molecular orientation. A field-free enhanced orientation via antialignment scheme is presented, which combines a linearly polarized femtosecond laser pulse with a half-cycle pulse. The laser pulse induces transient antialignment in the plane orthogonal to the field polarization, while the half-cycle pulse leads to the orientation. We identify two qualitatively different enhancement mechanisms depending on the pulse order, and optimize their effects using classical and quantum models both at zero and nonzero temperature.
-
(2006) Physical Review A. 73, 6, 061401. Abstract
We show that field-free molecular orientation induced by a half-cycle pulse may be considerably enhanced by an additional laser pulse inducing molecular antialignment. Two qualitatively different enhancement mechanisms are identified, depending on the pulse order, and their effects are optimized with the help of quasiclassical as well as fully quantum models.
-
(2006) Frontiers in Optics, FiO 2006. Abstract
We demonstrate single shot retrieval of coherent molecular field-free evolution by geometric space-time mapping combined with non-linear signal imaging.
-
(2006) Physical Review A. 74, 4, 041403. Abstract
We experimentally demonstrate isotope-selective alignment in a mixture of N214, N215 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.
-
(2006) Physical Review A. 73, 6, 063402. Abstract
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.
2005
-
(2005) Physical Review A. 72, 2, 023417. Abstract
We studied the focusing of atoms by multiple layers of standing light waves in the context of atom lithography. In particular, atomic localization by a double-layer light mask is examined using the optimal squeezing approach. Operation of the focusing setup is analyzed both in the paraxial approximation and in the regime of nonlinear spatial squeezing for the thin-thin, as well as thin-thick, atom lens combinations. It is shown that the optimized double light mask may considerably reduce the imaging problems, improve the quality of focusing, and enhance the contrast ratio of the deposited structures.
-
(2005) Physical review letters. 94, 15, 153002. Abstract
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.
-
(2005) Ultrafast Phenomena XIV. Kobayashi T.(eds.). Vol. 79. p. 819-821 Abstract
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.
-
(2005) 2005 Quantum Electronics and Laser Science Conference (QELS). p. 30-32 Abstract
Following alignment by a strong femtosecond pulse, we observe multiple revivals of molecular rotational wavepackets in isotopic mixtures. Pronounced separation of isotopic signals and their interferences are demonstrated paving the way for a new isotope separation technique.
2004
-
(2004) Physical Review A. 69, 3, p. 032102-1-032102-17 032102. Abstract
The semiclassical catastrophes and cumulative angular squeezing of a kicked quantum rotor were investigated. Semiclassical methods were applied to the time-domain cusp, fold and glory phenomena exhibited by the rotors subjected to strong kicks. It was observed that the wave function of the rotor developed a cusp at certain delay after a kick that resulted in a sharply focused rotational wave packet. The subsequent spread of the focused rotational wave packet was found to demonstrate a wave front fold leading to the moving rainbow-type singularities in the angular distribution.
-
(2004) Physical Review A - Atomic, Molecular, and Optical Physics. 69, 1, p. 10 013402. Abstract
We study molecular alignment by trains of strong, ultrashort laser pulses. Single-pulse alignment is analyzed both in classical and quantum-mechanical regimes. Moreover, we suggest multipulse excitation schemes leading to dramatically enhanced molecular alignment, and define conditions for femtosecond experiments of this kind.
-
(2004) Nonlinear Optics. Abstract
Temporal shaping of femtosecond laser pulses is used for the optimization of laser material processing. We find that the shortest pulse is not always the best in terms of ablation efficiency and quality.
2002
-
(2002) Journal of Physics A: Mathematical and General. 35, 36, p. 7803-7816 Abstract
The time evolution of wave packets (WPs) built from the eigenstates of the Dirac equation for a hydrogenic system is considered. We investigate the space and spin motion of wave packets which, in the non-relativistic limit, are stationary states with a probability density distributed almost uniformly along the classical, elliptical orbit (elliptic WP). We show that the precession of such a WP, due to relativistic corrections to the energy eigenvalues, is strongly correlated with the spin motion. We also show that the motion is universal for all hydrogenic systems with an arbitrary value of the atomic number Z.
-
(2002) Physical Review A. 65, 5, Abstract
We study the process of squeezing of an ensemble of cold atoms in a pulsed optical lattice. The problem is treated both classically and quantum mechanically under various thermal conditions. We show that a dramatic compression of the atomic density near the minima of the optical potential can be achieved with a proper pulsing of the lattice. Several strategies leading to the enhanced atomic squeezing are suggested, compared, and optimized.
2001
-
(2001) Physical review letters. 87, 16, p. 163601/1-163601/4 163601. Abstract
Generic features in the dynamics of a quantum rotor (molecule) drive by strong pulses are analyzed. In addition, a strategy is given for efficient squeezing of the rotor angular distribution by a sequence of pulses of moderate intensity.
-
(2001) Physical review letters. 87, 16, p. 163601/1-163601/4 163601. Abstract
Generic features in the dynamics of a quantum rotor (molecule) drive by strong pulses are analyzed. In addition, a strategy is given for efficient squeezing of the rotor angular distribution by a sequence of pulses of moderate intensity.
-
(2001) Journal of Chemical Physics. 114, 22, p. 9901-9910 Abstract
Coherence observation by interference noise (COIN) was used to measure fractional wave packet motion of thermal iodine. COIN interferograms were presented for different excitation wavelengths to analyze fractional vibrational revivals. The simulation results showed that the complex temporal structure of the observed fluorescence included rapid initial damping in short-time regime. The observation of the wave packets on long time scale indicated a delicate balance between rotational and vibrational molecular coherences.
-
(2001) Physical Review B. 63, 4, p. art. no.-45420 Abstract
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.
-
(2001) Physical Review A. 63, 4, p. 1-9 043407. Abstract
We present an optimal control approach to the process of molecular isotope separation by exciting vibrational wave packets with femtosecond laser pulses. In the weak-field limit, we developed an optimization procedure for designing shaped laser pulses leading to the best selectivity in the two-photon ionization processes. Several control scenarios are identified, which mainly belong to two groups. The first takes advantage of the revival phenomenon, which allows one to find times at which excited wave packets of different isotopes are well localized and spatially separated in the intramolecular space. The second is based on isotopically selective quantum interference between several wave packets excited in the same molecular potential by a designed sequence of laser pulses. Simulations have been done for the isotopic mixture of 79Br2 and 81Br2 molecules, which was used in the first experiments on wave-packet isotope separation.
2000
-
(2000) Journal of Chemical Physics. 112, 11, p. 5060-5069 Abstract
The principle of coherence observation by interference noise [COIN, Kinrot et al., 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 τ prepare interference fluctuations in the excited state population, so the correlated noise of fluorescence intensity-the variance varF(τ)-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.
-
(2000) Optics Communications. 174, 1-4, p. 33-41 Abstract
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 tip, 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.
1999
-
(1999) Physical Review A. 59, 3, p. 2163-2173 2163. Abstract
We analyze and further develop our method of quantum-state holography for reconstructing quantum superposition states in molecules or atoms [Phys. Rev. Lett. 80, 1418 (1998)]. The technique is based on mixing the unknown object state with a known reference state generated in the same system by two delayed laser pulses, and detecting the total time- and frequency-integrated fluorescence as a function of the delay. The feasibility of the method is demonstrated by reconstructing various vibrational wave packets in sodium dimers. Both the cases of completely controlled and noisy relative phase between the laser pulses are considered. In the latter case, we use the technique of coherence observation by interference noise to recover the interference component of the fluorescence signal. Our results clearly demonstrate the robustness of quantum-state holography and the high quality of reconstruction even in the presence of the external noise.
1998
-
(1998) Journal of Chemical Physics. 108, 15, p. 6057-6067 Abstract
We apply the method of "coherence observation by interference noise" (COIN) to molecular wave packet interferometry. In contrast to conventional wave packet interferometry, where one has to control precisely the relative phase between the pair of time-delayed laser pulses, COIN employs a sequence of randomly phased laser pulses. Whereas in this case the mean signal does not reflect anymore the coherent evolution of a localized wave packet, the fluctuations around this mean still mimic the dynamics of the propagating wave packet. We demonstrate the feasibility of wave packet interferometry without phase-locking by simulating fluorescence interferograms for the Na2-molecule.
-
(1998) Physical review letters. 80, 7, p. 1418-1421 Abstract
We introduce a method for reconstructing quantum superposition states excited in molecules or atoms by short laser pulses. The technique is based on mixing the unknown object state with a known reference state generated in the same system by an additional delayed laser pulse, and detecting the total time- and frequency-integrated fluorescence as a function of the delay. We demonstrate the feasibility of the method by reconstructing various vibrational wave packets in sodium dimers.
1996
-
(1996) Journal of Applied Physics. 79, 6, p. 2839-2845 Abstract
An analytical approach to the design of rugate filters with a smooth amplitude modulation of the sine-wave index is developed. The approach is based on the uniform WKB solutions (asymptotic expansions) of the coupled-wave equations. A closed-form solution for the inverse problem (finding the refractive index profile for a given reflectance shape inside the stop band) is found.
-
(1996) Physical review letters. 77, 17, p. 3518-3521 Abstract
We introduce and demonstrate a general wave packet method for laser isotope separation. In this scheme, laser excited molecular (or atomic) wave packets of differing isotopes become spatially separated in the course of their long-time free evolution. In order to overcome the quantum spreading of wave packets, we make use of the phenomenon of revivals. We demonstrate experimentally, with mixed 79Br2/81Br2 isotopes, that significant control over the isotope ratio can be achieved in a single shot.
-
(1996) Physical Review A. 54, 6, p. 5299-5312 Abstract
We study the temporal behavior of generic transient signals originating from multilevel quantum systems. Such signals typically arise in the physics of wave packets in atoms, molecules, cavity QED, and ion traps and consist of a sum of a large number of harmonics whose frequencies depend nonlinearly on the sequential number of the harmonic. In particular, we focus on the semiclassical limit. Here, quantum beats between individual terms in the underlying sum lead to characteristic features of the signal in different time regimes, such as collapse, fractional revivals, and full revivals. We present a universal recipe for describing analytically all of the details of these features. Our approach is based on a specific representation of the sum of harmonics, which is most convenient in each of these time regions of interest. This brings out in a most natural way the phenomenon of fractional revivals and full revivals and explains their fine structures observed in recent experiments.
1995
-
(1995) Physical Review Letters. 75, 21, p. 3822-3825 Abstract
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.
1994
-
(1994) Physical Review A. 50, 6, p. 5301-5308 Abstract
A general analysis of an oscillator with periodic frequency modulation is given. It is shown that squeezing and excitation energy exponentially grow with the number of modulation cycles under the condition of parametric instability. This condition yields a prescription for maximized squeezing when the frequency is periodically swept in an adiabatic fashion, with an abrupt return to the initial frequency at the end of each period. The type of modulation considered is shown to have remarkably broad instability domains near arbitrarily high ratios of the oscillator period to the modulation cycle duration. This property stands in striking contrast to the rapid narrowing of the squeezing domains with the ratio of the pump frequency to that of the signal in existing parametric processes. We discuss a possible realization of the proposed scheme, based on frequency modulation of a cavity mode in the microwave domain by a periodic train of optical pulses, and show that extremely strong squeezing is feasible under rather moderate requirements.
-
(1994) Physical review letters. 72, 4, p. 437-441 Abstract
We consider the focusing and deflection of a nonresonant atomic beam propagating through a spatially inhomogeneous quantized electromagnetic field. Different Fock states of the field deflect the atoms in different angles and focus them at different points. We find a regime in which individual foci corresponding to neighboring Fock states can be reoslved even for large average number of photons. In this sense this quantum lens for atomic waves leaves intact the discreteness of the photons even in the classical limit.
-
(1994) The Journal of chemical physics. 101, 11, p. 9295-9302 Abstract
We present an application of "optimal squeezing" theory to the design of laser pulses for generation of squeezed states of material waves (states whose localization in some variable exceeds that of the ground state) in Na2. Spatiotemporal evolution of the squeezed states during and after the laser pulse is studied. We show that the optimized laser pulses can affect squeezing via three basic scenarios whose realizations depend on the desired position of the wave packet and target squeezing times. These scenarios are alternations between momentum-space and coordinate-space squeezing, interfering collisions between wave packets, and overtaking of a slow front by a fast tail.
1993
-
(1993) Physical Review A. 47, 6, p. 5086-5092 Abstract
We present a practical method of designing laser fields for generation of spatially squeezed molecular wave packets. Our approach, based on optimal control theory, generalizes and improves previous, more intuitive suggestions for the optical generation of squeezed wave packets. Spatial-temporal evolution of the squeezing is studied and several underlying physical mechanisms of the effect are explored. Possible applications to laser femtosecond chemistry and experiments on laser modification of molecular adiabatic potentials are discussed.
1992
-
(1992) Physical Review A. 46, 5, p. R2205-R2208 Abstract
It is shown that the phenomenon of fractional revivals, which was observed in recent laser experiments on atomic wave packets, determines the long-time behavior of the Jaynes-Cummings model for a two-level atom interacting with a quantized cavity field. This provides an alternative mechanism for the generation of coherent superpositions of macroscopically distinguishable states of the field (so-called optical Schrödinger cats) via resonant processes. The emergence of these states is associated with the appearance of atomic inversion revivals following each other 2, 3, 4,... times faster than the usual ones.