Publications

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.

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.

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

We 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.

We report measurements of the optical transmission through a plasmonic flat surface interferometer. The transmission spectrum shows Fabry-Perot-like modes, where for each mode order, the maximal transmission occurs at a gap that grows linearly with wavelength, giving the appearance of diagonal dependence on gap and wavelength. The experimental results are supported by numerical solutions of the wave equations and by a simplified theoretical model that is based on the coupling between localized and propagating surface plasmon. This work explains not only the appearance of the modes but also their sharp dependence on the gap, taking into consideration the refractive indices of the surrounding media. The transmission spectra provide information about the phase difference between the light impinging on the two cavities, enabling interferometric measurement of the light phase by transmission through the coupled plasmonic cavities. The 1° phase-difference resolution is obtained without any propagation distance, thus making this interferometer suitable for on-chip operation.

We demonstrate composite, multiplexed 3D metamaterials for functional light manipulation. Applications include multi-wavelength achromatic metalenses in the visible spectral range, integrated elements for STED microscopy, and nonlinear holography. Prospects for novel applications are discussed.

Nanostructured metasurfaces offer unique capabilities for subwavelength control of optical waves. Based on this potential, a large number of metasurfaces have been proposed recently as alternatives to standard optical elements. In most cases, however, these elements suffer from large chromatic aberrations, thus limiting their usefulness for multiwavelength or broadband applications. Here, in order to alleviate the chromatic aberrations of individual diffractive elements, we introduce dense vertical stacking of independent metasurfaces, where each layer is made from a different material, and is optimally designed for a different spectral band. Using this approach, we demonstrate a triply red, green and blue achromatic metalens in the visible range. We further demonstrate functional beam shaping by a self-aligned integrated element for stimulated emission depletion microscopy and a lens that provides anomalous dispersive focusing. These demonstrations lead the way to the realization of ultra-thin superachromatic optical elements showing multiple functionalities- all in a single nanostructured ultra-thin element.

Plasmonic wakes are observed in a linear array of nanocavities in a gold film on glass substrate. The wakes are generated by the different propagation velocity of surface plasmons on the two sides of the film.

A hologram is an optical element storing phase and possibly amplitude information enabling the reconstruction of a three-dimensional image of an object by illumination and scattering of a coherent beam of light, and the image is generated at the same wavelength as the input laser beam. In recent years, it was shown that information can be stored in nanometric antennas giving rise to ultrathin components. Here we demonstrate nonlinear multilayer metamaterial holograms. A background free image is formed at a new frequency - the third harmonic of the illuminating beam. Using e-beam lithography of multilayer plasmonic nanoantennas, we fabricate polarization-sensitive nonlinear elements such as blazed gratings, lenses and other computer-generated holograms. These holograms are analysed and prospects for future device applications are discussed.

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.

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.

The laser-induced deformation of a typical commercial cantilever commonly used for scanning near-field optical microscopes was investigated by means of a software package based on the finite element method. The thermo-mechanical behaviour of such a cantilever whose tip was irradiated by a laser beam was calculated in the temperature regime between room temperature and 850K. The spatial tip displacement was simulated at timescales

We consider the optical properties of a gas of molecules that are brought to fast unidirectional spinning by a pulsed laser field. It is shown that a circularly polarized probe light passing through the medium inverts its polarization handedness and experiences a frequency shift controllable by the sense and the rate of molecular rotation. Our analysis is supported by two recent experiments on the laser-induced rotational Doppler effect in molecular gases and provides a good qualitative and quantitative description of the experimental observations.

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.

Localized plasmonic modes of metallic nanoparticles may hybridize like atomic orbitals forming a molecule. However, the rapid spatial decay of plasmonic fields outside the metal severely limits the range of these interactions to tens of nanometers. Herein, we demonstrate a strong coupling scheme between nanocavities carved in the same Silver metal films that is sustained by propagating surface plasmons within a hundreds of nanometers interval scale for a properly selected metal/wavelength combination. The nanotructures are patterned in Silver films by Focused Ion beam (FIB) with typical sizes in the 100 nm in all directions, also allowing to control the shape of the contours in different geometries [1]. Strong coupling drastically changes the symmetry of the charge distribution around the nanocavities, qualifying the highly symmetry-sensitive quadratic nonlinear optical response of the medium as a relevant probe [2,3]. We show by means of polarization resolved second-harmonic generation in a confocal microscope configuration that strongly coupled equilateral triangular nanocavities lose their individual three-fold symmetry to adopt the lower symmetry of the coupled system (see Figure).

We demonstrate strong coupling between molecular excited states and surface plasmon modes of a slit array in a thin metal film. The coupling manifests itself as an anticrossing behavior of the two newly formed polaritons. As the coupling strength grows, a new mode emerges, which is attributed to long-range molecular interactions mediated by the plasmonic field. The new, molecular-like mode repels the polariton states, and leads to an opening of energy gaps both below and above the asymptotic free molecule energy.

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.

We demonstrate a new approach to Four Wave Mixing spectroscopy involving simultaneous measurements at time and frequency domains, where spectral selectivity is achieved by phase matching filtering, and the time resolution is obtained within a single ultra-short pulse. We analyze the Four Wave Mixing signal, and show that our method is capable for discrimination between different spectroscopic pathways of vibrational coherences modulating the scattered signal.

A pulsed IR laser beam was used to align a peptide at the air-water interface. This peptide was designed to form a cyclic ß-strand dimer through Glu-Lys interactions in solution, which, when spread onto water, yielded a self-assembled ß-sheet bilayer (see picture) following solvent evaporation. During this process illumination with linearly polarized laser light induced formation of an aligned crystalline film, whereas circular polarization did not. (Figure Presented).

We present a new approach to nonresonant laser deceleration and cooling of atoms based on their interaction with a bistable optical cavity. The cooling mechanism presents a photonic version of Sisyphus cooling, in which the conservative motion of atoms is interrupted by sudden transitions between two stable states of the cavity mode. The mechanical energy is extracted due to the hysteretic nature of those transitions. The bistable character of the cavity may be achieved by an external feedback loop, or by means of nonlinear intracavity optical elements. In contrast to the conventional cavity cooling, in which atoms experience a viscoustype force, bistable cavity cooling imitates "dry friction" and stops atoms much faster. Based on this novel approach, we explore the prospects of using optical bistability for efficient radiation pressure cooling of micromechanical devices that are modeled as a Fabry-Perot resonator with one fixed and one oscillating mirror. In all cases, analytical results are presented, supported by realistic numerical examples.

By varying the polarization and delay between two ultrashort laser pulses, we control the plane, speed, and sense ofmolecular rotation. This control may be implemented to individual components within a molecular mixture.

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.

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.

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.

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.

We demonstrate single shot retrieval of coherent molecular field-free evolution by geometric space-time mapping combined with non-linear signal imaging. The method was experimentally tested to yield accurate spectrum of CHBr3 and CHCl3 molecules.

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.

We demonstrate a novel experimental approach to heterodyne detection where the local oscillator field is derived from the alignment of polarizable anisotropic molecules added in small amounts to the measured sample.

We demonstrate single-pulse polarization sensitive measurement of coherent vibrational dynamics of molecules by geometrical space-time mapping combined with non-linear signal imaging.

It is well accepted by now that nanoparticles of inorganic layered compounds form closed-cage structures (IF). In particular closed-cage nanoparticles of metal dihalides, like NiCl₂, CdCl₂ and CdI₂ were shown to produce such structures in the past. In the present report IF-NiBr₂ polyhedra and quasi-spherical structures were obtained by the evaporation/recrystallization technique as well as by laser ablation. When the nanoclusters were formed in humid atmosphere, nickel perbromate hydrate [Ni(BrO₄)₂(H₂O)₆] polyhedra and short tubules were produced, as a result of a reaction with water. Nanooctahedra of NiBr₂ were found occasionally in the irradiated soot. The reoccurrence of this structure in the IF family suggests that it is a generic one. Consistent with previous observations, this study showed that formation of the IF materials stabilized the material under the electron-beam irradiation. The growth mechanism of these nanostructures is briefly discussed.

Selective alignment by femtosecond pulses of molecules in multi-component mixtures is shown to be a powerful tool for the detection, identification, and separation of chemically close species.

We explore the prospects of optical shaking, a recently suggested generic approach to laser cooling of neutral atoms and molecules. Optical shaking combines elements of Sisyphus cooling and of stochastic cooling techniques and is based on feedback-controlled interaction of particles with strong nonresonant laser fields. The feedback loop guarantees a monotonous energy decrease without a loss of particles. We discuss two types of feedback algorithms and provide an analytical estimation of their cooling rate. We study the robustness of optical shaking against noise and establish minimal stability requirements for the lasers. The analytical predictions are in a good agreement with the results of detailed numerical simulations.

We propose a novel generic approach to laser cooling based on the nonresonant interactions of atoms and molecules with optical standing waves experiencing sudden phase jumps. The technique, termed "optical shaking," combines the elements of stochastic cooling and Sisyphus cooling. An optical signal that measures the instantaneous force applied by the standing wave on the ensemble of particles is used as feedback to determine the phase jumps. This guarantees a drift towards lower energies and higher phase-space density without the loss of particles typical of evaporative cooling.

Femtosecond laser ablation occurs on timescales faster than the thermalization of the excited electrons and the lattice in solid materials. The ultrafast deposition of energy competes with the slower electron-phonon energy redistribution, raising the question of what is the optimal pulse duration for efficient deposition of energy while minimizing peripheral damage, and whether the shortest pulse is always the most efficient. We studied femtosecond laser ablation of silicon and several metals, varied the pulse duration while keeping all other parameters equal, and looked for optimal conditions. The main findings in our study are that at low fluences, not too high above the ablation threshold, the shortest pulses are the most efficient, whereas under high fluence conditions, well above the ablation threshold, longer pulses ablate more efficiently. In order to facilitate eventual direct, real time optimization, we developed a diagnostics tool for the monitoring of the ablation efficiency over a wide range of pulse durations. The intensity of the emission at atomic lines (i.e. the 289 nm line in Silicon, calibrated by plasma emission at other wavelengths) provides such information, while optical and AFM microscopy provide reliable information about the quality of ablated structures.

A new method of laser-induced lithography for direct writing of carbon on a glass surface is described, in which deposition occurs from a transparent precursor solution. At the glass-solution interface where the laser spot is focused, a micro-explosion process takes place, leading to the deposition of pure carbon on the glass surface. Transmission electron microscopy (TEM) analysis shows two distinct co-existing phases. The dominant one shows a mottled morphology with diffraction typical of cubic (sp³) diamond. The other region shows an ordered array of graphene sheets with diffraction pattern typical of sp²-bonded carbon. The sp³ crystallites range in size from 9 to 30 Å and are scattered randomly throughout the sample. A UV Raman spectrum shows a broad band at the location of the expected diamond peak, together with a peak corresponding to the graphite region. We conclude that the patterned carbon is composed of a mixture of nanocrystalline sp³ and sp² carbon forms.

The monitoring and preparation of vibrational wavepackets with high quantum numbers was investigated by femtosecond time-delayed coherent anti-Stokes Raman scattering (TD)(CARS). A method utilizing two separate time delays between the pulses for the preparation of vibrational wavepackets in bulk gas-phase molecular iodine was demonstrated. The method was also used to investigate interference effects in femtosecond four-wave-mixing signals generated by molecular wavepackets.

We investigate the interaction of two molecules or nanosized particles with a nearly resonant laser field under the tip of an apertureless near-held microscope. We show that interference of several scattering channels provides means for enhanced spatial resolution. The visibility of two separate nano objects is considered, and a natural definition emerges for the resolution of the apertureless microscope operating under conditions of nearly resonant illumination. The probe tip creates an additional coupling channel between the two molecules, and thus affects the energy transfer between them. We demonstrate that the tip can either enhance or suppress this transfer. Two models fur the tip geometry are considered: a simplified pointlike dipole, and a more realistic elongated spheroid. Quantitative results are obtained for the dependence on irradiation frequency and tip position for dielectric as well as metallic tips. In particular, specific results are obtained for a silver tip under conditions of plasmon resonance, and we show that under fully resonant conditions the tip may enhance the intermolecular energy transfer by nearly two orders of magnitude.

The principle of coherence observation by interference noise [COIN, Kinrot 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.

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.

The excitation of molecules into specific wavepackets in high vibrational states in the ground electronic states was experimentally demonstrated. Two dimensional time delay coherent anti strokes Raman scattering method (TD)²CARS is based on exciting the molecule from the equilibrium ground state to an electronic state strongly coupled to ground state with a femtosecond ultrashort laser pulse. The generated wavepacket is evolved to a state strongly coupled to the desired ground state vibrations and induce an electronic transition to the final wavepacket in the ground state. This method provided the information for the generated wavepackets.

We present a new procedure for pretreatment seeding by ultrasonic agitation of silicon substrates in diamond nano-powder suspensions to which HF and KOH were added X-ray photoelectron spectroscopy (XPS) was used to measure the surface coverage by diamond nuclei immediately after the pretreatment. Coverage percentages of 70, 40 and 55% were obtained for the HF, KOH and the original diamond slurry, respectively. The seeding density (S_D) was calculated from the known nano-particles size, determined independently from X-ray diffraction of the powder. For nano-particle size of ~6 nm, we obtain nominal seeding densities of the order ~10¹² cm^-2. The advantage of the high coverage was most evident for films deposited at low substrate temperature (570 °C). The potential of the new seeding procedure and the XPS characterization method are discussed.

The interaction of laser light of arbitrary polarization with systems of high angular momentum is considered. We show that elliptically polarized light creates an anisotropic spatial distribution of atomic and molecular angular momentum which is qualitatively different from the alignment and orientation induced by light of circular or linear polarization. Multilevel coherent population trapping within a manifold of ground-state magnetic sublevels results in a nonclassical behavior of a high-[Formula Presented] molecular rotor. The classical approximation for the angular momentum distribution is compared with the exact quantum calculations, and is shown to fail in cases of long interaction times and high intensities of the exciting light. In these limits, the quantum uncertainty defines the spatial width of the angular distribution. The applicability of the classical treatment is analyzed and found to be different in the cases of [Formula Presented] and [Formula Presented] transitions. A biaxial spatial orientation with two preferential axes of rotation is experimentally created in sodium atoms via coherent population trapping by elliptically polarized light. A method for producing an arbitrary orientation of atomic angular momentum by magnetic field assisted coherent population trapping is proposed.

A novel scheme of coherent apertureless near field microscopy based on resonant excitation of the probe-sample junction and straightforward detection of the intensity of the coherently scattered light in the far-field zone is presented. A system can achieve a sub-nanometer resolution and single molecule sensitivity. The most general situation is considered where the external laser efficiently excites both localized plasmons in the metallized probe and the resonant molecular transitions. The principles of the microscope operation are illustrated by modelling the apex of the probe tip by an elongated spheroidal nanoparticle of different composition. Two modes of operation of the microscope, weak coupling and strong coupling schemes, were studied.

The proof for the existence of elliptical dark states (EDS), the multilevel analog of the well-known three-level dark states is presented. An ensemble of multilevel atoms when prepared by elliptically polarized light, becomes transparent to probe light of the same polarization. The effect stems from laser-induced coherences between many ground-state magnetic sublevels that reflect the polarization of the light that created the dark state. The novelty and essential character of the dark states are elucidated by the experimental demonstration of their creation by incoherent light.

A method to overcome the problem of phase noise in absolute distance measurements is suggested. The Coherent Observation by Interference Noise method allows recovery of the white light fringe even in the presence of artificially added phase noise. The method can be extended to other synthetic wavelength distance measurement schemes. Further, a similar method can be used to overcome incoherent scattering which smears the spatial features of an image.

We show that four-wave mixing in optically dense media is very different than in an optically thin medium. In time-resolved degenerate four-wave mixing we experimentally observe negative time delay response, fast decay rates at short delay times, and broad shoulders at long delays. The observations are explained very well in terms of pulse propagation effects, and a theoretical framework is presented for the analysis of the nonlinear interaction. We treat both the incident and generated fields self-consistently, and solve for the interaction of short pulses with a resonantly absorbing medium. The implications for the extraction of relaxation rates from four-wave-mixing experiments involving ultrashort pulses in optically dense matter are discussed.

Highly excited rotational states in electronically excited bromine molecules are measured with full hyperfine resolution, by means of sub-Doppler polarization spectroscopy. The dependence of molecular parameters such as predissociation rates and hyperfine coupling constants on the rotational quantum number is extracted, and found to be proportional to the rotational energy J(J + 1), extending previous data to much higher rotational states. By fitting a large set of hyperfine resolved rotational spectra from one vibrational band to a single set of parameters, very small dependencies may be extracted. The predissociation rates measured here in the frequency domain are compared to previous measurements in the time domain, and found to be smaller by more than an order of magnitude. We show that extracting decay times from lines of unresolved hyperfine structure may be misleading, and attribute this discrepancy to dephasing due to interferences between hyperfine components.

In the recently developed generalized jump model (GJM) for laser phase fluctuations, the amplitude is constant and the phase is assumed to jump where the size of the pump is correlated to the previous jump. The model is essentially described by three parameters: the characteristic phase jump size B, the correlation parameter γ, and the weighted average of time intervals between the jumps τ_av. The GJM has been applied to the resonance fluorescence (ResFl) of a two-level system for the case of highly anticorrelated phase jumps (γ ≈ -1). For fully anticorrelated jumps (γ = -1) the generalized telegraph model (GTM) is an extension of the phase telegraph model (PTM) to the case of a continuous distribution of the phase jumps. Here, the field spectrum consists of a δ-function and a broad component. The ResFl spectrum was derived analytically and verified by independent computer simulations. The field was assumed to be strong enough to produce a symmetric spectrum with three well-resolved components. The line shapes are Lorentzian for small phase jumps or low intensities of the field in the PTM case and consist of two Lorentzian components otherwise, broadening with the increase in the intensity. In the general case the spectrum depends on a number of parameters.

A unified approach is presented for the treatment of stochastic fluctuations in four-wave-mixing processes, including uncorrelated and fully or partially cross-correlated input laser fields. The theory allows for phase as well as amplitude fluctuations and covers Markovian and non-Markovian (non-Lorentzian) input line shapes. The analogy between laser field fluctuations and molecular dephasing processes is discussed, and several types of fluctuation-induced resonances are considered. Uncorrelated input fields give rise to resonances similar to those described previously by Agarwal, while correlated input fields cause extra resonances of the type presented by us in earlier publications. Partial cross-correlations are treated here for the first time, with predictions for new effects: Oscillations in the intensity of the generated four-wave-mixing signal as a function of the degree of cross-correlation. The conditions for the experimental observation of these effects are discussed.

The problem of coherent four-wave mixing is treated quantum theoretically in the dressed-atom picture using a set of "angular-momentum-like" operators. The phases of the radiation fields are considered by including a space-dependent phase factor in the raising (lowering) field operators. All possible transition pathways are considered, and their contributions to the total transition probability are added coherently. It is shown that interferences between the different pathways play an essential role and that the phases of the radiation fields should be properly handled. The result of this calculation is a directional, time-dependent transition probability that automatically expresses the phase-matching requirements of a coherent four-wave mixing process.