Dynamics of the fields of counterpropagating light pulses of a few oscillations in nonlinear dielectric media

Evolution equations have been derived for the fields of counterpropagating light pulses of a few oscillations interacting with dielectric media with nonresonant dispersion and inertialess electron nonlinearity. Changes in the field structure and spectrum of a low-intensity pulse colliding with a high-intensity counterpropagating light pulse are illustrated. It has been shown that such a collision can be accompanied by the generation of radiation at multiple frequencies with a duration smaller than the durations of the initial pulses.     

Nonlinear propagation of optical pulses of a few oscillations duration in dielectric media

Using a semi-phenomenological model of the polarization response of an isotropic solid dielectric that does not resort to the slowly-varying-envelope approximation, we have obtained a nonlinear wave equation for the electric field of a femtosecond light pulse propagating in the given dielectric. Evidence is presented that this equation possesses breatherlike solutions in the region of anomalous group dispersion and does not have any solutions in the form of steady-state traveling solitary video pulses. A universal relation is found linking the minimum possible duration of a breatherlike pulse with the medium parameters. It is shown that such a pulse contains roughly one and a half periods of the light-wave. Zh. éksp. Teor. Fiz. 111, 404–418 (February 1997)     

Optimized control of generation of few cycle pulses by molecular modulation

Production of subfemtosecond optical pulses or pulses with predetermined sub-cycle shape of electric field demands a broadband coherent light source of few octaves of bandwidth. Previous work has shown that such a broadband light source can be obtained by the molecular modulation technique. In this article, we review theoretical and experimental improvements in this area: from increasing the efficiency of the generation process by use of hollow waveguides to increasing the number of sidebands generated by the Raman additive technique, or by combined vibrational and rotational Raman generation. We find that stimulated rotational Raman scattering can be either enhanced or suppressed at proper detunings from vibrational Raman resonance in the same molecular ensemble.     

Quasi-phase matching of high-order harmonic generation at high photon energies using counterpropagating pulses

We extend all-optical quasi-phase matching of high-order harmonic generation into spectral regions where conventional phase matching is not possible. The high laser intensities required to generate harmonics at energy >130 eV, coupled with the resulting high level of ionization, preclude conventional phase matching in all nonlinear media. Selective enhancement factors between 40 and 150 in the flux of harmonics at photon energies around 140 eV are demonstrated using a train of two counterpropagating pulses.     

Intense nonlinear magnetic dipole radiation at optical frequencies: molecular scattering in a dielectric liquid

We report white-light generation and intense linear scattering from magnetic dipoles established by the time-varying magnetic flux of an incident light field in a dielectric medium. Large magnetic response is very unexpected at optical frequencies, and should lead to the discovery of new magneto-optical phenomena and the realization of low-loss homogeneous optical media with negative refractive indices.     

Characterization of femtosecond pulses via transverse second-harmonic generation in random nonlinear media

We study, both experimentally and theoretically, the process of second-harmonic generation by two noncollinear beams in ferroelectric crystals with a disordered distribution of ferroelectric domains. We show that this parametric process results in generation of second-harmonic wave in the direction transverse to the propagation of the fundamental beam. We demonstrate how this effect can be used for the femtosecond pulse characterization enabling the estimation of both width and chirp of the pulse.     

Electromagnetic shocks on the optical cycle of ultrashort pulses in triple-resonance Lorentz dielectric media with subfemtosecond nonlinear electronic Debye relaxation

The dynamical evolution of an intense ultrashort sub-10-fs two-cycle optical pulse is considered as it propagates through a transparent third-order dielectric medium characterized by three resonance lines and a finite sub-fs relaxation time of the electronic nonlinearity. Numerical integration of the full Maxwell's equations incorporating triple-resonance Lorentz linear dispersion and Debye nonlinear dispersion, for a linearly polarized electromagnetic pulse centered at lambda(0)=1.24 microm in the normal dispersion region near the zero dispersion wavelength, shows the formation of shocks occurring on the optical cycle due to the generation of optical harmonics. The finite relaxation time of the nonlinear electronic response (sub-fs time scale) (i) slows down the steepening rate of the optical cycle; (ii) does not limit the generation of strongly phase matched optical harmonics, and consequently the development of infinitely sharp edges on the optical cycle producing its breaking when linear dispersion is not included; (iii) reduces the production of phase matched harmonics and consequently the sharpening of the jumps when dispersion is present, compared to the case of an instantaneous nonlinear response; and (iv) reduces the harmonic spectrum spreading and modulation at later times on the appearance of self-steepening of the electric field envelope.     

Plasmonic generation of sound by counterpropagating light beams in semiconductors

The possibility of generating longitudinal sound waves on a deformation potential in a semiconductor medium has been investigated. This generation is due to the excitation of internal longitudinal electric field in the plasma electron subsystem of semiconductor by external counterpropagating light beams.     

Optical harmonic generation and laser light frequency mixing processes in nonlinear media

The essential results achieved during the ten years since the coming of lasers, in the fields of second, third and higher harmonic generation in reflected and transmitted, continuous and pulsed laser light, frequency mixing processes in various materials, and nonlinear optical properties of liquids, gases, crystals and other materials are reported concisely from the available literature. Stress is laid on the newer and more stimulating experimental results, and on their relation with other nonlinear phenomena.Invited review article given on 6 April 1970, at the IVth Polish Conference on Radiospectroscopy and Quantum Electronics in Pozna. This review covers only such papers as were directly accessible to the author.     

Interaction of counterpropagating discrete solitons in a nonlinear one-dimensional waveguide array

We experimentally investigate the interaction of counterpropagating discrete solitons in a one-dimensional waveguide array in photorefractive lithium niobate. While for low input powers only weak interaction and formation of counterpropagating vector solitons are observed, for higher input powers a growing instability results in discrete lateral shifting of the formed discrete solitons. Numerical modeling shows the existence of three different regimes: stable propagation of vector solitons at low power, instability for intermediate power levels leading to discrete shifting of the two discrete solitons, and an irregular temporal dynamic behavior of the two beams for high input power.     

Generation of unipolar pulses in nonlinear media

Methods recently proposed for generating unipolar pulses in nonlinear media in terahertz and optical electromagnetic ranges are reviewed. Such pulses have nonzero “electric area” (time integral of the field strength over the entire duration of a pulse) and, correspondingly, a significant component of the field with zero frequency, thus exhibiting quasistatic properties. Effective generation of unipolar pulses would allow, e.g., transferring mechanical momentum to charged particles and, thereby, controlling the motion of wave packets of matter, which can be useful for compact accelerators of charged particles and for other applications.     

Role of nonlinear refraction in the generation of terahertz field pulses by light fields

The generation of microwave (terahertz) pulses without any envelope in a four-level quasi-resonant medium is considered. Two intense quasi-monochromatic laser fields lead to a partial upper-level population. Microwave field pulses cause the transition between these levels. For appropriately chosen scales, the evolution of the fields is shown to be described by the pseudo-spin evolution equations in a microwave field with the inclusion of nonlinear refraction caused by an adiabatic upper-level population. The evolution of terahertz field pulses is described outside the scope of the slow-envelope approximation. When a number of standard approximations are taken into account, this system of equations is shown to be equivalent to an integrable version of the generalized reduced Maxwell-Bloch equations or to the generalized three-wave mixing equations. The soliton solution found by the inverse scattering transform method is used as an example to show that nonlinear refraction leads to a strong compression of the microwave (terahertz) field soliton.     

Experimental study on generation of high energy few cycle pulses with hollow fiber filled with neon

25 fs pulses with energy up to 0.8 mJ from a multi-pass amplifier system have been spectrally broadened from 460 nm to 950 nm due to strong self-phase modulation (SPM) effect in a gas filled hollow fiber. Using a set of chirped mirrors, the ultra-broadband dispersion compensation was achieved, and the compressed pulses reached their transform limit. Under optimized conditions we achieved pulses with duration of 5.1 fs and with energy of 400 μJ, corresponding to the peak power up to 80 GW. Supported by the National Natural Science Foundation of China (Grant Nos. 60608003, 60490280, 60225005 and 60621063)     

Parametric generation of high-energy 14.5-fs light pulses at 1.5 mum

High-energy light pulses that are tunable from 1.1 to 2.6 mum, with a duration as short as 14.5 fs were generated in a type II phase-matching beta-BaB(2)O(4) traveling-wave parametric converter pumped by 18-fs pulses obtained from a Ti:sapphire laser with chirped-pulse amplification, followed by a hollow-fiber compressor.     

Analysis of generation of terahertz radiation by nonlinear mixing of laser frequencies in GaAs crystal

Generation of terahertz (THz) radiation by nonlinear mixing of laser frequencies in GaAs crystal is considered for the case of free and guided propagation of THz waves. In the first case the strong diffraction of the THz radiation leads to the deviation from the known square-law growth of the generator power with increasing crystal length, In the second case the spatial divergence of the exciting laser beam results in the appearance of a maximum in the generated power dependence on the radius of the nonlinear waveguide. According to estimations, the optimal radius of the laser beam is 18 μm for the waveguide length of 6 mm and the maximal generated THz power is ～27 W at the laser beam power of 10 kW.     

Probe of high-order harmonic generation in a hollow waveguide geometry using counterpropagating light

We use counterpropagating light to directly observe the coherent buildup of high harmonic generation in a hollow waveguide geometry. We measure, for the first time, coherence lengths for high photon energies that cannot be phase matched using conventional approaches. We also probe the transition through phase matching, the ionization level at which different harmonic orders are generated, and the change in the coherence length as the driving laser is depleted. These results directly prescribe the optimal structures or pulse trains required for implementing quasiphase matching.     

Asymmetry of the scattering amplitude of atoms in the field of short counterpropagating light pulses

Rectification in bipotential scattering of a beam of atoms in the field of short pulses of traveling and standing waves is studied: As a result of the coherence induced by the traveling-wave pulse, the momentum transferred to the atomic beam during scattering by the standing wave is nonzero. The magnitude and sign of the asymmetry in the scattering amplitude are oscillatory functions of the duration of the traveling-wave pulse and the detuning of the frequency of the field from atomic resonance. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 12, 920–923 (25 June 1996)     

Frequency comb generation using femtosecond pulses and cross-phase modulation in optical fiber at arbitrary center frequencies

A technique is presented for generating optical frequency combs centered at arbitrary wavelengths by use of cross-phase modulation (XPM) between a femtosecond pulse train and a cw laser beam by copropagating these signals through an optical fiber. We report results from use of this method to place a 90-MHz frequency comb on an iodine-stabilized Nd:YAG laser at 1064 nm and on a frequency-doubled Nd:YVO(4) laser at 532 nm. XPM is verified to be the comb-generating process, and the width of the frequency comb is measured and compared with theory. The spacing of the frequency comb is compared with the femtosecond source, and a frequency measurement with this comb is demonstrated.