Spectra induced by interactions of atoms with ultrashort electromagnetic pulses

An ultrashort pulse of an electromagnetic field incident on an atom shakes the atom and gives rise to various electron transitions in it. These processes are accompanied by the reemission of the incident ultrashort pulse. This paper studies the relation between the spectra of reemitted photons of an ultrashort pulse and the transitions of atomic electrons into particular states. The obtained partial reemission spectra can allow one to relate direction patterns to the probability of the excitation of an atom into different states.     

Self-action dynamics of ultrashort electromagnetic pulses

The self-action dynamics of three-dimensional wave packets whose width is on the order of the carrier frequency is studied under fairly general assumptions concerning the dispersion properties of the medium. The condition for the wave field collapse is determined. Self-action regimes in a dispersion-free medium and in media with predominance of anomalous or normal group velocity dispersions are numerically investigated. It is shown that, for extremely short pulses, nonlinearity leads not only to the self-compression of the wave field but also to a “turn-over” of the longitudinal profile. In a dispersionless medium, the formation of a shock front within the pulse leads to the nonlinear dissipation of linearly polarized radiation and to self-focusing stabilization. For circularly polarized radiation, the wave collapse is accompanied by the formation of an envelope shock wave.     

Self-action dynamics of ultrashort electromagnetic pulses

The equation generalizing the nonlinear Schrödinger equation to the case of pulses with a duration of few field oscillation periods is analyzed. A change in the effective parameters (centroid, duration, and width) of the wave field on the pulse propagation path are determined by the moments method. The collapse of spatial structure is shown to occur, and its formation associated with the steepening of the pulse leading edge are numerically studied.     

Spatiotemporal structure of isodiffracting ultrashort electromagnetic pulses

We present a model of isodiffracting single-cycle and few-cycle ultrashort electromagnetic pulses. The model is based on exact solutions of the time-dependent paraxial wave equation with space-time coupling effects included. The spatiotemporal structure of these pulses is characterized by a scaling parameter which relates off-axis pulse shapes to the axial temporal waveforms. Depending on the spectrum a pulse may transform itself from a single-cycle pulse to a multicycle pulse along the radial coordinate. This model is also used to describe recirculating pulses in a curved mirror cavity resonator. The Gouy phase shift contributes an absolute phase that results in a pulse-to-pulse temporal instability.     

Interference effects during the reradiation of ultrashort electromagnetic pulses by polyatomic systems

A theory of the reradiation of ultrashort electromagnetic pulses by arbitrary polyatomic systems of isolated complex atoms has been developed. The technique used allows the spatial inhomogeneity of the field of an ultrashort pulse and photon momenta in reradiation processes to be accurately taken into account. The angular distributions of the reradiation spectra have been obtained for an arbitrary number of atoms in the system. The processes of interference between the photon emission amplitudes are shown to give rise to characteristic “diffraction” maxima. We consider one-dimensional, two-dimensional, and three-dimensional rectangular lattices as examples as well as planar and cylindrical structures as models of planar nanosystems and nanotubes.     

Scattering of ultrashort electromagnetic pulses on metal clusters

We have calculated and analyzed the probability of ultrashort electromagnetic pulse (USP) scattering on small metal clusters in the frequency range of plasmon resonances during the field action. The main attention is devoted to dependence of the probability of scattering on the pulse duration for various detunings of the USP carrier frequency from the plasmon resonance frequency. Peculiarities of the USP scattering from plasmon resonances with various figures of merit are revealed.     

Structural features of the self-action dynamics of ultrashort electromagnetic pulses

Self-focusing dynamics of electromagnetic pulses of arbitrary duration is analyzed numerically and analytically. The wave-field evolution is considered by the wave equation in the reflectionless approximation under quite general assumptions about the dispersion of the medium. Methods for qualitative investigation of the self-focusing dynamics of quasimonochromatic radiation are generalized to the case of wave packets with the length of a few oscillation periods. In particular, sufficient conditions for collapse and many other integral relations are obtained by the momentum method. A self-similar-type transformation is used to show that new structural features are primarily associated with the nonlinear dispersion of the medium (with the dependence of the group velocity of a wave packet on its amplitude). Numerical analysis confirms that the self-focusing of radiation is preceded by an increase in the steepness of the longitudinal profile.     

Study of correlation effects during reemission of ultrashort electromagnetic pulses by the helium atom

The calculation of the energy of the ground state of atoms is the criterion for the correctness of compact analytical atomic wave functions. However, such a test is in fact static. We studied the degree of taking into account electron correlations in various simple analytical wave functions in the dynamic process of reemission by the helium atom of ultrashort pulses of an electromagnetic field. Direct guidelines are given on the use of particular analytical wave functions, which can be helpful in simple calculations and estimations of dynamic processes.     

Formation of subwavelength periodic structures on tungsten induced by ultrashort laser pulses

The evolution of surface morphology of tungsten irradiated by single-beam femtosecond laser pulses is investigated. Ripplelike periodic structures have been observed. The period of these ripples does not show a simple relation to the wavelength and angle of incidence. The orientation of ripples is aligned perpendicularly to the direction of polarization for linearly polarized light. Surprisingly, we find that the alignment of the ripple structure turned left or right by 45 degrees with respect to the incident plane when using right and left circularly polarized light, respectively. The period of the ripple can be controlled by the pulse energy, the number of pulses, and the incident angle. We find a clear threshold for the formation as a function of pulse energy and number of pulses. The mechanism for the ripple formation is discussed, as well as potential applications in large-area structuring of metals.     

Generation of ultrashort pulses of electromagnetic radiation in the mid- and far-infrared ranges

Generation of ultrashort pulses of electromagnetic radiation in the range of wavelengths from 3 to 17 μm is experimentally demonstrated using difference-frequency mixing of the signal and idler waves delivered through an optical parametric amplification of a week broadband seed by high-power near-infrared ultrashort laser pulses. Cross-correlation measurements demonstrate that pulses as short as a few cycles of electromagnetic field are produced in the mid- and far-infrared ranges by means of difference-frequency generation.     

Multiphoton electron emission processes induced by different kinds of ultrashort laser pulses

Using ultrashort laser pulses of regular (gaussian shaped) time and spectral distribution, theoretically predicted, multiphoton electron emission processes have been observed. On changing this regular form of the ultrashort light pulses, however, the character of the emission process also changes, the explanation of which follows only partly from the evolution of the mode-locking train.     

Nonequilibrium phase transition in v-group semimetals,induced by ultrashort laser pulses

Coherent, totally symmetric large-amplitude phonons in bismuth and antimony were investigated by the pump—probe method using femtosecond laser pulses. The obtained results are compared to time-resolved Raman data. It is shown that intense photoexcitation by laser pulses with a duration shorter than the lifetime and reverse phonon frequency in Bi and Sb can lead to a nonequilibrium semimetal—metal phase transition, most likely caused by the instability of the crystal’s electron subsystem.     

Inelastic processes and interference effects during the interaction of positronium with ultrashort electromagnetic pulses

The excitation, breakup, and reradiation during the interaction of a positronium atom with ultrashort electromagnetic pulses are considered. The probabilities of inelastic processes and reradiation spectra have been obtained. The interference between the amplitudes of the photon emission by the electron and positron is shown to contribute noticeably to the reradiation spectra. The developed approach is applicable for describing the interaction of positronium with ultrashort pulses of attosecond or shorter duration.     

Influence of optical field emission on the nonlinear photoelectric effect induced by ultrashort laser pulses

A decrease in the order of nonlinearity of photoelectron emission from a metal cathode was found experimentally with mode-locked laser pulses at sufficiently high intensities. This phenomenon is interpreted as due to the theoretically predicted appearance of optical field emission.     

Reemission spectra and interference effects upon interaction of ultrashort electromagnetic pulses with nanosystems

The processes of reemission of ultrashort electromagnetic pulses by arbitrary nanosystems consisted of isolated complex atoms have been considered. The angular distributions of reemission spectra have been obtained for a series of regular nanosystems. It has been shown that the interference of the photon-emission amplitudes leads to the appearance of characteristic “diffraction” maxima. One-, two-, and three-dimensional nanostructures, as well as planar and cylindrical constructions as models of planar nanosystems and nanotubes, were used as examples that allow a simple analytical consideration.     

High quantum yield of photochemical reactions of protoporphyrin-IX induced by powerful ultrashort laser pulses

The action of powerful pulsed picosecond radiation from a Nd: YAG laser (λ=530 nm, pulse energy: 0.01 J, intensity: 2GW/cm²) and an argon laser (λ=515 nm, power: 50 mW) on protoporphyrin-IX dimethylether in three solvents (trichlormethane, carbon tetrachloride, dioxane) has been studied. Under continuous irradiation the quantum yield and resulting products do not differ materially from the ones produced under mercury lamp irradiation. When irradiation is performed by powerful laser pulses of picosecond duration the quantum yield of photodecomposition of protoporphyrin-IX dimethylether inereases substantially: by 10 in dioxane, by 4 in carbon tetrachloride and by 100 in trichlormethane. It is assumed that a quite different mechanism of multistep excitation is responsible for photodecomposition under powerful picosecond pulses.     

激光脉冲诱导的等离子体密度调制及其产生的相位反射

理论研究和数值模拟发现入射光和反射光在低密度等离子体中形成的干涉场可以产生深度的等离子体密度调制. 对于中等强度的入射光，譬如10¹⁵W／cm² ，产生密度调制的时间尺度在几十个光周期的范围. 这样的等离子体密度调制可以起类似布拉格反射镜的 作用，使得后面的入射光在临界面以下的区域产生相位反射. 因为密度调制的周期是光在等 离子体中波长的一半，其产生的反射率可以接近100％. 相位反射也可以在不均匀的低密度 等离子体中产生，它可以极大地减少等离子体对光的吸收，因此在惯性约束核聚变中需要考 虑到它的影响. 关键词： 相位反射 密度调制 激光等离子体 粒子模拟