Spectral dependence of the excitation of the “forbidden” second optical harmonic in an aqueous suspension of purple membranes by femtosecond laser pulses under electronic resonance conditions

The spectral dependence of the efficiency of generation of the forbidden (in the electric-dipole approximation) second harmonic (FSH) and its energy on the energy of the excitation pulses is investigated experimentally in a highly disperse suspension of purple membranes containing bacteriorhodopsin (BR) under excitation by femtosecond laser pulses into the single-photon absorption band of BR. The experimental data for the case of resonance excitation attest to an interference character of the interaction of optical nonlinearities of different orders in the process leading to the formation of the FSH signal. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 4, 251–255 (25 February 1998)     

Achromatic reconstruction of the wave front of femtosecond laser pulses

Wave-front reconstruction by geometric-optical reflection of the reconstructing radiation from interference surfaces of a structure recorded in the bulk of a medium by counterpropagating laser pulses is observed. The recording is done with the aid of a sapphire titanate laser. Pulse durations of 30–40 fs are used. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 5, 388–390 (10 March 1997)     

Influence of the Stark effect on multiphoton ionization of atoms when the dynamic polarizability depends strongly on the laser frequency

The influence of the Stark effect on multiphoton ionization of Ba atoms under conditions when the dynamic polarizability depends strongly on the frequency of the laser radiation is investigated. It is found that for some electric field strengths ε of the laser radiation this effect gives rise to resonance peaks in the Ba⁺ ion yield as a function of the laser radiation frequency at frequencies corresponding to single-photon transitions between the excited states. These frequencies can differ substantially from the frequencies corresponding to the conventional multiphoton excitation of these states from the ground states of the atoms. Peaks in the ion yield as a function of ε behave differently from the conventional Stark effect—their position on the frequency scale does not depend strongly on ε. The conditions under which such an induced resonance structure appears are determined. Zh. éksp. Teor. Fiz. 113, 499–512 (February 1998)     

Stimulated resonance scattering of light waves in laser crystals with a population inversion

A new type of stimulated scattering of light is discovered in a Nd:YAG laser crystal with a population inversion. The scattering occurs for light at the resonance frequency of the laser transition, with a small anti-Stokes shift. It is shown that the effect is due to the appearance of a wave of population of the metastable level and the associated traveling index grating in a laser medium with different polarizability of the excited and unexcited atoms. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 1, 13–18 (10 January 1996)     

Light-induced transition between states in a bistable regime at nonlinear magnetic resonance in (CH3NH3)2CuCl4

It is found that in (CH₃NH₃)₂CuCl₄ crystals under nonlinear magnetic resonance conditions in the magnetic-field range in which hysteresis is observed, pulsed laser irradiation induces a transition between different branches of the resonance curve. The effect exhibits a threshold with respect to the optical radiation power. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 12, 841–844 (25 December 1996)     

On the transverse distribution of a resonance field excited by a Gaussian electromagnetic beam on the critical surface of a radially nonuniform plasma sphere

The transverse distribution of the resonance field excited by a Gaussian electromagnetic field on the critical surface of a radially nonuniform plasma sphere is studied. Analytical expressions are obtained for this distribution. It is shown that when a laser beam is focused in front of or behind a spherical target, identical values of the integrated resonance coefficient can correspond to substantially different (in width) distributions of the resonance field over the spherical critical surface. Zh. Tekh. Fiz. 67, 125–128 (October 1997)     

Simultaneous self-focusing of two laser beams in a subthreshold coherent population trapping regime

The propagation of two-component laser radiation in a medium consisting of atoms with a Λ level scheme is investigated. The simultaneous self-focusing of two beams is considered. The main features of this phenomenon are: 1) lowering of the self-focusing threshold by several orders of magnitude in comparison with the known case of saturation of a transition in a two-level atom; 2) a strong dependence of the character of the propagation of the radiation on the difference between the detunings of the two frequency components of the field from resonance, which is associated with fulfillment of the two-photon resonance condition. Zh. Tekh. Fiz. 67, 126–129 (July 1997)     

Experimental check of the dispersion law for ultracold neutrons

An experiment to check the standard dispersion law for ultracold neutrons is described. The experiment is based on searching for a shift of the resonance line of a neutron interference filter as the neutron velocity component parallel to the filter surface is varied. The first results attest to a statistically significant effect. No mimicking effects were found in a control experiment, but their possible existence cannot be completely ruled out at present. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 10, 746–751 (25 May 1998)     

Observation of a single laser-excited center at the point of a crystalline needle

The points of lithium fluoride needles are investigated by laser photoelectronic projection microscopy. A situation in which a single atomic-size source of electrons — an F ₂ color center — is observed in the region near the point is realized. As a result of the good fluorescence properties of these centers, these needles can be used as the active element of a scanning fluorescence microscope employing resonance transfer of electronic excitation energy. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 6, 441–444 (25 March 1997)     

Light-induced conversion of nuclear spin modifications of molecules

We report a new effect in which one nuclear spin modification of molecules is converted into another in a resonance laser radiation field. The effect is based on selective (with respect to modifications) optical excitation and the difference of the conversion rates of spin modifications for excited and unexcited molecules. The effect is expected to be very substantial and should occur for all molecules capable of absorbing the radiation of existing lasers. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 8, 498–503 (25 October 1999)     

Mechanisms of resonant laser ionization

An experimental investigation and numerical simulation of resonant laser breakdown are performed. As a result, quantitative agreement between the experimental data on the parameters of a dense resonant plasma (the electron density and the electron temperature) and the results of calculations in the range of detunings of the laser radiation from resonance Δλ>2–2.5 nm, in which the spatial instability of the intense resonant laser beam and the absorption of radiation are minimal, is obtained for the first time. It is shown that the previously proposed mechanism of resonant breakdown associated with laser-induced associative ionization introduces only a small correction to the final extent of ionization of the resonant plasma and scarcely alters its temperature. The influence of quantum stimulated inverse bremsstrahlung processes, which are usually described as collisions of the second kind in the resonance case, on the energy gain by electrons is analyzed for the first time in reference to specific experimental findings. The numerical calculations show that at detunings of the order of the Rabi frequency, the mechanism by which electrons gain energy through the resonant system does not reduce to collisions of the second kind and can significantly increase the density of the resonant plasma. However, in this range of detunings the laser beam is still strongly perturbed by instability processes, precluding a proper comparison of the theory with experiment. At large Δλ the classical and quantum cases differ from one another only slightly, and the values of N _e calculated for both mechanisms lie within the measurement error. Zh. éksp. Teor. Fiz. 111, 1274–1296 (April 1997)     

Giant magnetoacoustic effect in KMnF3 due to nuclear spin waves

An explanation is proposed for the gigantic magnetoacoustic effect that we observed in KMnF₃ in previous work {Kh. G. Bogdanova, V. A. Golenishchev-Kutuzov, M. I. Kurkin et al., Zh. éksp. Teor. Fiz. 112, 1830 (1997) [JETP 85, 1001 (1997)]}. The effect entails a tenfold amplitude reduction of an acoustic pulse in a magnetic field that varies over the range 0–8 kOe. It is shown that this effect is due to the interference of two nuclear magnetoelastic waves propagating in the sample under magnetoacoustic resonance conditions, if this resonance occurs in the region of strong spatial dispersion of nuclear spin waves. The effect is said to be gigantic because it exceeds in magnitude the magnetoacoustic effects observed previously in magnetically ordered materials even though it is due to nuclear magnetism, which is 10⁵ times weaker than electronic magnetism. We observe a concomitant anomalous dependence of the dispersion of the velocity of sound on the external magnetic field. Zh. éksp. Teor. Fiz. 115, 1727–1739 (May 1999)     

Dominant role of dielectronic satellites in the radiation spectra of a laser plasma near the target surface

It is shown that in a dense, not very hot, multiply charged plasma the satellite structures of resonance lines can become more intense than the resonance lines themselves. Experimental and theoretical investigations show that the conditions under which the satellite structures dominate in the emission spectrum of the plasma are quite easily realized experimentally and, furthermore, apparently they will be the most typical case in investigations of compressed plasma regions in inertial-confinement fusion experiments and in the study of plasma produced by high-contrast pico-and femtosecond laser pulses. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 9, 679–684 (10 May 1997)     

Effect of four-photon interactions on coherent population trapping in Λ-systems

The resonance fluorescence spectrum of a Λ-system excited by two resonant light fields is calculated using a Markov analysis. Analytical formulas are derived in the strong-field limit within and beyond the rotating wave approximation. It is shown that the resonance fluorescence of the system does not vanish during coherent population trapping. Its spectrum consists of two multiplets which are similar to a triplet in the resonance fluorescence spectrum of a two-level atom and lie at the electronic transition frequencies, together with two triplets located at the frequencies of four-photon processes involving the optical excitation fields. The latter are fundamental in character and impose limits on the lower bound of the dephasing rate for the Raman resonance owing to the effect of radiative decay of the dipole transitions on the dynamics of the ground state. The effect of four-photon dephasing on the absorption spectrum of a Λ-system is analyzed and found to lead to a substantial reduction in the depth of a dip in the absorption spectrum which vanishes as the laser field strength is increased. Zh. éksp. Teor. Fiz. 113, 144–167 (January 1998)     

Laser acceleration of electrons in a thin metal film

A mechanism acceleration of electrons to relativistic velocities in a thin metal film irradiated with ultrashort (τ _L ≤1 ps) high-power (I>¹⁶ W/cm²) laser pulses is proposed. The acceleration is due to a resonance action of the nonuniform field on a portion of the electrons, viz., those which oscillate in the direction transverse to the film with a frequency close to the frequency of the field. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 1, 8–12 (10 July 1997)     

Above-barrier excitons: First magnetooptic investigation

An above-barrier localized excitonic state in a Bragg confining semiconductor superstructure based on an (In, Ga)As/GaAs heterosystem is observed experimentally. A sharp excitonic resonance corresponding to the interference mechanism of localization is observed in the absorption spectrum of this structure at 1.548 eV, i.e., 33 meV above the energy of a bulk exciton in GaAs. The oscillator strength of the above-barrier exciton is twice that of the main excitonic state in the system, and the above-barrier exciton gives rise to sharp Landau oscillations in the magnetoabsorption spectra. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 10, 727–732 (25 May 1999)     

Resonance structure of doubly-charged-ion production during laser dielectronic ionization of atoms

The resonance structure of doubly-charged-ion production during the ionization of Ba atoms by infrared radiation (color-center laser radiation) is identified. It is shown that this structure is due to the excitation of states of the neutral Ba atom which are strongly perturbed as a result of the Stark effect under conditions such that the dynamic polarizability has large absolute values and depends strongly on the frequency of the radiation. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 11, 796–800 (10 December 1998)     

Autodyne detection in a semiconductor laser as the external reflector is moved

An analysis is made of how the emission amplitude and frequency of a semiconductor laser with external optical feedback change as the external reflector is moved. It is shown that the influence of the external optical feedback on the interference signal from vibrations of the reflector can be lessened not only by lowering the feedback level but also through the choice of the distance to the vibrating object. Zh. Tekh. Fiz. 69, 72–75 (January 1999)     

Strong field effects in the system of two interacting Rydberg atoms

The ionization dynamics of two interacting Rydberg atoms in a strong laser field has been investigated. Each atom has been described in the “two discrete levels + continuum” model. Quasienergy states in this system, which describe field-dressed atoms, have been studied. It has been shown that one of the quasienergy states corresponds to the formation of an atomic state stable against ionization, which leads to the interference stabilization regime first observed in the case of individual atoms in [M. V. Fedorov and A. M. Movsesian, J. Phys. B 21, L155 (1988)]. Methods for creating entangled states in this system have been proposed and the dynamics of entanglement in the process of interaction with the laser field has been analyzed.     

Formation of periodic surface structures by ultrashort laser pulses

The formation of periodic surface structures by ultrashort laser pulses was observed experimentally and explained theoretically. The experiments were performed on graphite with picosecond laser pulses. The spatial period of the structures is of the order of the wavelength of the incident radiation, and the orientation of the structures is correlated with the direction of polarization of the light. The key point of the theoretical model proposed is resonance excitation of surface electromagnetic waves, which under conditions such that the temperature of the electronic subsystem is decoupled from the temperature of the crystal lattice causes a “temperature grating” to be written on the flat solid surface of the sample while the laser pulse is being applied on account of the temperature dependence of the surface impedance. The formation of a periodic surface profile from the temperature grating occurs by the volume expansion of a melted layer near the surface of the material. For typical values of the surface tension and viscosity for metals, there is not enough time for the periodic profile to be resorbed before the liquid layer solidifies. The formation of periodic surface structures is delayed in time relative to the laser pulse. Zh. éksp. Teor. Fiz. 115, 675–688 (February 1999)