Table of contents

The existence of temperature-noncritical phase matching in second-harmonic generation was discovered for the first time theoretically and confirmed experimentally. A temperature phase-matching bandwidth in excess of 200°C was found experimentally for a potassium titanyl phosphate (KTP) crystal of 7.7 mm length at the fundamental-radiation wavelength 1064.2 nm.

High-order Stokes and anti-Stokes generation in the visible and near-infrared in cubic laser crystal hosts Gd₃Ga₅O₁₂, Gd₃Sc₂Ga₃O₁₂, and Ca₃(Nb,Ga)₂Ga₃O₁₂ was observed for the first time. All scattering-laser components were identified and attributed to the SRS-active vibration modes of these garnet crystals.

An investigation was made of the characteristics of the optical Kerr effect in a spiropyran solution. It was found that this effect makes it possible to distinguish the coloured and uncoloured forms of spiropyran and that it represents a promising method for nondestructive data reading in three-dimensional optical memory systems based on photochromic materials.

A review is provided of the work on laser separation of Yb isotopes, carried out at the Institute of General Physics of the Russian Academy of Sciences and at the 'Lad' Scientific — Production Enterprise during the last 4 — 5 years. The processes of Yb isotope separation by the AVLIS (atomic vapour laser isotope separation) method were investigated both theoretically (by computer simulation) and experimentally. The main topics considered in the review are the ionisation selectivity, the formation of laser beams and of vapour flow in the cavity, and the extraction of ions from a plasma. A facility for producing highly enriched ¹⁶⁸Yb on an industrial scale is described. The rate of production of the enriched ytterbium is now 5 — 10 mg h^-1 (over 1 g per month). Commercially viable production of the enriched ¹⁶⁸Yb isotope by the AVLIS method was achieved for the first time anywhere in the world.

An investigation is reported of an acousto-optical system with optoelectronic feedback, ensuring stabilisation of the laser beam direction. Various modifications of the system based on isotropic and anisotropic diffraction of light are considered. An analysis is made of the influence of the acousto-optical interaction selectivity on the angular operating range and on the stabilisation coefficient. The results of an experimental study of a system with a paratellurite crystal cell are given.

An investigation is reported of external phase locking of optically coupled lasers, combined to form a ring and a linear array. A random scatter of the resonance frequencies and external noise is assumed. The theory of critical phenomena is used in simulation of the evolution of lasing in separate modules. It is shown that, depending on the real coupling coefficient, an increase in the detuning of the resonance frequencies of the modules and in the power of the external noise may result in either simultaneous suppression of lasing or in dephasing of the laser fields, leading to loss of the phase locking. The parameters of the injected signal and the laser characteristics essential for phase-locked operation are determined. A comparative analysis is made of a laser array with external phase locking and 'each with every other' coupling and also for an uncoupled laser array.

A dynamic model is proposed for collective operation of an array of diffraction-coupled waveguide lasers in an external cavity. The selective properties of a Talbot cavity predicted by this model match experimental results. A study is made of the transient process of evolution of collective lasing in one-dimensional and two-dimensional arrays. A statistical sampling method is used to show that the probability of establishing phase-locked operation as a result of this transient process is close to unity when the gain is low, but the probability falls on increase in the gain.

Reflection and transmission of a plane electromagnetic wave by a two-layer periodic dielectric structure with an arbitrary number of periods is investigated. A successfully selected method of obtaining the solution for an electromagnetic field inside the structure makes it possible to find the normal-incidence reflection and transmission coefficients in an exact and physically intelligible form. A systematic and detailed investigation is reported of the dependence of the reflection coefficient on the structure parameters.

An analysis was made of the operation of a metal — dielectric grating formed by a corrugated film located on a flat surface of a metal. It is shown that the high diffraction efficiency of such a grating for the TE waves is attainable not only in the autocollimation regime (Littrow mounting), but also for a grazing (Θ = 89°) incidence of a light beam on the grating (Littman — Metcalfe mounting). This high efficiency is observed when leaky modes are excited in the dielectric layer. An experimental check of the calculated results is reported.

Properties of InGaN/GaN/AlGaN light-emitting heterostructure diodes generating visible radiation were investigated at elevated temperatures (up to 450 K). The optical power fell relatively slowly above room temperature and the characteristic temperature constant was T₀* = 400 — 800 K. Auger recombination was not detected. The position of the spectral peak of Zn-doped diodes was practically independent of temperature. A red shift of the emission line of quantum-well diodes was observed with increase in temperature. This corresponded to a reduction in the band gap with a correction for the influence of the density-of- states tails.

A model of a solitary defect-generation wave is developed by analogy with the combustion wave model. This solitary wave is initiated and propagates in semiconductors and dielectrics as a result of intense laser generation of electron — hole pairs. The following characteristics of a defect-generation wave are determined analytically: the critical intensity of ignition of the wave; the profile and propagation velocity of the wave; the concentration of point defects generated by the wave. The results obtained are used to provide a quantitative interpretation of an experimental study of the damage to the surface of Si by repeated picosecond pulses.

Changes in the molecular structure of polymethyl methacrylate caused by irradiation with laser pulses of different powers were investigated by x-ray diffractometry methods. An analysis was made of the position and half- width of a halo in x-ray diffractograms. Coherent scattering regions and the distributions of the moduli of the interatomic vectors were determined. Action of laser radiation on this transparent dielectric induced molecular structure changes associated with an increase in the molecular order in the amorphous matrix and with the formation of clusters at certain specific laser radiation energies.

Numerical calculations are reported of thermooptical distortions of nonlinear ADP, LiNbO₃, KDP nonlinear crystals used in second-harmonic generation. The elasto-optical effect is taken into account. The calculations are made for crystalline samples in the form of circular cylinders in which a steady-state parabolic temperature field is established.

Equations describing second-harmonic generation under quasi-phase-matched interaction conditions in crystals with a regular domain structure are derived. It is shown that appropriate substitution of variables makes these equations exactly identical with those describing second-harmonic generation in the usual homogeneous crystals. This analogy is used to obtained an expression for the second-harmonic amplitude in the case of inexact quasi-phase-matched interaction.

Theoretical and experimental investigations were made of the characteristic features of doubling the frequency of Bessel light beams by the oe — e interaction in a KTP crystal. All possible and scalar interactions of plane—wave components of Bessel beams were observed experimentally. The spatial structure of the second harmonic in the far zone consisted of a central spot and a concentric ring. The diffraction divergence of the central beam was weak ( ~ 1.7 mrad). The energy conversion efficiency was ~ 21% for a Bessel beam, which was three times higher than the efficiency of conversion of the initial laser beam. A theoretical model was based on representation of the field of the second harmonic as a superposition of Bessel beams, which provided a correct description of the observed spatial and energy characteristics of the second harmonic.

A channel formed by self-focusing of 30-ps pump pulses of wavelength 1.06 μm and energy of up to 40 mJ in high-pressure hydrogen (with a pressure of up to 120 atm) was probed by second-harmonic pulses delayed in time with respect to the pump pulses. This revealed scattering of efficiency which increased from zero to almost total scattering with increasing delay time. The effect is interpreted as scattering by optical inhomogeneities formed by an acoustic perturbation excited under conditions of a nonuniform temperature distribution along the self-focusing channel.

Consecutive generation of the third harmonic (i.e. generation of radiation with the doubled frequency followed by frequency summation) radiation from a Q-switched Nd:YAG laser (λ = 1.064 μm) was observed in a Y:LiNbO₃ crystal polarised periodically along the z axis. The modulation period of the nonlinear susceptibility was ~ 60 μm. The second and third harmonics of the radiation were generated respectively in the ninth and thirty-third quasi-phase-matching orders.

An investigation is reported of the influence of losses and of thermal noise in a cubically nonlinear medium with a periodically inhomogeneous linear susceptibility on the generation of polarisation-squeezed light. Calculations are reported of the mean Stokes radiation parameters and of their variances. It is shown that the presence of losses is responsible for the existence of an interaction length optimal for suppression of fluctuations.