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The advantages and limitations of using highly bright laser diodes for pumping low- and moderate-power solid-state Nd3+:YAG lasers are considered. The practice of designing cw and pulsed, acousto-optically Q-switched solid-state lasers is analysed and their possible applications are discussed. The efficiency of a cw laser achieved 50%.

The metalorganic vapour phase epitaxy was used for growing GaInP/AlGaInP periodic structures with 25 quantum wells. The active elements based on these structures were prepared for a laser longitudinally pumped by a scanning electron beam. The structure is intended for resonance periodic gain in the case when the quantum wells are at the antinodes of the resonator mode corresponding to the peak of the gain line. The effect of the structural inhomogeneities over the thickness (up to 5%) on the lasing parameters is studied, as well as the temperature detuning from the resonant gain conditions. It is shown that the period of the structure must differ from the optimal value by no more than 0.7% for attaining a 10% uniformity in the lasing threshold along the active element.

The main factors determining the lasing threshold and efficiency of a laser cathode-ray tube at room temperature are considered. Recent achievements obtained by using laser screens made of the II — VI compound single crystals are discussed. It is shown that multilayer heterostructures allow the reduction in the lasing threshold by several times.

The temperature regime of a laser cathode-ray tube are studied upon the electron-beam raster scanning. Three main factors determining the temperature of the active region of a laser are distinguished: the adiabatic heating by the electron beam during excitation of an individual pixel, heating caused by adjacent lines, and heating averaged during previous scans, which is determined by cooling of the laser screen. The numerical estimates of the temperature rise caused by these factors are presented. The influence of the temperature of the active region on the lasing power is discussed.

Crystals of high-power laser diodes are directly mounted on copper heat-removing elements. The maximum output power of 25 W is obtained many times in a 808-nm cw laser with the 150-μm-wide strip contact at 20°C. After training tests at the output power of 6 W for 200 hours, the yield of acceptable samples was 80 %. No changes in the output parameters were observed after tests for 70 hours at the output power of 8.5 W. Tests are being continued.

Two-dimensional arrays of diode lasers were developed and investigated under quasi-cw operating conditions with an output power density of 1 kW cm-2 and an output energy density 0.45 J cm-2 when the pump pulse duration was 0.5 ms. A study was made of the profile of the output pulses, of the spectral composition of the output radiation, and of the far-field and near-field radiation zones when the duration and repetition frequency of the pump pulses were varied. Ways of increasing the peak and average output power and energy were analysed.

A superlattice with strained ZnCdSe/ZnSe layers was grown by the method of molecular beam epitaxy. This superlattice was used as the active layer in the screen of a laser cathode-ray tube. The room-temperature output power was 1.6 W in the form of one longitudinal mode at the 484 nm wavelength. The earlier results obtained at electron energies below 50 keV were improved.

Integrated linear injection-laser arrays were made from AlGaAs/GaAs heterostructures grown by MOCVD epitaxy. The total width of the emitting surface was 1 cm and the arrays were intended for the pumping of solid-state lasers. The watt—ampere characteristics, the overall efficiency, the emission spectra, and the shape of the light pulses emitted by the arrays were investigated in the 0.1–1 ms range of the pump pulse durations, best for the optical pumping of solid-state lasers. The maximum pulse power of 70 W, obtained for a current of 80 A, was limited by the source used to pump the injection lasers.

We have designed and fabricated submounts from synthetic diamond grown by microwave plasma chemical vapour deposition and developed an economical process for metallising such submounts. Laser diode chips having an 808-nm emission wavelength, 3-mm-long cavity and 130-mm-wide stripe contact were mounted on copper heat sinks with the use of diamond submounts differing in quality. The devices were tested for more than 150 h in continuous mode at an output power of 8 W on diamond with a thermal conductivity of 700 W m{sup -1} K{sup -1}, and no changes in their output power were detected. On diamond with a thermal conductivity of 1600 W m{sup -1} K{sup -1}, stable cw operation for 24 h at an output power of 12 W was demonstrated. (letters)

A cw injection laser emitting near 808 nm was constructed and investigated. Reduction of the internal cavity losses and of the series resistance of the laser made it possible to increase the external differential quantum efficiency to 0.67 and to achieve a total ('wallplug') efficiency of 50% while keeping the heat sink at 10°C (the total efficiency was 45% at 45°C). Accelerated life tests carried out at elevated temperatures indicated that the operating life of the laser should be in excess of 104h at 25°C.

An investigation was made of the charging of traps in the dielectric layers of a metal-siliconnitride/ silicon-dioxide-semiconductor structure subjected to electrical pulses. An analysis was made of the methods for optical switching of this structure by laser radiation and for photoelectric reading. The principal energy and time characteristics were determined for an optical memory based on this structure. Organization of an optical reversible memory of 108 bit capacity was considered.

An investigation was made of spatial light modulators utilizing the electrooptic effects in a liquid-crystal layer, acting as the insulator in a photosensitive metal–insulator–semiconductor structure. The semiconductor was a crystal of bismuth silicate or germanate. A high value of the energy sensitivity, up to 2 cm2μJ (for total modulation of light), was obtained; the intensity of the control (switching) light in the blue part of the spectrum was 25–30 μ W/cm2. The resolution was up to 200 lines/mm (deduced from the fall to half the maximum value of the frequency-contrast characteristic). The liquid crystals employed made it possible to reach a frame repetition frequency of 30–50 sec−1 at room temperature. A two-frequency supply of the modulator structure made it possible to achieve a controlled memory regime with a data storage time up to 2 sec, and with recording and erasure times of ~1 msec.

An investigation was made of the visible radiation emitted by planar metal–barrier–metal structures as a result of decay of plasma oscillations. A method was found for increasing the intensity of the output radiation using a barrier in the form of a multilayer insulator consisting of different oxides evaporated consecutively. It was found experimentally that the efficiency of emission by these structures was twice that of structures with a simple barrier. The rise time of an optical response of such metal–barrier–metal structures to a single short voltage pulse was at least 4 nsec. The output radiation intensity increased under the additional action of a microwave field of frequency up to 1.2 GHz. This property could be used for the purpose of fast-response visualization of microwave fields with a sensitivity threshold of ~1 mW/cm2.

An investigation was made of the use of a fast-response one-dimensional electrically controlled transparency as an input device in a system for data storage in an external holographic memory. The working material of the transparency was a memory-free modification of lead-lanthanum zirconate-titanate of the 9/65/35 composition.

A waveguide model of an injection laser is developed which allows for the dependence of the waveguide properties on the injected carrier density and for the nonuniform saturation of the gain over the resonator length. An analysis of this model shows that the steady-state watt-ampere characteristic of the laser may have a stimulated-emission termination point. This characteristic is used to explain the onset of "H-pulsing". An explanation is given of the dependences observed in the anomalous kinetics of stimulated emission.

The characteristics of laser screens made of II-VI semiconductor compounds (and designed to operate in electron-beam tubes) were investigated in the range 80–300°K. The spectral characteristics of spontaneous and stimulated emission and the threshold parameters were determined. It was concluded from these data that, in the temperature range, 80–300°K the laser action in these screens occurred as a result of radiative recombination in a degenerate electron-hole plasma.

An investigation was made of a coherent optical imaging system in which aperture synthesis was performed by additional optical elements (diffraction gratings). Sinusoidal gratings with a period of 150 μ. and a depth of modulation 4 π at the wavelength 630 nm were selected on the basis of an analysis of the instrumental function of this system. A twelvefold reduction in the size of the minimum resolvable element (from 200 to 17 μ) was achieved for a relative aperture of the objective 1:90. It was demonstrated experimentally that the requirements in respect of the alignment of the whole system, including diffraction gratings, were no more stringent than the requirements in respect of the alignment of the conventional optical systems and were governed by the objectives used.

A kinetic equation is solved to determine the distribution of the electric discharge power between elastic and inelastic electron collisions in argon, krypton, and xenon. The kinetics of the excimer emission is studied for krypton allowing for the processes of deactivation. An expression is derived for the growth time of a step-ionization instability which is found to be inversely proportional to the square root of the power supplied to the discharge. The maximum efficiencies and gain factors are found for inert-gas excimer lasers.

An investigation was made of a system for recording and reconstruction of Fourier holograms in which additional optical elements (diffraction gratings) were used in one-dimensional aperture synthesis. Optical data storage with a linear density of 1.6?104 bit/cm was achieved using Yupiter 36C photographic objectives with a relative aperture of 1:3.5 and a focal length of 250 mm. The requirements in respect of the alignment of the components of the system and in respect of the characteristics of the recording medium were considered. An analysis was made of the method of recording synthetic-aperture holograms. The results obtained demonstrated that the method of aperture synthesis is promising for the fabrication of archival holographic memories with a very large capacity.

An analysis is made of the evolution of high-power laser pulses in the case of coherent two-photon interaction with resonant media. It is shown that pulses of frequencies ω0 and ωs satisfying the equalit ω0±ω0=ω21 (ω21 is the resonance frequency of the medium) experience splitting into separate subpulses followed by a reduction in their duration and increase in their power. The efficiency of this process depends on the initial ratio of the amplitudes E0 and Es. It is shown that such effects can be observed in paramagnetic acoustic resonance.

An experimental investigation was made of spatial, temporal, and power characteristics of a streamer cadmium sulfide semiconductor laser. The shape and amplitude of the exciting pulses had a considerable influence on the formation of a streamer and on the velocity of its head. When the pulse amplitude was increased from 4 to 90 kV, the velocity of the streamer head increased from 2×107 to 3×109 cm/sec. The output power of the laser increased on increase in this velocity in accordance with a near-parabolic law. A study was also made of the influence of the voltage drop in the streamer channel on the spatial and temporal characteristics of the laser.

The principles of design of an optical processor with a variable logical structure are given. These principles are justified theoretically and it is shown that the tabular method is the most suitable for logic and arithmetic operations in such a processor. The processor in question has the facility for changes at the logic level and is characterized by a high degree of parallelism of data processing. It can be used to process signals in discrete or analog forms using the same elements. These features ensure much greater capabilities and performance of an optical processor compared with a conventional computer.

An analysis is made of a method for increasing the capacity of holographic memories. The method is based on the use of aperture synthesis principles and makes possible the increase of the memory capacity by ~ 104.

Methods of the realization of a tabular optical processor with variable operators are considered. The structure and functional links between the units of the processor are discussed. A detailed analysis is made of the operation of a unit of controlled light beams, units for recording and readout of holograms, immediate-access memory, and a data processing unit made up from optically controlled transparencies. Difficulties and prospects of the construction of optical and optoelectronic variants of this new high-performance processor are considered. It is noted that the optoelectronic variant of the processor may be realized within a few years.

A theoretical investigation was made of the propagation of a high-power light pulse in a semiconductor under interband absorption conditions. A self-consistent system of equations was obtained for the field and the medium allowing for nonequilibrium particle relaxation processes. Allowance was also made for the influence of a strong pulse field on the intensity of such processes. Analytic and numerical investigation of various interesting cases permit comparison between theory and experiment.

A theoretical analysis is made of the influence of doping of the active region on the threshold and temperature characteristics of GaAs lasers. A comparison is made with the experimental data obtained for electron-beam-pumped lasers. A satisfactory description of the experimental results obtained for active elements with the full range of dopant concentrations requires an allowance for the influence of the absorption by carriers and of the Auger recombination of nonequilibrium carriers on the laser characteristics. Doping of the active region of a GaAs laser makes it possible to reduce the threshold pump current only in the case of short nonradiative lifetimes τnr. If τnr ≈ 1 ns, the optimal acceptor concentrations in p-type GaAs are 4 × 1018–1019 cm−3 and the corresponding optimal concentrations of donors in n-type GaAs are (1–2) × 1018 cm−3.

A report is given of a theoretical and experimental investigation of the dynamics of the interaction between an avalanche process in a metal–insulator–semiconductor (MIS) structure and light. The case of a stable avalanche is considered in detail. The main transient effects responsible for the detection of light with the aid of pulsed avalanches are studied. A good agreement is obtained between the experimental results and a theoretical model. The results of the investigation make it possible to determine the potential applications of photodetectors in the form of MIS structures operating under avalanche conditions.

An investigation was made of the generation of light in II-VI semiconductors along a streamer channel. The establishment of a high gain amounting to (4?6)?103cm?1 inside a channel resulted in lasing in resonators with mirrors formed by optical inhomogeneities of a crystal. The radiation emitted in the direction of the streamer emerged from the lasing region and propagated through the bulk of the crystal, whereas in the opposite direction it traveled along an optical waveguide formed by the streamer channel. The angular distribution of the radiation in the forward direction was influenced by the Franz?Keldysh effect at the head of the streamer and by distortions of the light wave in secondary streamer channels. A considerable increase in the radiation energy in the forward direction observed at streamer velocities of at least 4?109 cm/sec was attributed to the attainment by the streamer head of a velocity higher than the group velocity of light at the wavelength of the emitted radiation.

A theoretical investigation is made of the coherent interaction between high-power optical radiation and excitons in impurity-doped molecular crystals. A self-consistent system of equations is obtained for the medium and the optical field. It is shown that, under certain conditions, the distances over which coherence effects are observed, decrease sharply. An analysis is made of the specific conditions for the experimental observation of these effects.

An analysis is made of the characteristics of erasable recording media intended for optical data storage, including thermomagnetic media used in disk memories. A new class of optoelectronic media based on multilayer structures is considered. It is shown that the density of the data recorded in such media can reach 105 bit/mm2 and the energy of an optical write pulse need not exceed 10−12 J. The use of such media is opening up extensive opportunities for optical programming and redundancy. The rate of exchange of data in an optoelectronic storage device can reach 1011 bit/sec.

This part of the review deals with coherent effects in the interaction of ultrashort light pulses with media characterized by degenerate transitions, including semiconducting media (in the direct band-band transition region), and with media exhibiting two-photon resonances. It is shown that the self-induced transparency can appear under such conditions and the properties of this effect are studied. A review is made of experimental investigations of the coherent bleaching of various media. The paper ends with a list of the most urgent problems requiring clarification.

A theoretical investigation is made of the evolution of a high-power light pulse in a semiconductor under interband interaction conditions for an arbitrary relationship between the effective masses of electrons in the bands. A self-consistent system of equations for the pulse field and medium are derived and some properties of the system are considered.

A review is given of the literature on optoelectronic methods for digital data processing. The possibilities of constructing various optoelectronic logic elements and of arithmetic devices based on these elements are considered. An analysis is made of the processing methods in optical memory systems and of the characteristics of optoelectronic computing devices in which nonpositional notation systems or systems with redundancy are used. The published principles are given for optoelectronic processes operating on two-dimensional data arrays. Their potentialities are considered and the competitiveness of optoelectronic computing systems is compared with that of electronic computers.

Theoretical and experimental investigations were made of the influence of the power of longitudinal resonator modes adjacent to the fundamental mode on the width of the emission line of injection lasers made from GaAlAs heterostructures with lateral optical confinement. The width of the emission line observed in the course of cw emission of real lasers was within the limits set by the homogeneous and inhomogeneous broadening of the gain profile. When the output power was ~ 5 mW from a face an increase (by up to 10%) in the relative power of the longitudinal modes adjacent to the fundamental one could broaden the line right up to the inhomogeneous limit. This effect was attributed to spatial depletion of the population inversion along the resonator axis.

An investigation was made of the photoluminescence spectra of cadmium sulfide and selenide single crystals (77 and 4.2°K) and of the spectra of the gain α(ħω) during excitation of the same crystals with electron beams of 20 keV energy (77°K). The experimental and calculated gain profiles were compared on the assumption of the exciton–phonon interaction between carriers. A correlation was found between the photoluminescence and gain spectra. Gains up to 70 cm–7 were obtained for CdS crystals whose 77°K photoluminescence spectra included a strong (compared with the edge luminescence) band due to A free excitons, and there was clear evidence of transitions assisted by one or two longitudinal optical phonons. In the case of CdSe crystals grown by the sublimation of a powder in an argon stream the maximum gain and full agreement between the experimental and calculated gain profiles were obtained when these crystals were grown in the temperature range 800–900°C.

The threshold characteristics of semiconductor lasers are considered as a function of the doping level and the temperature. Use is made of a model of the transitions from a parabolic conduction band to an impurity acceptor band with a Gaussian distribution of the density of states. The gain and the spontaneous recombination rate are calculated. The dependences of the threshold current, the quasi-Fermi levels, and the laser oscillation frequency on the impurity concentration and the temperature are obtained.

A theoretical, investigation is made of the laser gain associated with interband transitions in heavily doped semiconductors. Analytic expressions are obtained for the gain and quasi-Fermi levels considered as a function of the impurity concentration, temperature, and pumping power.

CONTENTS Introduction 702 I. Methods of Obtaining Negative Temperature States 703 1. Negative Temperature 703 2. Selection of Molecules in Molecular Beams by Means of Inhomogeneous Electric or Magnetic Fields 705 3. Excitation of Gas Molecules by Means of a Gas Discharge 705 4. Pulse Method of Obtaining Negative Temperatures in Semiconductors 708 5. Obtaining Negative Temperatures in Semiconductors between Levels in a Single Band ("Negative Mass Amplifiers") 713 6. Obtaining Negative Temperatures by Double-Resonance Techniques 715 II. Generation and Amplification of Electromagnetic Waves by Negative Temperature Systems 718 7. Interaction of Radiation with Negative Temperature Systems 718 8. Oscillation Conditions 719 9. Resonators 721 10. Amplifiers 723 11. Quantum-Mechanical Radiation Detectors 725 Conclusion 726 Cited Literature 726

A report is given of generation of laser radiation in the vacuum region by the excitation of condensed inert gases with an electron beam. Laser emission was achieved experimentally at λ = 1760 A by the excitation of liquid xenon with a beam of 800 keV electrons. The threshold current density (30–50 A/cm2) was determined. The divergence of the emitted radiation (~7°) and the half-width of the stimulated line (~20 A) were measured.