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The effects arising from the action of radiation from a laser operating in the free generation mode on optical glass are studied. The method of velocity interferometry is employed to establish that, when the laser radiation is focused within the specimen, optical inhomogeneities arise within the latter. The magnitudes of these inhomogeneities within the caustic of the focusing lens are measured. On the basis of the time lapse required for the rise of such inhomogeneities, the proposition is made that they are caused by thermoelastic stresses. Using the methods of thermoelasticity, the magnitudes of these strains and the temperature are calculated. It was also discovered that focusing the radiation on the surface of the sample produces evaporation of the specimen material. A qualitative description is given, and this effect is compared to the surface evaporation which occurs under laser action on condensed opaque media.

A description is given of a new method for generating high-power nanosecond pulses with steep leading edges and a high contrast. The method makes it possible to obtain beams with large transverse cross sections in a neodymium-glass laser system.

It is shown theoretically that a power resonance peak of a ring laser reproduces exactly the central frequency of an absorbing gas when the centers of the gain and absorption profiles coincide. A method is suggested for ensuring the coincidence of these centers: It is based on the existence of laser power resonances associated with the competition between the effects of spectral saturation of a line, spatial de-excitation of the medium, and phase interaction between traveling waves.

Experimental investigations were made of the processes occurring during the high-temperature heating of plasma by ultrashort laser pulses focused onto the surface of lithium deuteride. The following methods were used in these experiments: schlieren photography of a plasma illuminated with ultrashort pulses; photography of an expanding plasma with an image converter; investigation of the time dependences of the reflection of laser pulses; determination of the electron temperature of a plasma from its x-ray emission. The principal processes which accompany the heating of a plasma by ultrashort pulses are considered theoretically within the framework of approximations based on experimental results.

Introduction 641 I. Atomic and Molecular Frequency and Time Standards 642 1. Molecular and Atomic Generators 642 2. Cesium Frequency Standards 647 3. Other Frequency Standards 649 4. Frequency Stabilization 651 II. Use of Highly Stable Frequency and Time Standards to Check General and Special Relativity 653 1. Possible Experiments to Check General Relativity 653 2. First-Order Experiments to Check Special Relativity 666 III. Use of Atomic and Molecular Frequency Standards for the Investigation of Cosmological Effects 669 Conclusion 670 Appendix. Derivation of Formula for Relativistic Red Shift of the Frequency of a Spectral Line 670 References Cited 671

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

The results of experimental investigation of frequency stability of a He-Xe laser (? = 3.508?) with nonresonant feedback are reported. It is experimentally proved that the mean frequency of oscillations does not depend on the distance between the mirror and the scatterer. It is shown that the spectral width depends on the number N of modes interacting due to scattering. The greater N is, the narrower is the linewidth. In a He-Xe laser with a relatively low number of interacting modes (N = 10) the linewidth is about 15 mHz. The lasers with a large number of interacting modes (N~104 to 106) are considered. The linewidth in such lasers is expected to be 104 to 105Hz. The short-term stability of the laser with nonresonant feedback is determined by the linewidth, but the long-term stability should be much better; it is determined only by the stability of the atomic resonance center.

In this paper a method for obtaining states with negative temperature in semiconductors is discussed; as shown this method requires relatively small densities of excitation.

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.

A powerful light pulse propagation through an optical amplifier driven at saturation was studied theoretically and experimentally. It was shown that the maximum light pulse of a Q -switched laser propagates through amplifying medium with velocity much higher than the light velocity without any essential decrease of pulse duration. Conditions of reducing pulse duration by nonlinear amplification were found. It was found necessary to increase the leading edge rise time of the light pulse. "Superlight" velocity of propagation was measured in experiments. Appreciable reduction of pulse duration by nonlinear amplification was achieved experimentally when the leading edge of the input light pulse had been cut down by an additional Kerr cell.

A spectroscopic investigation was made of the stimulated radiation emitted by excited DF molecules generated by chemical reactions in D2 + F2, D2 + NF3, D2 + N2F4, and D2 + FNO2 mixtures at pressures of ~1 torr. It was found that the largest number of stimulated emission lines (including those in the 1–0 band) was emitted by the laser utilizing the D2 + NF3 mixture. A study of the time dependences of the intensities of various lines made it possible to explain the features associated with cascade transitions and with the rise of the temperature of the mixture during a reaction.

The results are given of calorimetric, time, spectral, and polarization measurements of radiation reflected back from a plasma heated by nanosecond laser pulses of λ = 1.06 and 0.53 μ at power densities exceeding 1014 W/cm2 at the target surface. It is shown that the absorption of the λ = 0.53 μ radiation by a plasma is three times as strong as the absorption of the λ = 1.06 μ radiation.

A review is given of chemical methods for the excitation of lasers. An analysis is made of the possibility of achieving population inversion of the vibrational levels of molecules in self-supporting chemical processes (chain and branched chain reactions, thermal explosions). Special attention is given to the problems arising in the theory of vibrational relaxation in relation to chemical lasers. The results are reported of experimental investigations of several chemically pumped laser systems. Some methods for the initiation of reactions in large volumes of a reagent are discussed qualitatively. A list is given of chemical lasers (as of August 1, 1970) and of their operating characteristics.

Power resonances in the emission of a gas laser are considered in the case of spectral hole burning (a laser with a Fabry–Perot resonator) and of interaction between the oscillation modes at frequencies close to the center of a gain or absorption line (a laser with a ring resonator). In the second case, the resonances may be much narrower and have a higher contrast than the resonances in the emission of a laser with a Fabry–Perot resonator. Stabilization is reported for a helium–neon laser with a Fabry–Perot resonator, emitting a peak associated with the saturation of the absorption in methane.

The luminescence spectra of condensed xenon, krypton, argon and some mixtures have been investigated between 3600 and 1000A. The characteristic feature of these spectra is the appearance of two bands of intrinsic luminescence for each of these gases. A transfer of excitation energy from atoms of lower atomic number to those of higher atomic number was observed. The experimental results are discussed on the basis of the concept of autolocalized excitons. The possibility of laser action in condensed inert gases excited by an electron beam is also discussed.

A D2 + F2 + CO2 + He pulsed chemical laser was put into operation at nearly atmospheric pressures in the mixture. The principal characteristics of the laser were the energy per pulse ~5 J, specific energy yield 1.210–2 J/cm3, and pulse duration ~ 10 μsec. It was found experimentally that the CO2 molecules inhibited the branched-chain self-ignition process. The use of photoinitiating components increased the utilization efficiency of the laser.

An experimental investigation was made of the gain of a CO2 electric ionization laser at pressures up to 8 atm. The dependence of the gain on the pressure and composition of the CO2+N2 mixture, the emission spectrum of the laser, and the pressure dependence of the pumping energy were determined. No new types of quenching due to the high pressure of the working gas were found in the active medium of the CO2 electric ionization laser.

A multistage neodymium-glass laser was constructed and its characteristics were determined. Introduction of Fabry–Perot interferometers into the mode-locked laser increased the pulse duration to 1 nsec. The output radiation was converted to the second harmonic in a KDP crystal (the energy efficiency of this conversion was in excess of 50%). The output emergy at the second-harmonic wavelength 0.53 μ was 10 J.

The results of the investigation of high-temperature non-stationary plasma produced at the spherical heating of the ball-shaped (CD2)n target by high-power irradiation of the 9-channel laser setup are reported. It had been found that the fraction of absorbed energy reached 70% of the incident radiation at flux densities ~ 1014 – 1015 W cm−2. About 10–25% of the energy absorbed in the target is reemitted in the X-ray spectral range, while the fraction of energy reflected from plasma was not greater than 10%. The study of gas-dynamic parameters of the hot plasma “corona” made it possible to measure the pressure in the dense plasma core. The pressure was about 106 – 107 atm at the flux density of about 1015 W cm−2. Experimental data on measuring density, velocity and pressure in the “corona” region made it possible to conclude about experimental realization of the compression value of the order of 30.

The first investigations of the interaction of laser radiation with opaque substances were undertaken shortly after the advent of solid state lasers. The main physical phenomena accompanying the heating of a substance under the action of laser radiation were analyzed in the theoretical papers [1–7].

The absorber method was used in an experimental study of the bremsstrahlung and recombination radiation emitted by (CD2)n and Fe plasmas irradiated with laser fluxes of ~1014 W/cm2 density. It was established that in the case of the Fe plasma the x-ray emission in the 1–10 keV range corresponded to a Maxwellian distribution with an electron temperature of ~0.8 keV. In the case of the (CD2)n plasma there were deviations of the distribution function from the Maxwellian form indicating an increase in the number of high-energy electrons (the electron temperature varied from 0.45 to 6 keV). Hard radiation with photon energies in excess of 100 keV was emitted from various targets. The intensity of this radiation and the deviation of the electron distribution function from the Maxwellian form increased with the density of the laser flux and decreased with the atomic number of the target material.

A review is given of the results of theoretical and experimental investigations of electrically excited preionization gas lasers in which the pressure in the working mixture may reach tens of atmospheres. The method of preionization by external agency is considered and it is reported that, under optimal conditions, practically 100% of the electrical pump energy can be converted into the energy of molecular vibrations. Experimental results are quoted to show that the output power of electrically excited preionization lasers increases proportionally to the square of the gas pressure. The results are given of experimental investigations of the threshold and output characteristics as well as of the efficiency and gain of electrically excited preionization (and, particularly, electron-beam-controlled) carbon dioxide lasers. High-pressure lasers of this kind are promising sources for thermonuclear fusion, selective stimulation of chemical reactions, and industrial processing. Some numerical data are given on the various modes of operation of electrically excited preionization carbon dioxide amplifiers and oscillators. The characteristics of some high-power laser units are briefly described. A discussion is given of potential application of the electrical preionization method in the excitation of the vibration-rotational molecular levels (CO2, CO, N2O, NO, N2, HF, and others) and of electronic state (N2, H2, CO, Cu, Pb, Xe2, and so on). The review is based on papers published up to May 1973.