Table of contents

A review of specific features and output characteristics of small-scale (l≤30 cm, d≤1.1cm) gas-discharge recombination He—Sr⁺ (λ=430.5 nm) and He— Ca⁺ (λ=373.7nm) lasers is provided. Such compact lasers are characterised by a relatively high reliability and durability. These lasers are easy to operate and are capable of generating laser pulses with high repetition rates f, a high specific mean power P_sp, and an improved quality of output radiation. A typical mean power P of self-heating sealed-off He—Sr⁺ (Ca⁺) laser tubes with l≈ 30 cm and d≈ 1 cm is ~ 0.5 W. The best specific characteristics of a He—Sr⁺ laser with tubes of this type were achieved with l=20 cm and d=0.6 cm (P_sp=73 mW cm^-3) and l=9 cm and d=0.55 cm (P_sp=65 W cm^-3). The best specific characteristics for a He—Ca⁺ laser were achieved with l=26.5 cm and d=0.7 cm (P_sp=50 mW cm^-3). The use of forced water cooling in a He—Sr⁺(Ca⁺) laser allowed the powers P=3.9 W and P_sp=137 mW cm^-3 to be achieved with a tube with l=30 cm and d=1.1 cm for f=29 kHz. A new method of inputting metal vapours based on cataphoresis is considered in detail. Using this method allowed the output characteristics of a He—Sr⁺ laser to be considerably improved and a record specific power P_sp=277 mW cm^-3 to be achieved for a tube with l=26 cm and d=0.3 cm with f=30 kHz and P=510 mW. A record gain, 0.15 cm^-1, was also achieved under these conditions. Some possible areas of applications of small-scale He—Sr⁺(Ca⁺) lasers in semiconductor microtechnology, holography, ecology, and medicine are outlined.

Spectral studies of induced quasi-crystal properties (which can be quantitatively characterised by the difference in the refractive indices of ordinary and extraordinary waves, Δn=n_o—n_e) in Rhodamine 6G and Rhodamine 4C solutions in glycerine excited in the visible and UV ranges of the absorption spectrum are presented. It is demonstrated that the observed spectral dependences of Δn of these dye solutions excited in the visible (long-wavelength) and UV (short-wavelength) ranges of the absorption spectrum can be interpreted in terms of an oscillator model of a molecule. The proposed method for the analysis of induced optical anisotropy in solutions of organic compounds allows the relative orientation of oscillators in a molecule and, thus, the relative orientation of electronic transitions in a molecule to be determined in a reliable way.

It is found that for SF₆—hydrocarbon (deuterocarbon) mixtures having a composition typical for nonchain HF(DF) lasers, the electric field strength reduced to the partial pressure of SF₆ (p_SF6) in the quasistationary phase of a volume self-sustained discharge (E/p_SF6)_st=92 V m^-1 Pa^-1 is close to the known critical value (E/p)_cr=89 V m^-1 Pa^-1, which is specified by the condition that the electron-impact ionisation rate of SF₆ is equal to the rate of electron attachment to SF₆ molecules. This testifies to the decisive role of these two processes and allows the use of the known approximations of the effective ionisation coefficient and the electron drift velocity for pure SF₆ when calculating the discharge characteristics. The oscilloscope traces of voltage and current calculated in this approximation for lasers with apertures ranging from 4 to 27 cm deviated from the experimental data by no more than 10%.

The spectral and amplitude—time characteristics of HF lasers pumped by a nonchain chemical reaction and initiated by radially convergent and planar electron beams were investigated. The principal channels leading to the formation of vibrationally excited HF molecules were analysed. It was confirmed that high efficiencies (~10%) of a nonchain HF laser may be attained only as a result of the simultaneous formation of atomic and molecular fluorine when the active mixture is acted upon by an electron beam and of the participation of molecular fluorine in population inversion. It was shown that a laser pulse has a complex spectral—temporal profile caused by the successive generation of P-lines and the overlap during the radiation pulse of both the rotational lines of the same vibrational band and of individual vibrational bands.

Radiative and photochemical properties of a number of laser dyes excited by focused radiation of a XeCl laser with intensity up to 200 MW cm^-2 were studied. A method for measuring the gain of organic molecules under high-power excitation is proposed. The dependence of the dye transmittance for the pump radiation on its intensity was studied. It is shown that changes in energy, spectral, and time characteristics of radiation and the photostability of compounds under high-power excitation are associated with the formation of superluminescence.

The laser ablation performed with an automated excimer XeCl laser unit is used for large surface cleaning. The study focuses on metal surfaces that are oxidised and are representative of contaminated surfaces with radionuclides in a context of nuclear power plant maintenance. The unit contains an XeCl laser, the beam delivery system, the particle collection cell, and the system for real-time control of cleaning processes. The interaction of laser radiation with a surface is considered, in particular, the surface damage caused by cleaning radiation. The beam delivery system consists of an optical fibre bundle of 5 m long and allows delivering 150 W at 308 nm for laser surface cleaning. The cleaning process is controlled by analysing in real time the plasma electric field evolution. The system permits the cleaning of 2 to 6 m² h^-1 of oxides with only slight substrate modifications.

One of the mechanisms of the inversion breaking in copper vapour lasers caused by a high prepulse electron density is considered. Inversion breaking occurs at a critical electron density N_{e cr}. If the prepulse electron density exceeds N_{e cr}, the electron temperature T_{e cr} cannot reach, during a plasma heating pulse, the temperature of ~2eV required for lasing. A simple estimate of N_{e cr} is made.

An efficient electric-discharge XeCl laser is developed, which is pumped by a self-sustained discharge with a prepulse formed by a generator with an inductive energy storage device and a semiconductor current interrupter on a basis of semiconductor opening switch (SOS) diodes. An output energy up to 800 mJ, a pulse length up to 450 ns, and a total laser efficiency of 2.2% were attained by using spark UV preionisation.

A brief review of the most interesting papers presented at the IV International Conference on Atomic and Molecular Pulsed Gas Lasers (AMPL'99), which was held in Tomsk, September 13–17, 1999, is provided.

Nonlinearity of the photorefractive response is studied experimentally and theoretically upon two-beam interaction of light waves in a bismuth silicate crystal placed in an external meander electric field. The experimental data are shown to be in good agreement with a model, taking into account the influence of the second harmonic of the space-charge field on a photorefractive grating. The concentration of acceptors in a crystal and a product of the mobility of charge carriers by their recombination time are estimated from a comparison of the experimental and calculated data.

Photoinduced variations in the refractive index of nitrogen-doped silica are directly measured in the wavelength range from 350 to 2500 nm. It is found that the change in the refractive index under the action of 193 nm laser radiation significantly increases in the long-wavelength region and amounts to ~10^-3 at λ=2.5 μm (for the energy fluence ~10⁴ J cm^-2). This means that the nature of the photorefractive effect in nitrogen-doped silica is related to photostructural transformations affecting the phonon part of the optical absorption spectrum.

It is shown that the resonant enhancements of the optical Kerr effect and the optical Stark effect of the same order are caused by the same change in the state of an atomic system. The analysis of the known experimental data demonstrates the identity of their influence on nonlinear optical processes. It is shown that a separate analysis of the influence of Stark shift and resonantly enhanced Kerr nonlinearities leads to incorrect results.

The effect of intensity, length, and wavelength of an ultrashort laser pulse on the formation of a hot electron component in a dense laser-produced plasma was first investigated in a single experiment. For a pulse length of 1 ps (or 200 fs, but with an energy contrast ratio of ~20), it was shown that the principal mechanism of generation of hot electrons is the resonance absorption of laser radiation and that the temperature of hot electrons depends on the laser pulse intensity I and the wavelength λ as T_h~(Iλ²)^1/3. The homogenisation of the nanostructures of porous silicon due to a poor contrast ratio or a long duration (1 ps) of the laser pulse lowers the yield of hard x-ray radiation compared to the case of high-contrast 200-fs pulses.

Mass spectrometry is used to investigate the emission behaviour and the characteristics of multiply charged ions in a plasma produced at small angles of incidence of laser radiation (α~20°) and also at grazing incidence (α~85°). It is found that upon grazing incidence of the laser radiation onto a target, the efficiency of production of multiply charged ions is reduced compared to that for α~20°. However, this geometry of laser irradiation of solids can be used for the elemental analysis of surface layers of a sample.

The effect of laser irradiation of heated Ti films on their phase composition is studied. The exposure of Ti films to a beam of linearly polarised 1.96-eV photons during their annealing in vacuum was found to suppress the oxidation reaction.

A two-frequency differential absorption lidar is proposed in which the reference beam is formed from CO₂ laser radiation and the radiation frequency of the working beam is tuned consequently to the lines of the NH₃ laser spectrum. It is shown that this lidar can be used to measure the concentration of freons and some other atmospheric pollutants as low as ~ 1 ppm with an error of about 20%.

The design of a miniature laser shock tube for the study of a wide range of hydrodynamic phenomena in liquids at pressures greater than 10 kbar and in supersonic flows with large Mach numbers (greater than 10) is discussed. A substance filling a chamber of quadratic cross section, with a characteristic size of several centimetres, is compressed and accelerated due to local absorption of 100 ns, 100 J KrF laser pulses near the entrance window. It is proposed to focus a laser beam by a prism raster, which provides a uniform intensity distribution over the tube cross section. The system can be used to study the hypersonic flow past objects of complex shape and the development of hydrodynamic instabilities in the case of a passage of a shock wave or a compression wave through the interfaces between different media.

The electron-impact excitation of holmium atoms was studied by the method of extended crossing beams. The cross sections and the optical excitation functions were obtained for odd levels of Ho I, including the 22014 cm^-1 laser level. Over 99% of the atoms were shown to reside in the ground level prior to collisions with electrons. Also measured were the excitation cross sections for six even levels, which presumably participate in the formation of inversion population in a gas-discharge holmium vapour laser.

The conditions for implementing a self-sustained exothermic nuclear (combustion) reaction in a system comprising long-lived metastable isomers and quasi-equilibrium high-temperature black-body radiation are examined. In this system the radiative decay of the metastable state is a result of an anti-Stokes process bypassing a strongly forbidden isomeric transition. The anti-Stokes transition in the reaction zone is triggered by the corresponding resonance spectral components of the quasi-equilibrium radiation, the temperature of which, sufficient to close the energy reaction cycle, is in its turn maintained by the absorption of hard photons, emitted by nuclei, in the reaction zone.

It is shown that the formation of a negative gas lens in a waveguide gas laser of medium pressure can break the symmetry and the number (classification) of waveguide modes. The violation of symmetry can be caused by imperfect waveguide construction or can occur spontaneously. The spontaneous violation of symmetry is caused by the optical nonlinearity of an active medium (the saturation effect) and leads to the appearance of new modes, which are absent in the linear waveguide with the same optical characteristics. The violation of symmetry of waveguide modes can cause, in particular, a decrease in coherence of laser radiation.

It is shown that, under certain conditions, the intensity of light in silica fibres is mainly limited by stimulated Raman scattering. An estimate is made of the threshold intensity that leads to irreversible destruction of the fibre due to overheating. The dependence of the threshold intensity on physicochemical properties of the material and the geometric dimensions of the fibre core is determined.