Table of contents

The processes of nonradiative energy transfer in calcium — niobium — gallium garnet (CNGG) crystals doped with Er³⁺ ions are studied. It is found that the energy of erbium ions in the Er:CNGG crystal with the erbium atomic concentrations C_Er=6% and 11% is transferred via the nonradiative co-operative processes ⁴I_11/2→ ⁴I_15/2, ⁴I_11/2→ ⁴F_7/2, ⁴I_11/2→ ⁴I_15/2, ⁴I_13/2→ ⁴F_9/2; and ⁴I_13/2→ ⁴I_15/2, ⁴I_13/2→ ⁴I_9/2, whose efficiency increases with increasing intensity of exciting radiation. It is shown that the cross-relaxation processes ⁴S_3/2→⁴I_9/2, ⁴I_15/2→⁴I_13/2, whose intensity depends on the concentration of Er³⁺ ions, are characteristic for Er:CNGG crystals with the Er atomic concentration above 1%.

Terawatt 248-nm UV pulses with an energy of 0.62 J, a width of no more than 1ps, and a divergence of 20 μrad have been obtained in the first experiments on the GARPUN-MTW Ti:sapphire — KrF laser system at double-pass amplification of frequency-tripled pulses from the Ti:sapphire front end in wide-aperture electron-beam-pumped KrF amplifiers. The contrast of short pulses relative to the amplified spontaneous emission is found to be ∼10⁶ for energy densities and ∼10¹¹ for intensities. The specificity of short-pulse amplification in an active medium with fast population inversion recovery and advantages of amplification of trains of short pulses or short and long pulses are discussed. The peak power of single subpicosecond pulses in this laser system can be increased to 30TW, and the focused beam intensity can be as high as 10²⁰ W cm^-2.

We have measured the fraction of holmium ions that relax nonradiatively to the ground level as a result of interaction at a metastable level in optical fibres with a silica-based core doped with holmium ions to 2 × 10¹⁹ — 2 × 10²⁰ cm^-3. The percentage of such ions has been shown to depend on the absolute active-ion concentration. The fibres have been used to make a number of 2.05-μm lasers, and their slope efficiency has been measured. The laser efficiency decreases with increasing holmium concentration in the fibres

A simple method to increase the output energy of a passively Q-switched erbium glass laser is proposed. Using the amplitude modulation of losses at the active element face, the fundamental mode was reliably suppressed and the laser operated in a selected higher-order mode. The output energy was experimentally increased by a factor of 2.1, and the range of allowable pump energy instability was extended threefold.

We report the possibility of creating high-power nonchain electric-discharge HF lasers with an all-solid-state pump source. The maximum energy stored in the pump source capacitors based on solid-state FID-switches is 990 J for the open-circuit voltage of 240 kV. The pulse energy of 30 J is obtained in the hydrogen-containing SF₆ mixture at the electric efficiency of the order of 3%.

Possibilities for increasing the active medium volume of a chemical oxygen — iodine laser (CCOIL) with a pulsed electric-discharge generation of atomic iodine are studied. The reasons are analysed of the low stability of the transverse self-sustained volume discharge in electrode systems with metal cathodes under the conditions of the electric energy input into gas-discharge plasma that are typical for CCOILs: low pressure of mixtures containing a strongly electronegative component, low voltage of discharge burning, low specific energy depositions, and long duration of the current pulse. An efficient electrode system is elaborated with the cathode based on an anisotropically-resistive material, which resulted in a stable discharge in the mixtures of iodide (CH₃I, n-C₃H₇I, C₂H₅I) with oxygen and nitrogen at the specific energy depositions of ∼5 J L^-1, pressures of 10 — 25 Torr, and mixture volume of 2.5 L.

The experimental and numerical studies of the transport properties and wave functions of electrons in GaAs/Al_xGa_1-xAs superlattices with weak barriers in a strong electric field demonstrate the possibility of terahertz lasing on transitions between Wannier — Stark ladders, which can be tuned by applying voltage. The calculated gain can be as high as 500 cm^-1, which exceeds several times the values characteristic of modern THz quantum cascade lasers. Thus, one would expect lasing to occur in a simple n⁺ — superlattice — n⁺ stripe structure. Such lasers, due to the possibility of wide tuning and simplicity of the superlattices they are based on, could compete with cascade lasers.

The dynamics of laser and X-ray radiation fields in experiments with spherical boxes was numerically investigated in a sector approximation using the SND-LIRA numerical code. The experiments were performed on the Iskra-5 laser facility at a wavelength λ=0.657 μm (the second harmonic of iodine laser radiation). The characteristics of X-ray generation were investigated with the inner surface of a converter box coated with different-Z materials (Au, Cu, Mg). With lowering Z, the laser energy absorption coefficient k_a decreases and there occurs a lowering of the effective X-ray radiation temperature. Our calculations reveal a strong dependence of the results on the electron free-streaming flux limitation f. In particular, on lowering f from 0.1 to 0.03 for a conventional box with a gold coating, the coefficient k_a decreases from 0.83 to 0.5 and the peak X-ray radiation temperature drops from 170 to 150eV. In these calculations, the rms nonuniformity of the X-ray irradiation of a capsule with thermonuclear fuel amounted to 1%—3%.

A model for the propagation of a focused light beam in a strongly scattering medium is used to analyse various factors that limit the capability of two-photon fluorescence microscopy (TPFM) to image deep sections of optically thick biological specimens. The TPFM imaging depth is shown to be limited by three main factors: (1) beam broadening as a result of multiple small-angle scattering, leading to a loss of submicron lateral resolution; (2) strong near-surface fluorescence at large imaging depths as a result of the increase in average source power in order to compensate for scattering losses; (3) reduction in useful two-photon fluorescence signal level because of the exponential attenuation of the excitation power. The influence of these factors is examined in a small-angle diffusion approximation of radiative transfer theory. The first two of them are shown to set a fundamental TPFM limit, whereas the last is an instrumental limitation and appears to be the most critical to state-of-the-art commercial two-photon laser scanning microscopy systems for the vast majority of fluorophores in current use.

The method for computer correction of images distorted by turbulent atmosphere is realised by means of the simplest optical system comprising a telescope and digital TV-camera. Real-time images with the diffraction resolution are obtained at ground paths of length up to 1800 m.

Design principles and the fabrication technique of highly efficient, high-speed photodetectors based on MSM nanostructures are developed. To efficiently confine light in the region of the strong field as well as to decrease light losses due to reflection from the diode contacts, use is made of a nanoscale interdigital diffraction grating and a multilayer Bragg grating. Measurements of the reflection coefficients and the quantum efficiency for a multilayer structure are in good agreement with theoretical estimates. A record-high quantum efficiency (QE = 46 %) is obtained for high speed MSM photodetectors. The detector has a high spectral selectivity (Δλ_1/2 = 17 nm) at a wavelength of 800 nm. Taking into account the diode capacitance and the drift time of photogenerated carriers, the performance of the detectors under study is ~ 500 GHz. The low level of the dark current density in the structures under study (j="1" pA μm^-2) makes it possible to realise on their basis highly sensitive, high-speed selective detectors of optical radiation.

We report an experimental study of the influence of pulsed 1064-nm laser light polarisation on the dc photovoltage response of nanographite films. The amplitude of the pulsed potential difference generated in the films across the plane of incidence is an even function of the angle between the plane of polarisation and the plane of incidence of the laser beam, and that along the plane of incidence is an odd function of this angle. We present empirical relations between the incident pulse power, parameters of elliptically polarised laser light and photovoltage amplitude. The results are interpreted in terms of surface electric currents due to the quasi-momentum transfer from the incident light to the electron system via interband quantum transitions and the surface photogalvanic effect.

We report fabrication of a new acousto-optic modulator (AOM-RN) operating purely in the Raman — Nath diffraction regime. This device can be used as an external phase modulator in frequency-modulation (FM) optical heterodyne spectroscopy for fast and broadband frequency control of diode lasers. The AOM-RN design is significantly simplified, and its dimensions are minimised due to a decrease (by almost an order of magnitude in comparison with the existing AOMs) in the acousto-optic interaction length and the absence of impedance matching circuit. The FM spectroscopy based on AOM-RN makes it possible to analyse both absorption and dispersion properties of optical resonances under study; this possibility is shown by the example of saturated-absorption resonances in cesium vapour. The possibility of detecting coherent population trapping resonances using FM spectroscopy with AOM-RN as an external phase modulator is experimentally demonstrated.

Excitation of nonlinear surface waves is studied at the SBN-75 crystal — air interface. The SBN-75 crystal is characterised by the diffusion-type photorefractive nonlinearity, the surface wave being excited in the surface layer and the depth of its penetration into the crystal being determined by the nonlinear crystal parameters. Because of a large refraction coefficient in SBN-75, the penetration depth of the surface wave into the air is small; therefore, the nonlinear surface wave is localised in the surface crystal layer several micrometers in thickness. The oscillating intensity distribution of the surface wave at the output end of the crystal is measured. The oscillation period is determined by the angle of incidence of the exciting beam. The nonlinear wave is excited not only at the crystal — air interface but also in the case when the active surface of the crystal is covered by an electrode, for example, an aquadag layer. This circumstance opens up a possibility of studying new properties of the surface waves when an external electric field is applied to the crystal.

It is shown that point defect modifications in hydrogen-loaded phosphosilicate glass (PSG) do not play a central role in determining its photosensitivity. Photochemical reactions that involve a two-step point defect modification and pre-exposure effect are incapable of accounting for photoinduced refractive index changes. It seems likely that a key role in UV-induced refractive index modifications is played by structural changes in the PSG network. Experimental data are presented that demonstrate intricate network rearrangement dynamics during UV exposure of PSG.

Two setups for manipulating (translocation, stretching, rotation) microscopic objects (with the dimensions from several nanometers to tens of micrometers) are developed based on optical trapping by femtosecond laser radiation. The possibility of single-cell translocation is shown. The possibility of destructing malignant cells as well as of cutting off a fragment from a malignant cell cluster due to the rapture of bonds is demonstrated in multiphoton absorption of femtosecond light pulses. The possibility of the holographic control to move several particles simultaneously is shown.

This paper presents numerical simulations of two-dimensional radiation transfer in a powder layer that resides on a substrate of the same material and is exposed to a normally incident laser beam with an axisymmetric bell-shaped or top-hat intensity profile. The powder layer is treated as an equivalent homogeneous absorbing/scattering medium with radiative properties defined by the reflectance of the solid phase, the porosity of the powder and its surface area. The model used is applicable when the laser beam diameter far exceeds the particle size of the powder. It is shown that the absorptance of an optically thick layer of opaque powder particles is a universal function of the absorptance of the solid phase and is independent of surface area and porosity, in agreement with experimental data in the literature. The fraction of laser energy absorbed in the powder-substrate system and that absorbed in the substrate decrease with an increase in the reflectance of the material, but the powder bed is then more uniformly heated.

The concept of neutron acceleration in a gradient magnetic field of a 'drifting' standing electromagnetic wave is presented. The promising fields of application of an accelerated directional beam of ultracold neurons, in particular, remote initiation of nuclear reactions, are suggested.

The mechanism of merging of shock waves produced by a pulsating energy source is considered for magnetised plasma. The criteria for the emergence of this mechanism are found and its high efficiency for producing low-frequency magnetosonic waves, which have the form of a jet and propagate at large distances without attenuation, is shown.