Table of contents

This paper analyses the development of active-region designs in quantum cascade lasers. Active-region designs have been demonstrated to date that employ various radiative transitions (vertical, diagonal, interminiband and interband). The lower laser level is depopulated through nonradiative transitions, such as one- or two-phonon (and even three-phonon) relaxation or bound state → continuum transitions. Advances in active-region designs and energy diagram optimisation in the past few years have led to significant improvements in important characteristics of quantum cascade lasers, such as their output power, emission bandwidth, characteristic temperature and efficiency.

A scheme of the experiment is described and the results of measuring the small-signal gain in the active medium of a pulsed chemical DF — CO₂ laser at a medium pressure in the range from 1 to 2.5 atm are reported. The values obtained (above 5 m^-1 at a pressure of 2.5 atm) make this laser a promising final amplifier of a multiterawatt laser system in the 10-μm wavelength region.

The results of the experimental study of a copper bromide vapour laser at high repetition rates of regular pump pulses are presented. A record-high pulse repetition rate of 700 kHz is attained for lasing at self-terminating transitions in copper atoms. To analyse the obtained results, use is made of the data of numerical modelling of the plasma kinetics in the phase of pumping and discharge afterglow.

The relation between the transport characteristics of subterahertz thermal phonons and the structural features of singlephase dielectric crystalline laser ceramics based on cubic oxides synthesised in different technological regimes is studied. The effect of plastic deformation on the formation of the grain structure and intergrain layers (boundaries), as well as on the thermophysical, acoustic, optical, and laser characteristics of the materials is analysed.

A new scheme of a cascaded converter of the first harmonic of broadband cw laser radiation into the second harmonic (SH) with compensation for the group walk-off in cascades is proposed and investigated. The conditions under which high conversion coefficients of broadband (∼33 cm^-1) single-mode fibre laser radiation with low peak power (∼300 W) into the SH are determined for frequency doublers based on the most promising LBO crystal. Conversion of cw radiation with an average power of 300 W and efficiency η = 4.5 % into the SH is obtained in a single LBO crystal. Effect of coherent addition of SH radiation excited in different cascades is demonstrated for two- and three-stage schemes. The expected conversion efficiencies, calculated disregarding loss but taking into account real aberrations of elements, are 18 % and 38 %, respectively. The effect of pumping depletion begins to manifest itself in the third cascade of a three-stage converter; it may reduce the latter value to ∼30 %.

The results of investigations into the generation of highorder harmonics (up to the 21st order) of picosecond (τ = 38 ps) Nd : YAG laser radiation in the plasma produced by laser ablation of metal and carbon-containing material surfaces are presented. We demonstrate the feasibility of generating high-order harmonics in the vacuum ultraviolet spectral range (with radiation wavelengths shorter than 120 nm) in plasmas with an efficiency of ∼0.7 × 10^-4. In carbon-containing plasma, the 7th harmonic intensity exceeded that of the 5th one by a factor of seven.

The nonlinear transmission of multilayer (∼12 layers) graphene at the wavelength λ ∼ 10 μm is measured for the first time. The absorption saturation intensity in graphene (∼330 kW cm^-2) and the ablation threshold of its outer layers (∼1 MW cm^-2, 0.11 J cm^-2) under action of a CO₂ laser pulse with a duration of 70 — 85 ns at λ = 10.55 μm are determined. The residual absorption of graphene at its partial saturation was 48 % of the initial value. This is significantly smaller than the value measured previously for samples with a close number of layers at λ = 1.55 μm (92.3 % — 93.8 %). It is shown that the ablation threshold of two graphene layers adjacent to the BaF₂ substrate (after successive ablation of outer layers) exceeds 0.27 J cm^-2.

We report the results of investigation of femtosecond laser pulse filamentation in fused silica by varying the wavelength in the range from 800 to 2300 nm. It is shown that in the case of the anomalous group-velocity dispersion, a sequence of 'light bullets' with a high spatial and temporal localisation of the light field is formed along the filament. The relation of the formation and propagation of light bullets with the formation of an isolated anti-Stokes wing of the supercontinuum spectrum is established.

We have studied experimentally and analytically the formation of the supercontinuum (SC) spectrum during femtosecond laser pulse filamentation in fused silica under conditions of zero and anomalous group velocity dispersions. It is found that with increasing centre wavelength from 1300 to 2300 nm, the SC spectrum of the anti-Stokes wing narrows, shifting to the blue. It is shown that the anti-Stokes (blue) shift of the SC spectrum increases with the multiphoton order of the medium ionisation by the light field in the filament. It is found that a broad minimum in the SC spectrum, separating the anti-Stokes wing from the centre wavelength, is a result of the interference of radiation from a moving broadband source, stemming from self-phase modulation of the high-intensity light field. The interference factors of the SC spectra obtained for this source, which moves along the emitting region of the filament in a dispersive medium, are in agreement with experimental and numerical results.

We have studied processes initiated in the bulk of singlecrystal silicon by local excitation of its electronic subsystem with femtosecond IR laser pulses (λ = 1.2 μm, τ_FWHM = 250 fs). IR femtosecond interferometry has been used for the first time to measure the refractive index of the irradiated zone. Using the interference images obtained in our experiments, we have reconstructed the dynamics of the electron — hole plasma generated near the axis of a focused laser beam. Experimental data have been compared to relevant numerical simulation results obtained in a simple, two-photon absorption model.

The radiation patterns of a leaky-wave InGaAs/GaAs/InGaP laser are studied. In the subthreshold regime, several peaks are found, corresponding to the emission of fundamental and excited modes. The dependences of the amplitude, position and width of the peaks on the pump current are investigated and explained.

Photorefractive surface waves propagating in a stationary regime along the surface of the metal — photorefractive crystal (SBN-75) interface are considered. The transverse structure of the optical field distribution is calculated in the near- and far-field zones, depending on the angle of incidence of the exciting wave. The calculation results are in good agreement with the published experimental results. It is shown that the photorefractive effect leads to a splitting of the spectrum of a surface plasmon polariton excited at the metal — photorefractive crystal interface.

We have studied the effect of variable valence impurities (cerium and iron) on the formation of bismuth-related IR luminescence centres and the optical loss between 1000 and 1300 nm in a magnesium aluminosilicate glass. The results demonstrate that additional doping of the glass with ceria leads to effective bleaching in a wide spectral range, including the luminescence range of the bismuth centres. At the same time, ceria reduces the concentration of luminescence centres. Gamma irradiation of the glass bleached by cerium restores the luminescence centres but leads to a background loss in a wide spectral range. Iron is shown to be a very harmful impurity in bismuth-doped active media: even trace levels of iron prevent the formation of bismuth-related active centres in the glass and produce a strong, broad absorption band centred near 1 μm.

The procedure of manufacturing silicon photodiodes with an integrated Zr/Si filter for extreme ultraviolet (EUV) spectral range is developed. A setup for measuring the sensitivity profile of detectors with spatial resolution better than 100 μm is fabricated. The optical properties of silicon photodiodes in the EUV and visible spectral ranges are investigated. Some characteristics of SPD-100UV diodes with Zr/Si coating and without it, as well as of AXUV-100 diodes, are compared. In all types of detectors a narrow region beyond the operating aperture is found to be sensitive to the visible light.

Application of a ring laser to determine the directions to the poles of Earth's rotation is considered. The maximum accuracy of determining the directions is calculated, physical and technical mechanisms that limit the accuracy are analysed, and the instrumental errors are estimated by the example of ring He — Ne lasers with Zeeman biasing.

A three-stage laser photoionisation scheme intended for enriching the ¹⁷⁶Lu isotope from natural lutetium was considered. An investigation was made of the hyperfine structure of the second excited state 5d6s7s → ⁴D_3/2 with an energy of 37194 cm^-1 and the autoionisation state with an energy of 53375 cm^-1 of the ¹⁷⁶Lu and ¹⁷⁵Lu isotopes. The total electron momentum of the autoionisation level and the constant A of hyperfine magnetic interaction were determined. Due to a small value of the isotopic shift between ¹⁷⁶Lu and ¹⁷⁵Lu, appreciable selectivity of their separation may be achieved with individual hyperfine structure components. The first tentative enrichment of the 176Lu isotope was performed to a concentration of 60 % — 70 %.