Bulletin of the Lebedev Physics Institute最新文献

Liquid crystal electro-optic modulators of mid-IR radiation have been developed, which can control the polarization state of CO laser radiation in the wavelength range of 5.1 to 5.6 μm with light aperture up to 2.5 cm². The use of a ferroelectric liquid crystal as an electrooptic medium located between crossed polarizer and analyzer made it possible to ensure thermally stable electro-optic modulation of CO-laser radiation at a frequency from 1 to 10 kHz and a contrast ratio from 15 : 1 to 25 : 1. The modes of electrically controlled half-wave and quarter-wave plates with switching times of polarization states from 20 to 50 μs are ensured as is the ability to remember the obtained polarization state after switching off control voltage. The minimum polarization state switching time was 1.4 μs, which was observed at a modulation frequency of 40 kHz and rotation of the liquid-crystal optic axis by an angle of a few degrees, which decreased the contrast ratio to 1.3 : 1.

We consider the problem of finding the spatial distribution of the complex dielectric constant of an object of arbitrary shape. An algorithm for solving this problem is constructed by processing phase diffraction patterns obtained by successive irradiation of the object with Gaussian beams. Formally, we deal with the coefficient inverse problem for the three-dimensional parabolic wave equation or the equivalent inverse problem of quantum scattering theory for a particle moving in a two-dimensional time-dependent potential. To solve the latter, the wave function of the system is expanded in terms of functions of Gaussian beams propagating in free space. The main advantages of the approach are the direct determination of the refractive index along with absorption, as well as the elimination of rotation or movement of the sample and radiation source; its further development can lead to the emergence of a qualitatively new nondestructive method for studying and testing materials and samples.

The results of studying the effect of metal coatings on the process of silicon crystallization on flexible polymer substrates are presented. The study was conducted using such metals as aluminum, tin, nickel, and molybdenum as examples. Crystallization was carried out by the method of metal-induced laser-stimulated crystallization (MILSC). To determine the degree of influence of a metal on the crystallization process, the silicon structure after processing was investigated by the Raman scattering method.

The design and application of reflectometers with a grazing incidence grating for the wavelength region shorter than 300 Å are discussed. The focus is on reflectometers with laser-produced plasma sources excited by a pulsed-periodic laser with “moderate” output parameters (pulse energy of 0.1 to 1 J, pulse duration of 10 ns or less). Our findings can be useful for the development of metrological facilities in the soft X-ray range of the spectrum, both laboratory-based and included in commercial production of integrated circuits by projection X-ray lithography.

The paper presents a concept of laser fragmentation of metal nanoparticles in liquids by nanoseconds pulses and demonstrates evolution of individual fractions in the nanoparticle size distributions. The concept is based on the capillary-wave mechanism for the formation of fractions of fragments of initial particles. The presented results of theoretical simulation are in good agreement with the experimental data.

The results of studies of the copper spectra in the soft X-ray range in two fundamentally different (they differ in the heating method) high-temperature dense objects with high energy densities, laser plasma and a hybrid X-pinch, are presented. The uniqueness of the conducted experiments is that both sources were created at the Lebedev Physical Institute, Russian Academy of Sciences (FIAN), and are currently actively operating. The use of the same measuring equipment, which was also invented at FIAN, in these experiments made it possible to completely eliminate the influence of the conditions for observing and recording spectra on the obtained results. Calculations of the spectra have been performed using the radiation–collision model, from which it follows that only if a large number of transitions followed by averaging the obtained numerical results are taken into account, the results can be compared with the experimental data. It has been shown that it is necessary to take not only the plasma temperature and density into account when comparing the theory and particular experimental results, but the plasma size and shape must also be considered. Only in this case, it becomes possible to simulate the experimentally obtained results with satisfactory accuracy. In this case, it is possible to obtain information on the size of the emitting plasma region, which was never done previously.

A selective laser etching (SLT) technology has been developed and successfully applied for precision processing of quartz glass up to 2 mm thick. The technology has demonstrated efficiency for two operations: cutting and punching holes. The following parameters have been achieved: roughness, Ra = 0.2 μm and Rz = 0.4 μm; deviation from perpendicularity, 1°; and cutting width, from 10 to 45 μm. Technological intervals for pulse energy, speed, and frequency at which selective etching is possible have been identified. The maximum etching rate of laser-modified quartz glass with a hydrofluoric acid solution is 225 μm/h, indicating a selectivity of about 47 compared to the original glass.

A consistent description of terahertz (THz) radiation generation by laser-accelerated electrons from a target irradiated by a short light pulse requires a complete understanding of the contribution of all processes of interaction between emitted relativistic electrons and the target, taking into account the arising self-consistent fields. Despite the fact that the dominant role in this radiation is assumed a priori to be due to transition radiation from electrons leaving the target, in addition to this mechanism, it is necessary to estimate a set of such processes as bremsstrahlung of trapped electrons accelerating and decelerating in the Debye sheath, as well as their transition radiation and synchrotron radiation during the rotation of the double sheath field. This paper, which relies on a physically justified qualitative analysis, does not pretend to be a strict quantitative calculation and uses methodological techniques that aid understanding. Estimates are also presented that allow one to assess the characteristics of the generated THz pulse.

Semiconductor lasers are actively used for optical pumping of various solid-state lasers such as Nd:YAG, Nd:YVO₄, Ti:Sapphire, Cr:Colquiriite, and Cr:Alexandrite among others. Diode pumping reduces considerably the overall dimensions of laser systems and cuts their cost. There have been repeated attempts to excite dye lasers with semiconductor lasers, but the efficiency of such systems was extremely low due to lack of pump radiation power. Acceptable results were produced only after the advent of high-power visible semiconductor lasers (445 nm and 520 nm, Nichia). Application of various methods of pumping by semiconductor lasers produced wide tuning ranges of laser wavelength (~200 nm), achieved high-efficiency (26%) lasing, carried out diode pumping of a continuous dye laser, and demonstrated mode-locking (pulse duration of about 200 ps). The following is an overview of research on diode pumping of dye lasers.

We report a study of electron beams and gamma radiation generation from targets with a preplasma and from targets with an extended plasma of near-critical density on the irradiated side. It is shown that for a laser pulse with an energy of about 1 J, a uniform layer of critical density makes it possible to generate a significantly larger number of high-energy electrons compared to targets with an exponentially decaying preplasma profile produced by the prepulse. The bremsstrahlung gamma radiation spectra of accelerated electron beams in a converter target are constructed, and the possibility of converting the laser pulse energy into gamma radiation energy (with photon energies above 1 MeV) at a level of 5% is demonstrated.