Bulletin of the Lebedev Physics Institute最新文献

We report an experimental investigation of a method for controlling the positions of energy subbands in a wide quantum well by introducing thin, tunnel-transparent barriers. Transmission electron microscopy and X-ray diffractometry measurements demonstrate the high epitaxial quality of the grown GaAs/AlGaAs heterostructures of various configurations, including those containing ultrathin layers (1–2 nm thick). Low-temperature photoluminescence measurements indicate a shift in exciton recombination energy toward higher energies as the number of introduced barriers increases. It is found that the experimental and theoretical energies of the lowest electron subband and the highest hole subband are in qualitative agreement and cover a wide range, extending from the bottom of the quantum wells up to the continuum.

Particle-in-cell (PIC) simulations of laser-plasma interaction using cylindrical geometry with azimuthal mode decomposition (AMD) significantly reduce computational costs compared with 3D simulations; however, for regimes in which axial symmetry is broken, the simulation accuracy depends on the number of modes (m) taken into account. This paper presents a systematic analysis of the AMD approach for two laser-plasma acceleration regimes: conventional laser wakefield acceleration (LWFA) in underdense plasma (n_e = 0.005n_cr) and self-modulated laser wakefield acceleration (SM-LWFA) in higher-density plasma (n_e = 0.03n_cr). It is shown that for axially symmetric LWFA, two modes (m = 2) are sufficient. For SM-LWFA, which is characterized by the development of symmetry-breaking instabilities, the requirements for the number of modes are more stringent: accurately reproducing the energy spectrum of accelerated electrons requires four modes (m = 4), while reproducing the angular spectrum requires twelve modes (m = 12). Even with twelve modes retained, the computational cost of the AMD simulation is approximately eight times lower than that of a full 3D simulation. These results are of particular importance for high-repetition-rate terawatt laser systems, in which SM-LWFA is the dominant acceleration mechanism, enabling higher acceleration gradients and larger beam charge.

We report a method for generating optical radiation with a rotating polarization vector and a controlled phase shift by varying the acoustic frequency during acousto-optic (AO) interaction. An AO cell fabricated from a uniaxial gyrotropic TeO₂ crystal is used in the experiments. The phase of polarization rotation of optical radiation with a wavelength of 0.63 μm changes from 0° to approximately 750° as the acoustic frequency varies from 30.8 to 31 MHz.

This paper demonstrates a several-fold increase in the productivity of femtosecond laser synthesis of elemental boron-10 nanoparticles in liquids by incorporating a cylindrical lens into the laser focusing system and employing a heterogeneous target fabricated by high-temperature sintering and pressing of the initial powder material. The enhanced productivity is achieved by optimizing the particle-size distribution of ablation products through suppression of cavitation-induced shock waves at the heterogeneous target surface while simultaneously increasing the ablation spot area, thereby enabling more efficient utilization of the laser energy.

Based on a simple model of radiation–matter interaction that ignores surface effects, we discuss the determination of optical constants in the soft X-ray range. For long-wavelength ranges, more rigorous models that take into account interfaces are considered: a plane-parallel plate and a free-standing surface. Since coherent imaging methods require calculating the field on the output surface of the sample, we perform a numerical experiment simulating the image of a standard test object. The purpose of the numerical experiment is to evaluate the accuracy of the methods and the required computational effort.

We report an experimental study of highly transient stimulated Raman scattering (SRS) in a BaWO₄ crystal pumped by a subpicosecond ytterbium fiber laser at a wavelength of 1030 nm, with adjustable pump-pulse chirp and pulse durations ranging from 0.25 to 6 ps (positive and negative external chirp). It is shown that, in agreement with theoretical predictions, SRS is observed when the chirped pump pulse duration exceeds 0.73 ps—allowing the system to overcome the suppressive effect of Kerr nonlinearities; otherwise, up to 62% of the pump energy is converted into supercontinuum. The optimal pulse-duration ranges for chirped pump pulses are found to be 2–5 ps and 1–4 ps for positive and negative external chirp, respectively. In these ranges, efficient multiwavelength SRS generation occurs, involving both a high-frequency shift associated with the strong totally symmetric vibrational mode (925 cm^–1) and a low-frequency shift associated with the weak bending vibrational mode (332 cm^–1), with a total SRS conversion efficiency of 31.5% for the first Stokes components (wavelengths of approximately 1138 and 1066 nm). The shift of this optimal range toward lower pulse durations with negative external pump-pulse chirp is attributed to a reduction in the resulting chirp caused by the presence of positive internal chirp induced by nonlinear self- and cross-phase modulation effects. Adjustable pump-pulse chirp also enables the controlled tuning of the wavelength of the first Stokes component of high-frequency-shifted SRS radiation within the 1136–1141 nm range.

This study investigates methods for enhancing the ultimate sensitivity of sensors based on passive ring resonators by utilizing the properties of PT-symmetry. A simple PT-symmetric system is considered, consisting of two parallel waveguides—one providing optical gain and the other introducing optical loss—where the passive waveguide forms part of a ring resonator coupled to the system. This configuration enables an increase in the ultimate sensitivity of sensors based on passive ring resonators. The system can operate in several regimes. The first regime is realized when the gain of the active waveguide does not exceed the coupling coefficient between the parallel waveguides. This regime makes it possible to increase the ultimate sensitivity by more than an order of magnitude—specifically, by a factor of 12 in the example analyzed. The second regime is obtained at an optimal ratio of system parameters. Theoretically, this regime enables an almost unlimited increase in the slope of the transmission coefficient of the system near frequencies lying between the resonator eigenfrequencies, thereby increasing the ultimate sensitivity. However, the amplification of the light passing through the system also increases almost without limit, which may hinder the practical implementation of this regime. Nevertheless, in practice, the use of the second regime can be appropriate when operating the sensor near the resonator eigenfrequencies, where the system does not amplify the transmitted light. In the example in question, this enabled a 76-fold increase in the ultimate sensitivity. The influence of the system parameters on the adjustment of these regimes is also analyzed.

The penetration into GaAs of an ultrashort laser pulse with a carrier frequency close to the absorption edge has been studied. It is shown that the frequency dispersion of the dielectric permittivity must be taken into account when describing penetration of such a pulse. It has been established that due to a sharp increase in the absorption coefficient with frequency increasing, as the ultrashort laser pulse penetrates into GaAs the maximum of the spectral energy density of the pulse shifts to the low-frequency region. Consequently, the shorter the pulse, the deeper it penetrates into GaAs, since a significant portion of the short pulse energy is concentrated at low frequencies. As the pulse penetrates deeper into the semiconductor, its shape is primarily determined by harmonics with frequencies lower than the frequency corresponding to the bandgap. In the considered frequency range, the group velocity in GaAs at high frequencies is lower than at low frequencies. Because of this, a small peak in the tail of the ultrashort laser pulse is formed, the main contribution to which comes from high-frequency harmonics that have a lower group velocity. The change in both the spectral composition and the shape of the reflected pulse is minor.

Atypical effects have been discovered in the optical spectra of a series of single-mode vertical-cavity surface-emitting lasers near the threshold lasing current, including a large number of relaxation sidebands of comparable amplitude, a change in the nature of the spectrum asymmetry with increasing pump current, and distortion of the Lorentzian profile and its transformation into a double-peaked structure in the initial stage of evolution for one of the laser samples. A qualitative explanation is provided for some of these effects. The possibility of developing a theory for an appropriate description of the spectra in the near-threshold current region is discussed.

This paper presents numerical simulation and performance analysis of a rotary valve in a single-stage Gifford-McMahon cryocooler with a mass flow rate of up to 9.5 g/s. The study aims to improve the system’s heat-transfer efficiency and reliability by optimizing the valve geometry and phase parameters. It is shown that using a symmetric window geometry with 105° inlet and 115° outlet phase angles ensures stable cycle operation at 1.2 Hz. The proposed configuration minimizes wear on rubbing surfaces and increases the service life of the mechanism compared to traditional asymmetrical solutions.