Bulletin of the Lebedev Physics Institute最新文献

We report numerical analysis of the effect of pump divergence on thermo-optical distortions in an Nd:YAG crystal. Thermally induced distortions are shown to depend on the focusing depth, power, and divergence of the pump beam under longitudinal diode pumping. The focal length of a thermally induced lens increases by ∼36% when the pump beam divergence is taken into account. Increasing the pump focusing depth by 1–2 mm into the crystal leads to an increase in the optical power of the thermal lens by approximately 12%. The obtained results can be used in laser development.

The comprehensive investigation of scanning strategies of the multitrack layers produced by laser metal deposition (LMD) with coaxial powder feeding is presented. The first two steps of the part additive production are optimized using a previously developed hydrodynamic model in search for low waviness and microstructure control. We verify the numerical model for the single-track LMD showing rather good correspondence. The main parameters of the scanning strategy i.e. scanning path, overlap ratio and idle time are optimized for additive manufacturing (AM) applications. We show that the optimal overlap ratio is dependent on processing parameters and multitrack investigation is needed for its estimation. A new method for optimizing layer growth parameters is proposed, which is in the adjusting of the geometry of a single track when the process conditions change instead of searching for new hatch spacing. Feasibility for modifying the average crystalline size and the preservation of the layer geometry and scanning strategy is presented both numerically and in experiments. The possibility of controlling the grain size of similar layers is another step for building parts with a defined quality.

Laser lithotripsy of gallstones, despite its effectiveness, carries the risk of thermal tissue damage due to heating of irrigation fluid (saline solution) by laser radiation. The paper presents the results of an experiment aimed at evaluating the dynamics of fluid temperature during 30 and 60 seconds of continuous laser emission under active irrigation (60 mL/min) and in a standing water environment at various average radiation power values. Laser exposure was performed using a new generation Urolase+ Premium thulium fiber laser (TVL) (VPG LaserONE, Russia). The average power varied between 5 and 70 W. Provided that irrigation fluid was continuously supplied at a temperature of 37°C, the maximum permissible radiation power that did not cause hyperthermia of the duct wall was 25 W for 30 and 60 seconds of activation. However, the use of room-temperature saline solution as an irrigation fluid at a flow rate of 60 mL/min ensures thermal safety during intraductal contact laser lithotripsy at all power levels of the Urolase+ Premium laser device, even during 60-second activations. In the case of intermittent water supply, it is recommended to continuously interrupt the laser radiation (emission duration no longer than 10 s) and follow it with subsequent irrigation to restore the optimal temperature.

Generation of Fourier-limited pulses with repetition rate higher than 1 GHz from a periodically phase-modulated CW laser field by phase manipulation of its spectral components is considered. Phase modulation in the form of modulus function of sine (MFS) is proposed. It allows producing pedestal-free pulses with controllable repetition rate and duty cycle. Sidelobes of these pulses can be excluded by choosing a proper modulation index. The proposed method allows producing pulses with 50% duty ratio. It is shown that three-tone modulation of phase allows simulating MFS phase modulation.

We report a study of the intensity distribution of spiral light beams in the form of a closed curve, constructed using a holographic method and a spatial light modulator. The limitations imposed on the spatial spectrum of the beams formed by a liquid crystal phase spatial light modulator are determined. Along with a qualitative assessment of the experimentally obtained intensity distributions of the studied beams, the values of the mean square error, peak signal-to-noise ratio, and structural similarity coefficient are presented.

We report a study of the vibrational spectra of ortho-xylene in pure form and in solutions with chloroform at room temperature and atmospheric pressure. Raman spectra of binary mixtures of ortho-xylene and chloroform with concentrations of 0.9, 0.7, 0.5, 0.3 and 0.1 mole fractions are analyzed. It is found that the 733 cm^‒1 band of symmetrical stretching vibrations of CH₃ groups in pure ortho-xylene shifts toward lower frequencies (728 cm^‒1) when chloroform is added to the solution. The main reason for the shift is the interaction of the π-electrons of the aromatic ring with the hydrogen of the CH-group of chloroform (CH…π interaction), leading to a decrease in the vibration frequency. Furthermore, a noticeable shift of the band associated with C–H stretching vibrations is observed in the region of 3011 cm^–1, which also indicates a change in the nature of the intermolecular interactions between the components of the solution. Furthermore, the spectrum reveals an additional shift in the region of 3044 cm^–1, confirming the influence of solvation effects: the C–H dipole of chloroform interacts with the π-electrons of ortho-xylene, changing the C–H bond length and, consequently, shifting the vibrational frequency toward higher frequencies. The exchange of charges between atoms during the formation of the complex is analyzed. The structural parameters of molecules, molecular electrostatic potential (MEP), and Mulliken distribution of atomic charges are studied using the density functional theory (DFT) method.

It is shown that many anomalies observed in underdoped cuprates, including anomalous spectral weight transfer and a large pseudogap, appear to have a common nature due to both the cluster structure of the underdoped phase and the specific mechanism of superconducting pairing. The combined action of these factors leads to the fact that at a temperature T lying in a certain temperature range T_c < T < T*, the crystal contains small isolated clusters that can exist both in superconducting and normal states, randomly switching between them. In this case, below T_c with a very high probability the cluster is in a superconducting state, and above T* it is in a normal state, and the interval T_c < T < T* is the region of the so-called pseudogap phase. At a given T in the same temperature range, the time sequence of randomly arising superfluid density pulses from each cluster can be represented as a random process. The effective width Δω_eff of the spectrum of such a random process will be determined by a correlation time, i.e. the characteristic time between successive on/off superconductivity in two different clusters. This time, according to the estimate, is ~10^–15 s, which corresponds to Δω_eff ~ 1 eV and explains the effect of spectral weight transfer to the high-frequency region. This approach also makes it possible to explain other anomalies observed in the vicinity of T_c: the reversibility of magnetization curves in a certain temperature range below T_c, the anomalous Nernst effect and anomalous diamagnetism above T_c.

The effect of the duration of ultrashort laser pulses with a wavelength of 1030 nm on the process of their nonlinear absorption in polymethylmethacrylate is studied. It is found that the dependence of absorption on pulse duration is nonlinear, with an increase in the range from 250 to 450 fs, a decrease from 450 to 1500 fs, and insignificant changes after 1500 fs.

The paper examines the evolution of weak perturbations excited at the boundary of a “warm” coronal loop in a strong magnetic field. The primary focus is on the effect of thermal misbalance caused by heating and radiative cooling processes on the dispersion properties and energy distribution between slow magnetoacoustic and entropy modes. An exact analytical solution to the boundary value problem is obtained using the Fourier method and Duhamel’s principle, allowing one to analyze the dependence of the energy distribution on the characteristic times of the thermal misbalance. It is shown that local heating and cooling parameters determine not only the phase velocities and wave decrements but also the initial energy contribution of each mode. The obtained results can be used to interpret observational data and diagnose plasma parameters in the solar corona.

Integral cross sections and mean energy loss of slow protons and electrons in water are calculated in the framework of the photo-absorption ionization model. The calculation results are compared with experimental data. Practical applications of the model are discussed.