Bulletin of the Lebedev Physics Institute最新文献

The paper demonstrates the possibility of laser printing with hyaluronic acid-based hydrogel using femtosecond laser pulses and donor substrate without absorbing metal coating. Optimum parameters of laser exposure were determined to provide stable transfer wherein the initial velocity of microjets lies in the range of 60 to140 m/s. It is shown that when laser radiation is focused into the bulk of glass for the whole range of used laser pulse energies of 10 to 1000 μJ only an insignificant “swelling” of the hydrogel layer occurs without transfer. It is shown that photon impulses should be taken into account when using femtosecond radiation for printing. A needle hydrophone was used to estimate shock and acoustic waves that will affect living microsystems during the laser-based bioprinting process. When a femtosecond laser pulse with an energy of 250–1000 μJ is absorbed, a shock wave is formed, the amplitude of which can reach 10⁴ atm at a distance of ~20 μm from the optical axis. These findings may be useful for the development of laser bioprinting technology and its promising area of laser engineering of microbial systems.

The generation of optical radiation with a broadband comb spectrum in a ring fiber cavity with harmonic mode locking is discussed. The main element of the cavity is an active fiber Bragg structure whose spectral characteristics depend on the ambient temperature and mechanical actions. The influence of these parameters on the time and spectral characteristics of generated pulses was investigated within the framework of numerical modeling for the steady-state generation mode.

The sum-frequency generation of broadband CO laser radiation (λ ≈ 5–6 μm) in a new nonlinear BaGa₂GeS₆ crystal at type I and II phase matching at the variation of polar and azimuth angles of this crystal has been experimentally investigated. A small mismatch (~3°) between the values of measured and calculated phase matching angles was found. Also, based on experimental data and their comparison with calculations, a set of coefficients was determined of the nonlinear susceptibility tensor of BaGa₂GeS₆, which best describes the experimental results.

The study of scattering plane waves by a periodic array of parallel cylinders utilizes the method of cylindrical wave decomposition, thereby reducing the problem to a system of linear algebraic equations. This method proved to be particularly efficient when the diameter of cylinders is significantly less than the wavelength of incident wave, which results in a rapid diminution of expansion coefficients as the number increases, and facilitates computation in near field. Subsequent comparison with the results of other methods demonstrates a qualitative concordance thereby showing the efficacy of cylindrical wave decomposition in simplifying and accurately modeling wave scattering phenomena in near field in structured media.

The process of oxidation of cobalt (Co) nanowires leading to the formation of different oxides (CoO an Co₃O₄) was investigated. The transition from CoO to Co₃O₄ was studied by X-ray diffraction analysis and Raman spectroscopy techniques. Only Co₃O₄ reflexes were detected in the X-ray diffraction patterns after annealing nanowires at 360°C, which confirms the complete conversion to cobalt tetroxide. A similar result was observed upon exposure to laser radiation, where local heating induced by the laser also led to the transition from CoO to Co₃O₄. Two Raman spectra of cobalt tetroxide were demonstrated with a relative shift of 20 cm^–1 depending on the degree of crystallization at different excitation laser powers.

The paper describes an erbium fiber laser mode-locked with a semiconductor saturable-absorber mirror (SESAM) and made up entirely of polarization-maintaining fiber components. With the focal spot correctly adjusted on the semiconductor mirror, the laser operates in the harmonic mode-locking (HML) regime available over the entire pump power range up to ~355 mW with the supermode suppression level of less than 25 dB. In the HML regime, the laser can generate linear polarized pulses and provide a pulse repetition rate up to ~1145 MHz. We have shown experimentally that by injecting radiation from an external continuous laser directly into the fiber cavity we can improve the stability of laser operation in the HML regime while the level of supermode suppression is increased by 20 or 30 dB. In addition, it is shown that external injection can extend the pump power range available for the laser in the HML regime increasing the maximum pulse repetition rate up to ~2195 MHz. It is important to note that optical injection does not affect the high purity of the laser’s polarization state. The presented results of numerical modeling can qualitatively explain the effects observed experimentally.

Using numerical simulation in COMSOL Multiphysics software we demonstrate for the first time the possibility of selecting a fundamental whispering gallery mode in a spherical microresonator with a radius of 80 μm based on the absorption effect in a thin metal film with a gap located at some distance from the resonator surface. Q-factors of radial and azimuthal modes were calculated for modes with the azimuthal index m = 455. The results showed a high Q-factor ≈ 10⁹ of the fundamental mode including to internal loss and the possibility of ~100-times and 10-times suppression with a 5-nm layer of the Q-factor of higher radial and azimuthal modes, respectively. In addition, the paper shows the possibility of using this method for selecting the fundamental mode in a wide range of index m values while maintaining high Q-factor and suppression factor of higher modes.

We propose a new approach to simulating fiber-optic communication lines (FOCLs) using an amplifier model as a section of active fiber with saturated gain and a nonuniform spectrum, and report the development of an algorithm for constructing a train of dissipative solitons with a phase-shift keying (PSK) modulation format for transmitting data over FOCLs. By establishing periodic dynamics of the parameters of carrier pulses in the amplification section, it is possible to transmit signals over long distances without reducing efficiency. The stability of this approach to adding amplifier noise is demonstrated.

We discuss the mechanism of generation at the С³Π_u → B³Π_g transition of the second positive system of nitrogen molecule bands when pumped by a pulsed longitudinal inductive discharge. Based on an analysis of amplitude-time characteristics of spontaneous emission and lasing pulses from nitrogen molecules and of longitudinal inductive discharge current compared to similar characteristics at nitrogen pumping by a classical longitudinal electric discharge, it was found that the main mechanism for the formation of population inversion and laser generation at the С³Π_u → B³Π_g 2⁺ transition of the N₂ band system is direct excitation of nitrogen molecules by electron impact from the ground state.

An argon gas-discharge fiber laser is demonstrated for the first time. Laser generation at wavelengths of 2208 and 2397 nm corresponding to the 3d → 4p' transitions in argon atoms was achieved by using microwave radiation (2.45 GHz) to maintain the gas discharge in a revolver-type hollow-core fiber filled with a He–Ar gas mixture. The spectral and temporal characteristics of laser output radiation depending on pump power and gas mixture pressure were investigated.