Bulletin of the Lebedev Physics Institute最新文献

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.

We report an efficient noniterative numerical algorithm for solving a system of linearly coupled nonlinear Schrödinger equations (NLSE) describing the propagation of light pulses in multicore optical fibers, with optical losses and saturated gain taken into account. The method can also be used in the case of a single scalar NLSE. The proposed algorithm is based on the split-step Fourier method and has the second order of accuracy in the evolutionary variable and the exponential order of accuracy in time. To solve the system of coupled NLSEs, we propose to approximate the matrix exponential using the Pade approximation.

Using the literature data on the calculated ab initio interatomic interaction potentials, we determine transport frequencies of collisions of Cs atoms in the ground and excited states with atoms of inert gases He, Ne, Ar, Kr, and Xe (a temperature range from 100 to 2000 K). We report the calculated data for the relative difference in the transport frequencies of collisions, which are necessary for a quantitative description of the effect of the light-induced drift of Cs atoms in a buffer medium of inert gases. Analysis of the experimental data on the light-induced drift of Cs using the obtained results will make it possible to confidently select the most correct models of interaction potentials.

The processes of continuous photoabsorption by weakly bound NeAr⁺ molecular ions at room and elevated temperatures have been theoretically investigated. A comparative analysis of the relative contributions of four types of phototransitions (photodissociation, bound-bound transitions, free-free transitions and photoassociation) with different initial and final states of internuclear motion to the total absorption coefficient at room and elevated temperatures was made. It is shown that it is possible to describe under certain assumptions the above contributions under quasi-equilibrium conditions without resorting to direct calculation of photoabsorption coefficients for each of the channels. It is demonstrated that the relative efficiencies of different types of phototransitions in the NeAr⁺ ion radically differ from previous results for homonuclear noble gas ions and for ArXe⁺ and KrXe⁺ ions. The dependences of these contributions on the wavelength of the incident radiation and on the gas temperature of the plasma were investigated. It was found that there are well-defined regions corresponding to the predominance of a particular channel of phototransitions within the wavelength range where efficient absorption occurs. The presented results are of interest for the construction of kinetic models of pumping and excitation relaxation in gas lasers based on active media containing Ne/Ar mixtures.

We have performed a theoretical analysis of the extinction spectra of light in hybrid plasmon–exciton nanosystems. Within the framework of the quasistatic approximation, an analytical formula is derived that explicitly defines the dependence of the effective cross section of light extinction by a system of coupled oscillators on their eigenfrequencies and damping constants as well as on the coupling constant and the frequency of the incident light. The theory has been applied to the analysis of light absorption by specific plasmon–exciton nanosystems. These systems consist of a spherical and rod-shaped metal core coated with an outer layer of an isotropic molecular aggregate of a dye. We have shown that the results of the work can be used for quantitative interpretation of the available experimental data on the light extinction spectra in the regimes of weak and strong plasmon–exciton coupling. Studying the effects of plasmon–exciton coupling is of particular interest for the development of hybrid nanowaveguides, highly sensitive photodetectors, and optical sensors.