Bulletin of the Lebedev Physics Institute最新文献

A scheme for transferring frequency stability from an ultrastable laser at a wavelength of 871 nm to a laser at a wavelength of 1550 nm has been implemented. The results of comparing the laser frequency at a wavelength of 1550 nm with the frequencies of two reference lasers showed that the developed system ensures a relative frequency instability less than 4 × 10^–15 for intervals from 0.4 to 2 s and less than 10^–14 for averaging intervals from 0.2 to 500 s. A femtosecond optical frequency comb and a laser with a reference cavity are designed to meet the requirements of compactness and portability for use in field and onboard applications.

Photonic analogs of graphene (waveguide lattices that imitate the electronic properties of graphene in the optical range) demonstrate the presence of special topological states (flat bands) due to the presence of conical Dirac points, which form a linear dispersion relation. Using the coupled-mode method for a graphene-like photonic lattice formed by a periodically repeating cell of six waveguides and taking into account the interaction of each waveguide with its nearest neighbors, with each other, and between lattice cells, we derive a dispersion relation and analytical expressions for the amplitudes of propagating waves. This allows the emergence of light localization to be accurately predicted as a functions of the values of the coupling constants that describe highly localized radiation propagating without transverse diffraction along the graphene-like lattice. It is shown that the spectrum of linear waves in the system (in the strong coupling approximation) consists of four dispersion bands and two flat bands.

We report a detailed study of line X-ray emission from a nanosecond laser-produced copper plasma. The plasma is generated by a laser pulse (0.53 μm, 3 J, 2 ns) focused onto a massive target. The spectra are measured using a focusing crystal spectrometer. It is shown that the main contribution to the spectrum intensity is made by the satellite lines 1s2sⁿ2p^m – 1s²2sⁿ2p^{m – 1}, occupying a narrow wavelength range (Δλ/λ = 0.035) near the resonance line of the [He]-like ion CuXXVIII (λ ∼ 1.5 Å). Using X-ray spectroscopy methods, the electron temperature of T_e = (1.4 ± 0.1) keV is measured, the relative intensities of lines of different ionization multiplicities (from the [He]-like ion CuXXVIII to the [O]-like ion CuXXII) are determined, and the absolute X-ray yield of X = 3 × 10⁹ photons/sr is measured.

We propose using two acousto-optic (AO) Bragg diffraction regimes, arising in a single AO cell at the same sound frequency but at different angles of light incidence, for processing two-dimensional images in two channels. It is shown that these regimes are most easily implemented in the region of extrema of the frequency-angular function f(Θ), where f and Θ are the sound frequency and the angle of incidence of light, respectively. For channels to be formed, it is proposed to use the Bragg polarization splitting (BPS) regime, whose parameters lie in the region of the minimum of the f(Θ) function. Image contours in the +1st and –1st diffraction orders are experimentally formed using a TeO₂ AO cell operating in the BPS regime. The wavelength of the optical radiation is 0.63 μm, and the sound frequency is 11.2 MHz.

The numerical values of the van der Waals interaction constant C₆ are determined between alkaline earth atoms in singlet Rydberg |n¹S₀〉 states with large principal quantum numbers, n > 20. The results of calculations using the semiempirical quantum defect theory (QDT) method are approximated by quadratic polynomials. The polynomial coefficients are tabulated and can be used for simplified quantitative estimates of interatomic interaction-induced energy shifts, determination of the Rydberg state excitation blockade radius, and limitations on the wavelengths of radiation forming Rydberg lattices for group IIA atoms. The numerical values of the contributions of various diatomic states to the constant C₆ are calculated. States providing the main contribution, analogous to that of the Förster resonance, are identified.

We report an experimental and theoretical study of the mechanisms of optical losses in a quantum-well waveguide structure for indium phosphide-based electro-optical modulators. It is shown that the presence of an InGaAs cap layer with a high refractive index and high optical loss can lead to anomalously high attenuation of the fundamental TE₀ mode. The attenuation is observed near the degeneracy of the dispersion curves of the first two TE polarization modes, which manifests itself at InGaAs cap layer thicknesses of approximately 0.31 μm. The numerical simulation data are in good agreement with the experimentally measured optical losses in such structures.

The paper presents the first study of the formation of disordered arrays of dynamic microcavities under four-pulse excitation of a two-level resonant medium by a train of single-cycle attosecond pulses. The pulses act on the medium similarly to 2π pulses of self-induced transparency. It is shown that multiple pulse collisions in different regions of the medium result in the formation of localized dynamic structures arranged in a disordered manner. The properties of these structures can be rapidly modified by controlling the time delays between the pulses. The results open up new possibilities for research in the field of disordered media, where localization of light and control of dynamic structures are of particular interest.

We examine the effect of electron–ion collisions on the generation of THz radiation from a magnetoactive plasma exposed to a femtosecond laser pulse when the laser frequency exceeds that of the plasma. The frequency ranges for which collisions must be taken into account are identified. The total energy of low-frequency radiation is found to reduce down to 30% due to collisions in the most interesting case for generation, where the laser pulse duration is on the order of the inverse plasma frequency.

We examine the main mechanisms limiting the optical power of InGaAsP/InP semiconductor lasers operating at 1.55 μm at high pump currents. Analysis is performed using a numerical calculation of a two-dimensional laser diode model, taking into account transverse drift-diffusion transport and nonuniform distribution of charge carriers and photons along the cavity axis. The calculation results demonstrate the dominant influence of internal losses from scattering by free carriers in the waveguide layer on saturation of the output laser power. As the pump current increases, the primary saturation mechanism becomes leakage current, generated by electron transport into the p-emitter and reducing the internal quantum efficiency. Analysis also shows that the uneven distribution of photons and gain along the cavity axis contributes to the limitation of the output laser power; however, this effect does not significantly affect the primary mechanisms—leakage current and free-carrier losses.

We report the results of studying X-ray emission spectra in the energy range 0.2 keV ≤ hν ≤ 20 keV from the plasma of a laser-induced spark discharge with a current of up to 30 kA. The X-ray energy spectra are measured using an absorber method with a set of lithium fluoride LiF(Mg,Ti) thermoluminescence detectors. X-ray measurements by a pin diode are performed as a supplementary diagnostic. It is shown that the X-ray spectra exhibit noticeable differences at different voltage polarities on high-voltage triggering electrode, with intensity and stability being higher across the entire studied range upon initiation at cathodes.