Bulletin of the Lebedev Physics Institute最新文献

The spatial distribution of the electric field of a light wave in ensembles of silicon nanowires (SiNWs) coated with gold nanoparticles (AuNPs) is calculated. It is found that, compared to the case of nanowires without nanoparticles, the electric field of a light wave in a SiNW/AuNP composite structure significantly increases, which is explained by the combined contribution of the enhancement of the local electric fields due to Mie scattering on silicon nanowires and the fields of localized surface plasmons in gold nanoparticles. It is shown that the local field enhancement is nonmonotonic and strongly dependent on the excitation wavelength in the 600–800 nm range, which is due to the manifestation of spectral maxima of elastic scattering of the excitation light in the silicon nanowire array. Enhancement also depends on the type of nanoparticle size distribution (monodisperse or lognormal) and is maximal at a nanowire space filling factor of approximately 50%. The obtained results explain the experimental data on giant Raman scattering in SiNW/AuNP structures excited by 633 nm light and point to further opportunities for improving the parameters of semiconductor–metal structures for use in optical molecular sensing methods.

NaGdF₄:Eu nanocrystals are known to be able to convert UV radiation into the visible range with a quantum yield close to 200% via the quantum cutting mechanism. According to the literature, quantum-cutting luminescence in NaGdF₄:Eu nanocrystals is caused by excitation at a wavelength of 202 nm. This way of excitation, however, is not suitable for creating solar light converters due to the absence of this ultraviolet component in the solar spectrum near the Earth’s surface. We have experimentally demonstrated and described the mechanism of conversion by NaGdF₄:5% Eu nanocrystals of ultraviolet radiation at a wavelength of 252 nm into visible emission of Eu³⁺ ions with a quantum efficiency of 188%. These quantum-cutting parameters are of interest for the development of light-converting coatings, for example, in green photovoltaics to improve the efficiency of solar panels.

Films consisting of arrays of MoS₂ crystallites (nanowalls), which are oriented perpendicular to the substrate, attract the attention of researchers as a promising material for creating photonic, optoelectronic, and sensor devices. The additional ordering of the structure of such layers by orientating nanowalls along a certain direction in the substrate plane allows creation of materials with unique characteristics. For their formation, we use the method of selective laser ablation of MoS₂ films formed via deposition from the vapor phase and consisting of nanowalls on the surfaces of silicon substrates. The parameters of a laser pulse, which lead to the emergence of the maximum degree of ordering of nanowalls in the film, are determined. The photoluminescence (PL) and Raman scattering (RS) have been studied. In particular, the degree of polarization of PL and RS signals was determined as a function of the angle between a selected direction in the film plane and the polarization of exciting radiation. It was revealed that exactly nanowalls are responsible for the optical properties of such films but not the underlying film. The possible applications of the materials obtained using the selective ablation method in photonics are discussed.

The paper examines the influence of the delivery scheme and spectral characteristics of remote optically pumped amplifiers (ROPA) on the maximum length of unrepeatered communication lines. It is established that in the backward ROPA (BROPA) scheme, signal amplification due to stimulated Raman scattering (SRS) in the working telecommunication fiber causes a small shift (~5 nm) of optimal pump wavelength toward shorter wavelengths relative to the maximum gain-to-noise ratio of the erbium-doped fiber (max[Gain_EDF(λ_pump)/NF_EDF(λ_pump)]). The pump delivery through the working fiber in the case of BROPA has been found to increase the transmission distance due to additional signal amplification caused by SRS and the absence of significant depletion of pump power delivered to the erbium-doped fiber. Conversely, when the signal and pump propagate forward (FROPA) along the working fiber, the pump power decays due to SRS, which makes its delivery via an additional fiber more efficient. It has been determined that for a pump spectral width exceeding 4 GHz, the effect of the Mandelstam-Brillouin stimulated scattering on pump power can be disregarded. Approximate formulas are derived for calculating the dependence of the BROPA gain on the microparameters of the erbium-doped fiber, the signal spectrum, and on the delivered power and pump spectrum subject to the amplified spontaneous emission.

We report the results of measurements of the specific output power of an optically pumped rare-gas metastable laser using an argon–helium mixture. Data obtained at two different facilities at the Russian Federal Nuclear Center (VNIIEF) (136 W cm^–3 at an average power of 4.5 W) and at the Samara Branch of the Lebedev Physical Institute (150 W cm^–3 at a peak power of 2.1 W) experimentally confirm for the first time the existing theoretical estimates of this parameter.

A holmium fiber laser and its ring cavity with the air path embedded inside is investigated, the path being a confocal system of two lenses. Different samples of a saturable absorber were placed at the center of the air path waist, including random single-walled carbon nanotubes (SWCNT) and those aligned along one of the directions in the SWCNT application plane. Alterations of lasing regimes and of the central radiation wavelength were demonstrated, and the self-starting characteristics of the holmium fiber laser were investigated using the intracavity z-scanning method on different SWCNT samples. In the case of a sample of randomly oriented SWCNTs in the cavity, a set of little different regimes was obtained with an average radiation power of ~7 ± 0.1 mW and the central wavelength of ~2075 nm. It was altered but slightly (less than 1 nm). When a sample of aligned SWCNT was used, the average radiation power varied from 7 to 7.5 mW, the central wavelength at the position of the optical waist was ~2038 nm, and it was altered in the range of ~2 nm. The pulse repetition rate in both cases was ~20.7 MHz. The minimum duration of the transfer process during the transition to continuous mode locking was 25 μs in the case of the cavity with a sample of aligned SWCNTs located at the optical waist, the average power density along the beam cross-section on the sample being ~1.1 W/mm². The position of the SWCNT samples relative to the optical waist determined the boundaries of mode locking with or without self-starting capability and the mode locking regimes of passing into continuous lasing.

A laser on a CdSSe single crystal with a wavelength of 623.5 nm with longitudinal two-photon pumping by radiation from a Q-switched Nd : YAP laser has been studied. When the single crystal was cooled to liquid nitrogen temperature, a peak power of up to 80 kW was achieved with a pulse duration of 7 ns and an efficiency of ~3%. The full divergence angle of the laser depended on the pump spot diameter and was approximately 30° at a 1-mm diameter. The power was limited by the crystal-surface destruction. Ways to improve the laser performance are discussed.

We report the method of modulation piezoelectric resonance laser calorimetry (MPLC) for studying the interaction of nonlinear optical crystals with laser radiation. The kinetics of the crystal heating power is measured during laser radiation propagation, and the method is based based on periodic irradiation of the sample and the isolation of the amplitude of its equivalent temperature oscillations. The theoretical justification for the MPLC technique and its approbation was performed by studying LBO crystal degradation under UV irradiation.

We report a comprehensive study of the spatiotemporal dynamics of miniature, broad-area, vertical-cavity surface-emitting lasers (VCSELs). The main attention is paid to the development of methods for stabilizing their output using external optical injection. A detailed analysis demonstrates that free-running VCSELs exhibit chaotic dynamics caused by modulation instability. This leads to beam filamentation and a significant deterioration in its spatiotemporal characteristics. An improved mathematical model is considered that takes into account key features of semiconductor active media, including the Henry factor, which allows the observed nonlinear effects to be adequately described. It is found that external optical control not only effectively suppresses the development of instabilities but also ensures the formation of controllable spatial structures. In particular, the possibility of generating regular patterns (stripe, ring, and hexagonal structures) and controlled switching between them is demonstrated. The obtained results are important for the development of stable compact laser systems for applications in photonics, optical information processing, and communication systems, where precise control of the spatial characteristics of radiation is needed.

A kinetic model has been developed for the optical cooling of the rotational levels of the CaO⁺ (X²Π, ({v}) = 0) molecular ion ground state. This was achieved by placing the molecular ion in an ion trap and subjecting it to laser and thermal radiation. The model includes (1) excitation of the electron level (B²Π, ({v}) = 8) by tunable broadband laser radiation with a frequency cutoff option and overlapping the bandwidth of spectral transition, (B²Π, ({v}{{'' }}) = 8) ← (X²Π, ({v}) = 0), with the ground state rotational quantum number values, J > J_m; (2) interaction with the thermal radiation from the medium; and (3) spontaneous radiative relaxation of excited states. The laser cutoff frequency coincided with the frequency of transition from the state of (X²Π, ({v}) = 0, J = J_m. The simulation includes 50 rotational levels for three electron states of X²Π, A²Σ⁺, and B²Π, 42 vibrational levels for X²Π and A²Σ⁺, and 9 vibrational levels for B²Π. The levels of the lower electron excited state were dispersed by inducing the radiation of the second laser at the transition (A²Σ⁺, ({v}{{'}}) = 1) → (В²Π, ({v}{{'' }}) = 8). The rates of radiation transitions between states were determined based on calculated values of Einstein coefficients and flux densities of laser and thermal radiation. It is shown that the rotational level populations are depleted by laser radiation in tens of milliseconds, simultaneously with a significant accumulation of the population of “dark” levels with J < J_m. At a spectral cutoff of 33256 cm^–1 (J_m = 0.5), the population of the X²Π state (({v}) = 0, J = 3.5) is eight times higher than the thermal state (T = 300 K).