S. K. Evstropiev

Spectral properties of ZnO and ZnO-Al2O3 coatings prepared by polymer-salt method

Abstract. Experimental results show that the use of film-forming solutions based on zinc and aluminum nitrates and high molecular polyvinylpyrrolidone allows obtaining thin oxide coatings having unusually large bandgap values and transparent in a wide spectral range. The values of the bandgaps of coating materials are significantly higher than the bandgap of bulk ZnO, and it is varied in the range from 3.46 to 4.16 eV, resulting from the metastable structure of the coating.

2-Fold inner cladding symmetry design for actualizing 100μm-core class double-clad large-pitch fibers with the preferential fundamental mode amplification property

Silica/air large-pitch photonic crystal fibers, known simply as large-pitch fibers (LPFs) due to their hole-to-hole spacing 10 times larger than the wavelength, performing mode-field diameters (MFDs) about 50 μm along with nearly diffraction-limited output beam quality provide a versatile platform for realizing compact and stable multi-kW average power and multi-mJ pulse energy ultrashort pulse fiber laser systems [1]. Major efforts have been devoted to improving mode delocalization phenomena in Yb-doped double-cladding LPFs with a view to reduce the spatial overlap of higher-order modes (HOMs) with the doped area [2]. However, the thermo-optic effect which changes the refractive index profile and reconfines a group of the most competitive HOMs (TE01, HE21, TM01) in the gain region jeopardizes the acceptable conditions for the practical fundamental mode (FM) operation and consequently power scaling capabilities of such fibers [3]. For this reason, development of new double-clad LPF designs providing extended HOMs delocalization properties under the extreme heat load is an urgent challenge in the field of fiber laser technologies.

Optical limiting of laser radiation in semiconductor monocrystals, quantum dots and multilayer microresonators in the visible and near infrared spectral range

Investigations of nonlinear optical properties and mechanisms of optical limiting in semiconductor materials are of persistent interest for the design of laser radiation protection elements. Studies into nonlinear optical properties of ZnS semiconductor monocrystals in visible range and InP in NIR range allowed to determine the main optical limiting mechanisms: two-photon absorption, free-carrier absorption and absorption in forming microplasma (at high fluencies). Theoretical (based on Z-scan measurements of nonlinear constants) and experimentally obtained values of the limiting threshold for monocrystals are: 2 J/cm2 and 0.5 J/cm2 correspondingly (ZnS, λ = 532nm), 0.2J/cm2 and 0.1J/cm2 correspondingly (InP, λ = 1540nm). Polyvinylpirrolidone-stabilized ZnS quantum dots (QD)-based composite media synthesis helped to enhance optical limiting characteristics, in particular, to shift the UV edge of transmittance due to an increased band gap from 3.6eV for ZnS monocrystal up to 3.9 ÷ 4.7eV for Zn QD owing to sizes of synthesized QD (1.4-2.0nm). Introduction of C60 fullerene, perylene, tetramethylbenzidine and their combinations into CdSe/ ZnS QD colloid solutions led to formation of hybrid photoactive structures (photoreactors) in the form of donor-acceptor complexes in the ground state, and thereby to widening of spectral rage into the IR region, and also to a decrease of the limiting threshold on the grounds of “light quenching” mechanism. To solve an issue of low-threshold limiting, we worked out multilayer microresonators with a nonlinear layer visible and NIR limiters. To set up a low-threshold NIR limiter we employed a technique of GaAs/AlAs layers molecular epitaxy on a GaAs substrate. It gave way to construction of a resonator with transmittance line width 2.5nm at λ = 1117nm. A shift of the transmittance line maximum induced by the laser radiation owed to changes of the refraction index, the latter phenomena being connected with two-photon absorption and free-carrier generation. The said shift helped to get optical limiting with a low threshold value at a level of 10-3J/cm2.

Spectral and optical limiting properties of ZnS nano- and bulk crystals

Some features of spectral and non-linear optical properties of ZnS quantum dots stabilized by high-molecular polyvinylpyrrolidone have been studied. It is shown that the absorption spectra of ZnS composite materials (sols, coatings) in UV spectral region are determined by quantum confinement effect, exhibiting the dependence of the absorption edge of the size of the ZnS nanocrystals.