Raman Scattering on the Effective Soft Mode for Lithium Niobate Crystals

The evolution of Raman spectra in a wide range of temperatures that includes the ferroelectric transition point in lithium niobate single crystals is studied for polarization geometry X(ZZ)Y. In this geometry, the soft mode responsible for the phase transition distinguished by 1A₁(TO)-type symmetry should appear in the spectra. Experimental studies show that the 1A₁(TO) mode interacts resonantly with nonfundamental modes in the low-frequency region of the spectrum. Near the ferroelectric phase transition point, an isofrequency opalescence effect is observed that consists of an abrupt increase in Raman signal intensity at fixed frequencies near the excitation line.     

Optical properties of lithium niobate and lithium tantalate crystals with impurities and defects

The photoinduced and Raman scattering in lithium niobate and lithium tantalate crystals with impurities and defects have been studied. An exciting laser beam propagated either along the ferroelectric Z axis or perpendicular to it. The conditions for exciting transverse and longitudinal polar optical modes in Raman spectra are established. The regularities of the excitation of Raman spectra in several polarization geometries (X(ZZ)Y, Z(XX, Y Y)Z, Z(XX, Y Y)Z, X(ZX)Y, X(ZX)X and X(ZX)X) have been investigated. Additional (extra) spectral lines are interpreted as a manifestation of a biphonon enhanced by the Fermi resonance and the result of violation of selection rules for pseudoscalar modes of the A ₂ type due to the reduction of the point symmetry group caused by the presence of impurities and defects in real crystals. The conditions for exciting coherent longitudinal and transverse modes in lithium niobate and lithium tantalate single crystals upon stimulated Raman scattering are analyzed. The temperature evolution of the spectra recorded in the X(ZZ)Y geometry near the ferroelectric phase transition point is explained based on the concept of effective soft mode and analysis of the isofrequency opalescence effect. Strong photoluminescence is found in copper-doped lithium niobate crystals.     

Reflection spectra of 1D photonic crystals based on aluminum oxide

Optical properties (transmission and reflection) of 1D photonic crystals (PCs) based on mesoporous anodic aluminum oxide with the lattice periods of 188 and 194nm are investigated. The experimentally measured reflection spectrum is compared in its first bandgap region with the theoretical dependence obtained from the dispersion relation for the 1D PC. Angular dependence is established for spectral positions of bandgaps in the 1D PC. A possibility of using mesoporous aluminum-oxide-based 1D PCs as narrow-band filters, narrow-band mirrors, and refractive sensors of molecular compounds is analyzed.     

On the possibility of detecting low-energy scalar and pseudo-scalar bosons

The properties of the conversion of “cold” extra-light scalar or pseudo-scalar bosons weakly interacting with material media to photons with energies of 0.001–1.0 meV are analyzed. Various possible experimental schemes including closed resonant cells at low temperatures and highly sensitive receivers of radio-frequency photons are presented. The existence of such elementary particles is predicted in various expansion versions of the “standard model”. Their direct or indirect detection would make it possible to clarify the nature of the “dark matter” phenomenon.     

Vibrational spectrum of Li2B4O7 crystals

The polarized infrared reflection spectra of Li₂B₄O₇ were studied in the spectral range 80–1600 cm^?1 and compared with Raman spectra. From the spectrum dispersion analysis, the frequencies, damping, and dielectric oscillator strengths were determined for all vibrational modes observed. A calculation of the effective charges and an analysis of the chemical-bond types of the Li₂B₄O₇ crystal structural units were carried out on the basis of the obtained data.     

Localized Second Optical Harmonic in Nonlinear Optical Ceramics Excited by a Femtosecond Laser

Optics and Spectroscopy - Second harmonic generation is studied in samples of LiTaO3 powder ceramics with different lithium concentrations (48.8 and 49.3%) under excitation by a femtosecond laser...     

Analysis of photoluminescence spectra of anthracene traces in photon traps

Photoluminescence spectra of small amounts of anthracene being in the photon trap, a specific type of the resonant cavity, were measured. In photon traps, radiation is input and output by optical waveguides. The use of ultraviolet light-emitting-diode sources of exciting radiation and a minipolychromator allowed photoluminescence spectra measurements in the range of 340–500 nm at an exposure of ～ 10^?3 s.     

Secondary emission of globular photonic crystals based on the opal-POPOP composite

The spectra of secondary emission of a globular photonic crystal such as the opal matrix filled with the POPOP aromatic compound (a known luminophore) and initial materials excited by semiconductor light-emitting diodes were studied. It was found that the luminescence spectrum of opal filled with POPOP significantly differs from luminescence spectra of POPOP itself and initial opal. It was shown that the observed luminescence in the visible region is mostly caused by three-photon parametric light scattering. In this case, the spectral shape is controlled by the photon density of states, differing from the photon density of states of pure opal. The shape of the secondary emission spectrum of artificial opal filled with POPOP was calculated. The effect of the photonic bandgap position on the intensity distribution of spontaneous emission of used luminophore was established.     

Two-photon-excited fluorescence in KTiOPO4 polycrystals

The spectra of two-photon-excited fluorescence in KTiOPO₄ were obtained at room temperature. A coppervapor laser was used as a source of excitation light and provided two emission lines (λ=510.6 and 578.2 nm). The laser operated at a high pulse-repetition rate (～ 10⁴ Hz) and featured a peak power of about 10⁴ W, average power of 1 W, and pulse duration of 20 ns. The fluorescence spectra of crystalline KTiOPO₄ are compared with the resonance fluorescence spectra of KTiOPO₄ at 4.2 K. The measured decay time of fluorescence was found to be less than 16 ns. The efficiency of conversion of the laser radiation to fluorescence was about 10^?10 under saturation conditions.