Edge photoluminescence spectra and the intensity of the intracenter <Emphasis Type="Italic">f</Emphasis>-<Emphasis Type="Italic">f</Emphasis> transitions in Er-and Sm-doped GaN crystals

Doping of GaN crystals prepared by various methods (HVPE and MOCVD) with various degrees of perfection of the mosaic structure, using rare-earth (RE) ions has been studied. An analysis of the shape of the photoluminescence spectra obtained before and after the doping showed that, as the defect concentration decreases, the intracenter f-f transitions characteristic of RE ions, at 1.54 and 0.54 μm in Er³⁺ and 0.72 μm in Sm²⁺, become observable. The intracenter f-f transitions of RE ions are seen, as a rule, in epitaxial layers with well-aggregated and relaxed domains and are absent in the case of a mosaic structure containing domains in the near-surface part of the epitaxial layer that are not fully coalesced. RE doping of the crystals under study was observed to initiate defect gettering. __________ Translated from Fizika Tverdogo Tela, Vol. 46, No. 5, 2004, pp. 814–819. Original Russian Text Copyright ? 2004 by Krivolapchuk, Lundin, Mezdrogina, Nasonov, Rodin, Shmidt.     

Dynamics of erosion plasma formation and formation of a plasma flame with a condensed disperse phase when quasi-continuous laser radiation acts on bismuth

The dynamics of near-surface plasma formation and the formation of an erosion plasma flame was investigated for quasi-continuous laser radiation (λ=1.06 μm, q=0.1–10 MW/cm², τ∼1.5 msec) acting on bismuth targets in air. The absence of low-threshold plasma formation at q<2 MW/cm² was established and explained. Instabilities in the plasma formation were revealed and the range of laser radiation power densities (2≤q≤4 MW/cm²) at which these instabilities take place was determined. At q≥5.4 MW/cm², brightening of the flame was noted. The dependence of the luminescence temperature of the flame on the laser radiation power density q was determined. Institute of Molecular and Atomic Physics, National Academy of Sciences of Belarus, 70, F. Skorina Ave., Minsk, 220072, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 1, pp. 126–133, January–February, 1998.     

Program for Experimental Investigations of the Mechanisms for the Emission of Coherent Radiation by Relativistic Electrons with Energies of 10–500 MeV in Oriented Crystals

A program of experimental investigations of the mechanisms for the emission of coherent radiation by electrons in periodic structures is proposed. The program is aimed at developing sources of energy-tunable quasimonochromatic x radiation and radiation.     

Suppression of the polarization bremsstrahlung from a fast charged particle in an amorphous medium

Based on a consistent quantum-mechanical approach, we calculate and analyze the suppression of the polarization bremsstrahlung (PB) from a fast charged particle moving in an amorphous medium. The suppression effect is attributable to destructive interference between the contributions from atoms of condensed material to the bremsstrahlung due to their finite sizes and chaotic arrangement. We calculate the spectral and angular PB dependences on various targets that demonstrate characteristic features of the phenomenon under consideration. We have established the region of the clearest manifestation of the suppression effect. The results obtained by various methods of calculation are compared.     

Enhancement of Optical Second Harmonic Generation in Hybrid Plasmonic–Photonic Microcavities

A one-dimensional combined plasmonic–photonic system tailored for the enhancement of nonlinear optical effects is proposed and implemented. In this structure, a layer of silver plasmonic nanoparticles is placed in the cavity region of a photonic-crystal microcavity made of porous silicon. A multifold increase in an optical second harmonic generation signal in comparison to the structure without plasmonic nanoparticles is demonstrated.     

Anisotropy of torsional rigidity of sheet polymer composite materials

Wide application of polymer composite materials (PCM) in modern technology calls for detailed evaluation of their stress-strain properties in a broad temperature range. To obtain such information, we use the dynamic mechanical analysis and with the help of a reverse torsion pendulum measure the dynamic torsional rigidity of PCM bars of rectangular cross section in the temperature range up to 600 K. It is found that the temperature dependences of the dynamic rigidity of the calculated values of dynamic shear moduli are governed by the percentage and properties of the binder and fibers, the layout of fibers, the phase interaction along interfaces, etc. The principles of dynamic mechanical spectrometry are used to substantiate and analyze the parameters of anisotropy by which the behavior of a composite can be described in the temperature range including the transition of the binder from the glassy into a highly elastic state. For this purpose, the values of dynamic rigidity are measured under low-amplitude vibrations of the PCM specimens with a fiber orientation angle from 0 to 90°. It is shown that for unidirectional composites the dependence between the dynamic rigidity and the fiber orientation angle is of extreme character. The value and position of the peak depend on the type of the binder and fibers and change with temperature. It is found that the anisotropy degree of PCM is dictated by the molecular mobility and significantly changes in the temperature range of transition of the binder and reinforcement from the glassy into a highly elastic state (in the case of SVM fibers). The possibility of evaluating the anisotropy of composites with other reinforcement schemes, in particular, of orthogonally reinforced PCMs, is shown.Translated from Mekhanika Kompozitnykh Materialov, Vol. 35, No. 3, pp. 291–308, May–June, 1999.