Mechanisms of loss formation in nonlinear optical crystals ZnGeP2 in the terahertz frequency range

Physical mechanisms of formation of radiation losses in the terahertz range in ZnGeP₂ crystals have been experimentally studied in the wave number range of 5–350 cm^?1 at temperatures of 10–300 K. The dominant contribution of two-phonon difference processes to the loss formation in the given frequency range has been shown.     

基于GaSe和ZnGeP2晶体差频产生可调谐太赫兹辐射的理论研究

基于非线性光学频率变换理论,采用已报道的利用非线性光学差频方法产生可调谐太赫兹波的实验条件作为理论分析的实验模型,计算模拟出在不同相位匹配条件下,GaSe和ZnGeP2晶体差频的相位匹配角、走离角、允许角和有效非线性系数,并对计算结果进行了分析比较,总结出对应输出不同太赫兹波长的最佳相位匹配方式.计算结果为利用非线性晶体差频产生可调谐太赫兹辐射的实验研究提供深入和全面的理论基础.     

Behavior of impurity copper in ZnGeP2 single crystals with diffusion doping

Results from a study of electrophysical and luminescent properties of zinc-germanium diphosphide single crystals, diffusion doped with copper, are presented. The nature of the dominant defects formed upon copper diffusion in ZnGeP₂ is determined using a thermodynamic analysis of defect formation processes in ZnGeP₂ and a ZnGeP₂: Cu solid solution performed within the framework of the quasichemical analysis method. It is demonstrated that by choosing the copper diffusion method the hole concentration in the ZnGeP₂ can be varied over the range 10¹²–10¹⁶ cm^–3. A retrograde character was observed in copper solubility in ZnGeP₂ compounds. Defect formation processes in ZnGeP₂ upon copper diffusion depend on the degree of atomic ordering in the cation sublattice at the diffusion annealing temperature.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 64–69, February, 1985.     

Measurements of electron energy losses and VUV-reflectivity of anthracene single crystals

Anisotropic behaviour of the electron energy loss spectra is discussed in terms of the oriented gas model. These measurements are compared with electron energy loss spectra in the vapour phase and normal incidence VUV-reflectivity from the (ab) plane for 2 directions of polarization. The VUV measurements were made at DESY using synchrotron radiation.     

The effect of electron irradiation on some alkali halide single crystals

The investigated alkali halides (Nacl, KCl, RbCl and CsCl) decompose under electron irradiation into alkali metal colloids and chlorine gas. This decomposition is a special case of radiation damage effects, which strongly influence the crystalline structure and the physical properties of the crystals under investigation. The decomposition process consists of the breaking up of the single crystalline material into polycrystalline aggregates; at the same time cavities, or larger aggregates of vacancies are produced, which, together with the boundaries of the polycrystalline aggregates, serve as the nuclei of growing dark platelets, which appear to be precipitated alkali metal crystals.We wish to thank Prof. Dr. S. Lengyel, director of our laboratory, for his helpful assistance in securing the necessary means, and his interest. We further thank Mrs. L. Fülöp, who carried out the electron-microscopic preparatory work and helped most effectively in taking and evaluating the electron micrograms and diffractograms. Finally we wish to thank R. Voszka, head assistant of the University, and J. Kis, research physicist, for valuable discussions.     

Effect of high-energy electron irradiation on forsterite laser crystals

The effect of 21-MeV electron irradiation on the optical absorption characteristics of Czochralski-grown forsterite (Mg₂SiO₄) single crystals (both undoped and chromium-doped) has been investigated. The irradiation is found to induce additional optical absorption (AOA) in the crystals in the range of 225–1200 nm due to the formation of color centers based on intrinsic host point defects and the change in the oxidation state of chromium ions. The AOA spectra have been decomposed into elementary bands. The influence of the chromium concentration in crystals, the oxygen content in the growth atmosphere, and additional doping with lithium on the behavior of these bands has been analyzed. A possible structure of the color centers responsible for the AOA is discussed. It is shown that the electron irradiation somewhat decreases the intensity of the characteristic absorption bands of tri- and tetravalent chromium ions and gives rise to a new absorption band in Mg₂SiO₄:Cr and Mg₂SiO₄:Cr,Li crystals heavily doped with chromium.     

Effect of carbon substitution on low magnetic field AC losses in MgB2 single crystals

The DC magnetization and AC magnetic susceptibilities were measured for MgB₂ single crystals, unsubstituted and carbon substituted with the composition of Mg(B_0.94C_0.06)₂. AC magnetic losses were derived from the AC susceptibility data as a function of the AC amplitude and the DC bias magnetic field. From the DC magnetization loops critical current densities were derived as a function of temperature and DC field. Results show that the substitution with carbon decreases critical current densities at low external magnetic fields, in contrast to the well known effect of an increase of the critical current densities at higher magnetic fields.     

光子晶体对太赫兹波的调制特性研究

利用传输矩阵方法研究了掺杂半导体n-GaAs/聚碳酸脂一维光子晶体的太赫兹波透射谱.研究结果发现,与一般由两种介电材料组成的一维光子晶体不同,由于掺杂半导体中自由载流子对太赫兹波存在较强的吸收,所以这种材料组成的一维光子晶体除可形成光子带隙外,还可以增强n-GaAs对太赫兹波的透射.同时还提出了一种基于这种一维光子晶体的太赫兹波调制器,通过外加电压控制半导体中电子浓度的大小可实现对太赫兹透射波幅度的调制. 关键词： 掺杂半导体光子晶体 太赫兹波 太赫兹波的调制     

The temperature coefficients of delay of surface acoustic waves in LGS and LGN crystals in a wide temperature range

Numerical analysis of the first-and second-order temperature coefficients of the delay of surface acoustic waves (SAWs) in LGS and LGN crystals was carried out. The calculations were performed along thermostable directions in a wide temperature range. The effect of an aluminium layer having a finite thickness on the SAW temperature characteristics is shown.     

Effect of Ar ion irradiation on the room temperature ferromagnetism of undoped and Cu-doped rutile TiO_2 single crystals

Remarkable room-temperature ferromagnetism was observed both in undoped and Cu-doped rutile TiO_2 single crystals(SCs).To tune their magnetism,Ar ion irradiation was quantitatively performed on the two crystals in which the saturation magnetizations for the samples were enhanced distinctively.The post-irradiation led to a spongelike layer in the near surface of the Cu-doped TiO_2.Meanwhile,a new CuO-like species present in the sample was found to be dissolved after the post-irradiation.Analyzing the magnetization data unambiguously reveals that the experimentally observed ferromagnetism is related to the intrinsic defects rather than the exotic Cu ions,while these ions are directly involved in boosting the absorption in the visible region.     

Effect of high temperature electron irradiation in gold

Pure gold is electron irradiated with 2.8 MeV electrons at 450±50°C. Both electrical resistivity and transmission electron microscopy showed damage which anneals at about 800°C.     

Multiphonon absorption in a MgO single crystal in the terahertz range

The reflection and transmission spectra of a MgO single crystal at frequencies of 10–1000 cm⁻¹ in the temperature range 10–300 K have been measured using submillimeter and infrared Fourier spectros-copy. The evolution of anharmonic absorption with variations in temperature has been investigated in the frequency range below the frequency of the transverse optical phonon. The parameters of the dispersion model that adequately describes the lattice absorption of terahertz radiation in single-crystal magnesium oxide have been determined.     

Elastic,piezoelectric and dielectric properties of ZnO and CdS single crystals in a wide range of temperatures

A complete set of elastic, piezoelectric and dielectric constants of ZnO and CdS at room temperature was determined by the method of resonance-antiresonance. Elastic constants s^E₁₁, s^E₁₂, s^E₅₅, c^D₃₃, c^D₅₅, coefficients of electromechanical couplingk₃₁, k₁₅, k_t and dielectric constants ε^T₁₁, ε^T₃₃ of ZnO single crystals were determined in the temperature range 4.2–800 K. Elastic constants s^E₁₁, s^E₁₂, s^D₃₃, s^E₅₅, s^D₃₃, s^D₅₅, coefficients of electromechanical coupling k₃₁, k₃₃, k₁₅, k_t and dielectric constants ε^T₁₁, ε^T₃₃ of CdS single crystals were determined in the temperature range 4.2–300 K.     

Propagation characteristics of two-dimensional photonic crystals in the terahertz range

Based on the plane wave expansion method, the complete band gaps of two-dimensional terahertz photonic crystals with typical structures are optimized through varying structural parameters. During our calculation, two kinds of lattice structures that are very promising for the materials of terahertz components are found and some interesting phenomena that were not reported in the former papers are also presented. Using the finite difference time domain method, we simulated the electromagnetic field distribution of THz waves in a PC splitter. And by the plane wave expansion method, we achieved the dispersion relation and defect modes in a rotated PC waveguide. These results provide a useful guide and a theoretical basis for the developments of THz functional components. PACS 42.72.Ai; 42.70.Qs     

Josephson-vortex-flow terahertz emission in layered high-Tc superconducting single crystals

We report on the successful terahertz emission (0.6-1 THz) that is continuous and tunable in its frequency and power, by driving Josephson vortices in resonance with the collective standing Josephson plasma modes excited in stacked Bi2Sr2CaCu2O8+x intrinsic Josephson junctions. Shapiro-step detection was employed to confirm the terahertz-wave emission. Our results provide a strong feasibility of developing long-sought solid-state terahertz-wave emission devices.     

Surface tension and equilibrium shape of crystals in a wide range of angles

We have found the angular dependence of the surface tension using simplified models including short-range interaction as well as long-range interaction. The shape of a rounded surface is found in total range of angles as well as sizes of rectilinear (smooth) faces depending on parameters of a model and the temperature. Some dimensionless ratios have occurred to be universal.     

Accumulation features and TL of TLD-500 detectors in a wide temperature range at pulsed and continuous high-dose irradiation

The results of a comparative research of thermoluminescence (TL) of TLD-500 detectors based on anion-defective corundum irradiated with continuous and pulsed X-ray and pulsed electron beams in a range of doses of 0.3 ÷ 10⁷ Gy, dose rates of 0.02–2.6·10¹¹ Gy/s, and in a temperature range of 300–950 K are presented. It is found that, in contrast to continuous irradiation, upon pulsed irradiation with a duration of 10 ns and dose rate of P_P ≥ 5·10⁶ Gy/s, the first linear region of dose dependences for TL peaks at 450, 580 and 830 K is, instead of saturation, followed by a second one with a smaller slope at doses near 2, 200 and 10³ Gy. Moreover, the slope of the second region increases with growing P_P. It was also found that dose dependence for the peak at 830 K in the area of the first linear region at 10–10³ Gy remains invariable at P_P ≤ 10¹⁰ Gy/s. It is shown that the upper limit of doses registered by TLD-500 detectors can be increased to 2·10³ and 6·10⁶ Gy for continuous and pulsed irradiation, respectively. New broadband UV luminescence with a maximum hν = 4.1 eV and half width H = 0.85 eV was registered within the TL peak spectrum at 830 K. Besides, the optical depletion spectrum in which a single band with hν = 5.2 eV and H = 1.6 eV is observed was investigated for a trap causing a peak at 830 K.     

H+ irradiation effect in Co-doped BaFe2As2 single crystals

We report the suppression of the critical temperature T_c in single crystalline Ba(Fe_1?xCo_x)₂As₂ at under-, optimal-, and over-doping levels by 3 MeV proton irradiation. T_c decreases and residual resistivity increases monotonically with increasing the dose. The low-temperature resistivity does not show the upturn in contrast with the α-particle irradiated NdFeAs(O,F), which suggests that proton irradiation introduces nonmagnetic scattering centers. Critical scattering rates for all samples obtained by three different ways are much higher than that expected in s±-pairing scenario based on inter-band scattering due to antiferro-magnetic spin fluctuations.