Fresnel formulas for the forced electromagnetic pulses and their application for optical-to-terahertz conversion in nonlinear crystals

We show that the usual Fresnel formulas for a free-propagating pulse are not applicable for a forced terahertz electromagnetic pulse supported by an optical pulse at the end of a nonlinear crystal. The correct linear reflection and transmission coefficients that we derive show that such pulses can experience a gain or loss at the boundary. This energy change depends on linear dielectric constants only. We also predict a regime where a complete disappearance of the forced pulse under oblique incidence occurs, an effect that has no counterpart for free-propagating pulses.     

Optical detectors on GaSe and InSe layered crystals

Fast uncooled GaSe and InSe detectors that can record ultrashort (10^?12–10^?9 s) laser pulses in the visual and near-IR ranges are developed. The quick response of the detectors is due to rapid recombination channels with a high capture cross section present in the crystals.     

Optical properties of GaSe: Mn single crystals

To identify the manganese related defect levels in GaSe, GaSe:Mn single crystals were grown with various Mn dopant levels using the Bridgman technique and the photoconductivity and photoluminescence properties were investigated. Peaks introduced by the manganese related defects were observed at 1.916 and 1.724 eV in the photoconductivity spectra and at 1.804 eV in the photoluminescence spectra at 80 K. These results allow the calculation of the energies of the A₁ and A₂ centers at 0.348 and 0.156 eV, respectively, above the valence band and a donor level at 0.112 eV below the conduction band. Also, we find that the A₁ and A₂ centers are pinned within the conduction band from measurements of the temperature dependence of the photoconductivity spectra.     

Hole traps in GaSe:Co single crystals

In order to clarigy the nature of the cobalt related hole traps (CRHT) in GaSe single crystals, GaSe:Co single crystals were grown by the Bridgman method and the optical absorption spectra, the photoconductivity spectra and the thermally stimulated current (TSC) were investigated. The energy levels of the CRHT in GaSe:Co single crystals were located at 0.18, 0.28, 0.38, 0.49 and 0.57 eV above the valence band. The optical absorption peaks and the photoconductivity peaks corresponding to the CRHT were observed in the wavelength range from 650 to 950 nm and these peaks are respectively attributed to the carrier transitions from the CRHT to the indirect conduction band of GaSe:Co single crystals.     

Sensitive elements of pressure transducers made of layered intercalated InSe, GaSe, and Bi2Te3 crystals

It is shown that layered InSe, GaSe, and Bi₂Te₃ semiconductors are promising for sensitive elements of pressure transducers. Two ways for measuring pressure with layered crystals are suggested: from the pressure dependence of the intercalation parameter (current) and from the pressure dependence of the intercalate electromotive force.     

基于GaSe和GaSe0.7S0.3单晶的单脉冲CO2激光倍频器

描述了使用电感储能发生器和半导体转换开关泵浦的工作波长为10.6μm的高效CO2激光器。给出了激光泵浦的非线性晶体GaSe和GaSe0.7S0.3的二次谐波振荡的实验数据和理论估算结果。结果显示,GaSe晶体在输入能量为180mJ时,最大能量转换效率为0.38%,倍频激光的峰值功率为8 kW。     

Low-temperature thermoluminescence study of GaSe:Mn layered single crystals

Mangan doped GaSe single crystals have been studied by thermoluminescence measurements performed with various heating rates between 0.4 and 1.0 K/s in the temperature range of 10?300 K. Thermoluminescence spectra exhibited four distinguishable peaks having maximum temperatures at 47, 102, 139 and 191 K revealing the existence of trapping levels in the crystals. Curve fitting and initial rise methods were applied to observed peaks to determine the activation energies of four trapping levels. Capture cross-sections of each level were also evaluated using the obtained energy values. Moreover, heating rate dependencies of the obtained peaks were investigated. It was shown that increase in the heating rate resulted in the decrease in thermoluminescence intensity and shift of the peak maximum temperatures to higher values. Discrete, single trap behaviour was established for acceptor level related with the peak at 191 K by analysing the sequentially obtained peaks with different stopping temperatures between 15 and 65 K.     

The nature of negative linear expansion in layer crystals C,Bn, GaS,GaSe and InSe

Experimental investigations of thermal expansion parallel and perpendicular to the layers plane of layer crystals GaS, GaSe and InSe are described. The obtained results are analized together with known thermal expansion data on graphite, C, and boron nitride, BN. Theoretical calculations of linear expansion coefficients are carried out on the basis of the model of highly anisotropic crystal. It is shown, that the negative thermal expansion in the layer plane, typical of layer crystals, is due to “bending” waves, acoustic waves propagating in the layer plane and polarized perpendicular to this plane (TA_⊥ mode)     

Age-induced oxide on cleaved surface of layered GaSe single crystals

It is shown that a long-term keeping of a layered gallium monoselenide at room temperature results in formation of the intrinsic oxide at a cleaved surface of semiconductor. It is found that the chemical compositions of the intrinsic oxide at the surfaces of the intentionally undoped and doped samples of GaSe are different. The electrical properties of the GaSe-intrinsic oxide system are presented. It is established that intrinsic oxide films at the surface of GaSe are characterized by current instability with N-type current-voltage characteristic. The influence of relative humidity on changes of capacitance and surface resistivity of the intrinsic oxide is also discussed.     

Temperature-dependent optical properties of GaSe layered single crystals

Optical properties of GaSe single crystals have been investigated using temperature-dependent transmission and room temperature reflection measurements in the wavelength range of 380–1100 nm. The analysis of the absorption data at room temperature showed the existence of indirect transitions in the crystal with energy band gap of 1.98 eV. Temperature dependence of the transmission measurements revealed the shift of the absorption edge toward lower energy as temperature is increased from 10 to 280 K. The rate of change of the indirect band gap was found as γ = ?6.6 × 10^?4 eV/K from the analysis of experimental data under the light of theoretical relation giving the band gap energy as a function of temperature. The absolute zero value of the band gap energy and Debye temperature were calculated from the same analysis. The Wemple–DiDomenico single-effective-oscillator model applied to refractive index dispersion data was used to determine the oscillator energy, dispersion energy, oscillator strength and zero-frequency refractive index values.     

Optical and defect properties of S-doped and Al-doped GaSe crystals

S-doped and Al-doped GaSe crystals are promising materials for their applications in nonlinear frequency conversion devices. The optical and defect properties of pure, S-doped, and Al-doped GaSe crystals were studied by using photoluminescence(PL) and Fourier transform infrared spectroscopy(FT-IR). The micro-topography of(0001) face of these samples was observed by using scanning electron microscope(SEM) to investigate the influence of the doped defects on the intralayer and interlayer chemical bondings. The doped S or Al atoms form the S_(Se)~0 or Al_(Ga)~(+1) substitutional defects in the layer GaSe structure, and the positive center of Al_(Ga)~(+1) could induce defect complexes. The incorporations of S and Al atoms can change the optical and mechanical properties of the GaSe crystal by influencing the chemical bonding of the layer structure. The study results may provide guidance for the crystal growth and further applications of S-doped and Al-doped GaSe crystals.     

Optical nonlinearities in GaSe and InSe crystals upon laser excitation

The nonlinear absorption of light and its temporal evolution in the vicinity of exciton resonance in layered GaSe and InSe crystals under high optical excitation have been experimentally investigated. The decisive factor for the observed temporal dependence of the absorption coefficient and its dependence on the excitation intensity is screening excitons by nonequilibrium-carrier plasma. It is shown that the increase in the transmittance in the absorption-band edge in GaSe with a simultaneous blue shift of the band edge is caused by filling the energy bands under high optical excitation.     

Fractal structure formation in GaSe crystals grown from melt

The structure of GaSe single crystals prepared by rapid cooling of melt has been studied. These crystals are shown to contain dendrites due to the nonequilibrium conditions of single-crystal preparation. Dendrites have a fractal structure with the Hausdorff measure equal to 1.7.     

Al、S和Te掺杂GaSe晶体的光学及倍频特性

对不同浓度Al、S和Te掺杂GaSe晶体的透过率和倍频特性进行了研究.结果表明,适宜浓度的掺杂晶体在透明波段内吸收系数约为α≤0.1~0.2cm-1,适于非线性应用.GaSe∶S晶体透过率曲线向短波方向移动,且更适于高浓度掺杂,而Al和Te掺杂晶体透明范围的波段截止波长向长波方向移动,且移动幅度小于S掺杂晶体,当掺杂浓度达到GaSe∶Al(0.5mass%)和GaSe∶Te(5mass%)时,晶体光学质量明显下降.通过fs Ti∶Sapphire激光和CO2激光泵浦下I类倍频实验发现,S掺杂晶体相位匹配曲线向短波方向移动,倍频输出功率比纯GaSe晶体有明显提高,最佳掺杂浓度为2mass%.Al和Te掺杂晶体相位匹配角与纯GaSe晶体相比没有明显变化,实验结果与理论曲线符合较好.利用非线性方法研究发现当掺杂浓度不超过5mass%时,GaSe∶Te晶体与纯GaSe同样属于六角形结构.三种掺杂方式最佳掺杂浓度分别为GaSe∶Al(0.1mass%),GaSe∶S(2mass%)和GaSe∶Te(0.5mass%),在CO2激光泵浦下,三者频率转换效率之比约为1∶0.6∶0.5.在所使用的晶体样本中,GaSe∶S(2mass%)晶体性能最佳,其频率转换效率可达纯GaSe晶体的3倍左右.     

Nonlinear absorption in undoped and Ge doped layered GaSe semiconductor crystals

P type-GaSe and 0.01 at% Ge doped GaSe crystal were grown using the conventional Bridgman–Stochbarger method from a stoichiometric mixture of starting materials in quartz ampoules, and the nonlinear absorption properties of both crystals have been studied by using an open aperture Z-scan technique under 1064 nm wavelength with 4 ns or 65 ps pulse durations. GaSe crystal exhibits two-photon absorption (TPA) at all input irradiances. On the other hand, at low input irradiance the Ge doped GaSe crystal exhibits saturable absorption (SA). At higher input irradiances TPA becomes dominant. A monotonic increase of the nonlinear absorption coefficient with increasing laser pulse duration from 65 ps to 4 ns is observed for the GaSe and Ge doped GaSe crystals.     

Spectral anisotropy of a photoresponse from heterojunctions based on GaSe and InSe layered crystals

The object of investigation is photoresponse spectra taken from the cleaved end face of heterojunctions formed by GaSe and InSe anisotropic crystals. Spectra taken from the as-prepared and chemically processed faces of the heterojunctions are compared. A modified method of growing GaSe crystals with a virgin end face is suggested, and the surface of GaSe crystals thus grown is examined by atomic force microscopy.     

Injection and thermodepolarization currents in GaSe <Co> single crystals

The current-voltage characteristics (CVC) at different temperatures, the temperature dependence of electric conductivity [σ(T)] and the currents of thermostimulated depolarization (TSD) have been studied in GaSe <0.05 at.% Co> on a combined basis.The location depth (E_t=0.57eV) and the concentration of traps (N_t=2.7x10¹²cm^?3) have been determined from the temperature dependence of the trapping factor. In the course of TSD investigations, levels with location depths of 0.28±0.02 and 0.57±0.03 eV have been revealed. It is noted that traps with the energy of 0.57±0.03 eV are found both with TSD measurements and on the basis of the temperature dependence of the electric conductivity and the trapping factor.It has been established that the hole centres above the valence band are responsible for the CVC, σ(T) and TSD. The location depths, concentrations and trapping cross-sections of these centres have been determined.     

Widely linear equalization for IQ imbalance contaminated multicarrier systems

In a multicarrier modulated (MCM) wireless communication system, the received bandpass signal is downconverted to the baseband signal for ease of receive processing. During downconversion, a slight mismatch in the amplitude and phase of locally generated sinusoidal signal in the In-Phase and Quadrature (IQ) branches of downconverter results in IQ imbalance of the baseband signal. The IQ imbalance leads to mirror interference between subcarriers of an MCM system and degrades the performance. To compensate for the effects of IQ imbalance, this paper presents widely linear minimum mean square error (WL-MMSE) based equalization technique and compares it with the traditionally used linear minimum mean square error (L-MMSE) based equalization technique. A performance comparison between WL-MMSE and L-MMSE based estimation techniques in terms of mean square error (MSE) and bit error rate (BER) suggests that the former outperforms the latter in the presence of IQ imbalance.