Investigation of symmetries of second-order nonlinear susceptibility tensor of GaSe crystals in THz domain

Following our measurements and analysis made on several GaSe crystals, we demonstrated that terahertz (THz) generation from ultrafast laser pulses can be developed into a sensitive technique for investigating symmetries of second-order nonlinear susceptibility tensor of a nonlinear crystal. Indeed, for GaSe crystals, both Kleinman's symmetry condition and spatial symmetry were violated due to the contribution of ionic displacement to nonlinear polarization and deviation of GaSe lattice from hexagonal symmetry. When the pump photon energy was increased from that below the bandgap of GaSe to that above it, the mechanism for the THz generation was switched from optical rectification to photocurrent surge.     

Widely linear and non-phase-matched optical-to-terahertz conversion on GaSe:Te crystals

We demonstrate the widely linear and broadband terahertz (THz) generation on GaSe:Te crystals by femtosecond laser pulses. It was found that the dopant, Te atoms, in GaSe crystals significantly enhances the efficiency of THz generation, and its central frequency can be tuned by varying the crystal thickness through non-phase-matched optical rectification. Moreover, the wide-ranging linearity for the optical-to-THz conversion and central-frequency-tunable THz generation promise for GaSe:Te crystals to be potential materials for high-power (>1.36 μW) THz applications.     

Tellurium and sulfur doped GaSe for mid-IR applications

Centimeter-sized Te:GaSe (??10?mass%) ingots have been grown by the vertical Bridgman technique and studied to reveal the potentials for phase matching and frequency conversion. Less than 5 mass% Te-doped crystals show the hexagonal structure like ??-GaSe. Te:GaSe (??2?mass%) crystals were suitable for non-linear applications. The optimal Te-doping level between 0.1 and 0.5 mass% has been clearly observed in CO₂ laser SHG experiment. The CO₂ laser SHG efficiency in Te:GaSe (0.1?C0.5 mass%) is ??20?% higher than that of GaSe due to better optical quality. Phase matching conditions in Te-doped crystals have been shown to be identical with those of GaSe.     

Frequency conversion of CO laser radiation in the ZnGeP<Subscript>2</Subscript> nonlinear crystal

The experimental results of the study of CO laser frequency doubling in ZnGeP₂ and GaSe nonlinear crystals of high optical quality are presented. A multifrequency low-pressure Q-switched CO laser and a selective pulsed electroionization mode-locked CO laser with cryogenic cooling were used in the experiments. The highest external factor of radiation conversion into the second harmonic of the pulse-periodic Q-switched CO laser and pulsed mode-locked CO laser was 1% and 3.5%, respectively. Theoretical calculations showed the possibility of obtaining far IR and terahertz radiation using difference frequency generation of CO laser lines in these crystals.     

差频可调谐太赫兹技术的新进展

太赫兹技术在最近30年来得到快速发展, 并在医学、生物、农业、材料、安检、通信、天文等领域得到广泛应用. 从太赫兹源的频谱特性可以分为窄带(单频)太赫兹源和宽带太赫兹源. 从频谱技术方面来说, 相干的宽带和窄带太赫兹谱是一种互补性关系, 具有各自的技术特点和应用范围. 宽带太赫兹谱可以用于快速获取较宽频谱范围的分子振转谱, 实现混合特征谱的快速检测或成像. 窄带太赫兹源具有很好的光谱灵敏度和分辨率, 适用于太赫兹抽运-探测、分子振转能级谱精细结构分辨 以及太赫兹远程探测和成像. 因此研制具有可调谐的高峰值功率的窄带太赫兹源是适用于探测和识别分子振转能级指纹谱的应用需求, 而差频技术是获得高功率和宽调谐窄带太赫兹源最重要的技术之一. 为了突出该技术的最新进展, 本综述引证论文仅仅限于近5 年来基于差频技术产生太赫兹波的研究进展, 分为光学激光差频源和量子级联激光器差频源两大部分. 对于光学激光差频源, 分别对目前文献报道的各种双波长差频源和太赫兹产生用的非线性晶体进行分类介绍, 并给出所采用的技术和实验结果; 对于量子级联激光器差频源, 分别介绍了量子级联激光器中的差频产生技术和波长调谐技术的最新进展. 量子级联激光器差频太赫兹源是目前实现量子级联激光器在太赫兹波段室温运转的惟一技术, 是实现小型化、窄带宽调谐和室温运转太赫兹源的新发展领域, 值得关注.     

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.     

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. 关键词： 太赫兹波 GaSe晶体 2晶体')" href="#">ZnGeP₂晶体 差频     

AgGaS2- and Al-doped GaSe Crystals for IR Applications

We report a systematic study of AgGaS₂- and Al-doped GaSe crystals in comparison with pure GaSe and S-doped GaSe crystals. AgGaS₂-doped GaSe (GaSe:AgGaS₂) crystal was grown by Bridgman technique from the melt of GaSe:AgGaS₂ (10.6 wt.%). Its real composition was identified as GaSe:S (2 wt.%). Al-doped GaSe (GaSe:Al) crystals were grown from the melt of GaSe and 0.01, 0.05, 0.1, 0.5, 1, 2 mass % of aluminium. Al content in the grown crystals is too small to be measured. The hardness of GaSe:S (2 wt.%) crystal grown from the melt of GaSe:AgGaS₂ is 25% higher than that of GaSe:S (2 wt.%) crystal grown by a conventional S-doping technique and 1.5- to 1.9-times higher than that of pure GaSe. GaSe:Al crystals are characterized by 2.5- to 3-times higher hardness than that of pure GaSe and by extremely low conductivity of ≤ 10^− 7 Om^− 1 cm^− 1. A comparative experiment on SHG in AgGaS₂-, Al-, S-doped GaSe and pure GaSe is carried out under the pumps of 2.12-2.9 μm fs OPA and 9.2−10.8 μm ns CO₂ laser. It was found that GaSe:S crystals possess the best physical properties for mid-IR applications among these doped GaSe crystals. GaSe:Al crystals have relatively low conductivity which have strong potential for THz application.     

On the choice of nonlinear optical and semiconductor converters of femtosecond laser pulses into terahertz range

We carried out an experimental study of the use of nonlinear crystals ZnTe, GaP, GaSe, DAST, and LiNbO₃ and semiconductors GaAs and InAs for the rectification of femtosecond optical pulses. This conversion produces broadband terahertz pulses consisting of a single period of the field oscillation. Here, we discuss features of different types of the terahertz converters, their efficiency, and spectral range.     

Generation of backward terahertz waves in GaSe crystals

We have observed a backward-propagating terahertz wave by mixing two coherent infrared beams in two GaSe crystals. For the 47 mm long crystal, the output wavelength can be tuned in the wide range of 167.6 to 2060 microm (0.146 to 1.79 THz), whereas the output peak power reaches 217 W. The corresponding power conversion efficiency is approximately 0.03%.     

Down-conversion in the 4–18 μm range with GaSe and AgGaSe2 nonlinear crystals

The signal and idler outputs from a LiNbO₃ optical parametric oscillator have been down-converted in GaSe and AgGaSe₂ crystals. A many hundred watt peak power output continuously tunable in the range 4–18 μm has been achieved.     

Optical Properties of Boron-Doped Gallium Selenide

The electronic-absorption and luminescence spectra and the kinetics of relaxation of photocurrent in GaSe single crystals and thin films doped with boron atoms are experimentally investigated. Crystals are doped either in the course of synthesis or upon growing single crystals by the Bridgman method. Thin GaSe films are obtained by thermal evaporation of the compound preliminarily doped with boron. An absorption band with the maximum at λ = 925 nm, which is attributed to boron contaminations, is revealed from the comparison of the electronic-absorption spectra of a GaSe crystal and thin films doped with boron. When the crystals are excited with Nd:YAG laser pulses of a duration of 12 ns, it is established that recombination of nonequilibrium current carriers in pure crystals occurs through rapid and slow recombination channels and in crystals doped with boron only through rapid channels. In the region near λ = 932 nm, photoluminescence is observed in the crystals doped with boron, and its half-bandwidth is 15 Å.     

Generation of coherent terahertz phonons by sharp focusing of a femtosecond laser beam in the bulk of crystalline insulators in a regime of plasma formation

The generation of coherent terahertz phonons in a regime of plasma formation by a femtosecond laser radiation with an intensity of 10¹³ W/cm² in the bulk of crystalline quartz has been detected by the method of probing by a probe pulse of the third harmonic. A smooth increase in the frequency of coherent terahertz phonons from 2.2 to 5.5 THz has been detected, along with its subsequent sharp decrease down to 2.2 THz due to an α-β phase transition in crystalline quartz. The generation of 1-THz coherent phonons has been detected in BaF₂ crystals. A smooth variation of the frequency of coherent phonons from 2 to 2.5 THz has been detected in leucosapphire. The generation of coherent phonons during local laser excitation in CaF₂ and LiF crystals develops at the frequencies of 2.3 and 0.1 THz, respectively.     

Linear and nonlinear absorption,SHG and photobleaching behaviors of Dy doped GaSe single crystal

The linear absorption, nonlinear absorption (NA), second harmonic generation (SHG) and carrier mobilities of undoped and 0.1 at% Dy doped GaSe single crystals grown by modified Bridgman method have been studied by UV–vis, open aperture Z-scan, SHG and transmission ultrafast pump probe spectroscopy experiments. Both linear absorption and SHG measurements clearly showed that the doping of GaSe crystal with Dy³⁺ leads to a shift in the linear absorption edge. The mechanisms contributing to the NA behavior were explained and discussed. The photobleaching behaviors of the crystals were investigated by delaying of the probe signal in transmission pump-probe spectroscopy experiments.     

Investigation of terahertz generation from passively Q-switched dual-frequency laser pulses

When two Nd:YLF crystals share a Cr:YAG crystal functioning as a single passive Q switch, the timing jitter between each pair of dual-frequency pulses generated by the two crystals has been reduced by a factor of 20. Such a reduction in the timing jitter allows us to generate terahertz pulses by focusing such a passively Q-switched laser beam onto a nonlinear crystal. Such a result represents the first step for us to eventually implement a compact terahertz source based on ultracompact microchip lasers.     

Compact and portable terahertz source by mixing two frequencies generated simultaneously by a single solid-state laser

We have demonstrated a compact and portable terahertz (THz) source, based on difference-frequency generation in a GaSe crystal. The two input frequencies, required for achieving frequency mixing, are generated by a single Q-switched Nd:YLF laser incorporating two laser resonators. The average power of the THz output reaches 1 μW at 1.64 THz (182 μm) within a linewidth of 65 GHz. Such a THz source can be packaged into a compact and portable system.     

Modified GaSe crystal as a parametric frequency converter

Physical and optical properties of GaSe, doped with indium and sulphur has been discussed. Generation of tunable mid-infrared radiation by the technique of second harmonic generation (SHG) of tunable CO₂ laser radiation has been demonstrated. Conversion efficiency of SHG in GaSe:In and GaSe:S has been compared. PACS 42.65.k; 42.65.ky     

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.     

Efficient, tunable, and coherent 0.18-5.27-THz source based on GaSe crystal

Continuously tunable and coherent radiation in the wide range 56.8-1618 mum (0.18-5.27 THz) has been achieved as a novel and promising terahertz source based on collinear phase-matched difference frequency generation in a GaSe crystal. This source has the advantages of high coherence, simplicity for tuning, simple alignment, and stable output. The peak output power for the terahertz radiation reaches 69.4 W at a wavelength of 196 mum (1.53 THz), which corresponds to a photon conversion efficiency of 3.3%. A simple optimization of the design can yield a compact terahertz source.     

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.