ZnTe晶体中光学整流产生的THz辐射及其电光探测研究

借助抽运-探测技术研究了ZnTe晶体中光学整流产生的太赫兹(THz)辐射，利用ZnTe晶体的线性电光效应探测THz辐射场分布，观察到了较窄(约为0.2 ps)的THz场分布及相应较宽(响应超过4 THz，半峰宽约为2.4 THz)的THz频谱，并运用琼斯矩阵对实验结果进行了理论拟合. 研究了飞秒激光脉冲波长(750—850 nm)、脉冲宽度(56—225 fs)和晶体旋转与THz辐射产生的关系. 同时改变探测光偏振方向进行偏振调制，并从理论上分析了偏振调制对THz辐射探测的影响. 关键词： THz辐射 光学整流 电光探测 ZnTe     

MBE-grown MCT hetero- and nanostructures for IR and THz detectors

We present an overview of our technological achievements in the implementation of detector structures based on mercury cadmium telluride (MCT) heterostructures and nanostructures for IR and THz spectral ranges. We use a special MBE design set for the epitaxial layer growth on (013) GaAs substrates with ZnTe and CdTe buffer layers up to 3” in diameter with the precise ellipsometric monitoring in situ. The growth of MCT alloy heterostructures with the optimal composition distribution throughout the thickness allows for the realization of different types of many-layered heterostructures and quantum wells to prepare the material for fabricating single- or dual-band IR and THz detectors.We also present the two-color broad-band bolometric detectors based on the epitaxial MCT layers that are sensitive in 150–300-GHz subterahertz and infrared ranges from 3 to 10 μm, which operate at the ambient or liquid nitrogen temperatures as photoconductors, as well as the detectors based on planar HgTe quantum wells. The design and dimensions of THz detector antennas are optimized for reasonable detector sensitivity values. A special diffraction limited optical system for the detector testing was designed and manufactured. We represent here the THz images of objects hidden behind a plasterboard or foam plastic packaging, obtained at the radiation frequencies of 70, 140, and 275 GHz, respectively.     

Terahertz wave generation and detection from a cdte crystal characterized by different excitation wavelengths

Terahertz (THz) wave generation and detection from a (110)-oriented CdTe crystal via optical rectification and electro-optic sampling has been performed with laser wavelengths ranging from 710 to 970 nm. Three optical rectification regimes are studied with various excitation wavelengths: a resonance-enhanced regime above the bandgap, a highly phase-mismatched regime near the band gap, and a semi-phase-matched regime. A CdTe crystal generates more THz power than a ZnTe crystal at 970 nm.     

Determination of third-order nonlinearity in ZnTe by use of femtosecond laser pulses

Third-order nonlinearity induced by femtosecond pulses in ZnTe is investigated experimentally. We employ the transmission z-scan technique, using femtosecond pulses at 810 nm, to determine Kerr-type nonlinearity in ZnTe. The implications of our results to the terahertz (THz) radiation generation are discussed.     

Far Infrared Properties of Electro-Optic Crystals Measured by THz Time-Domain Spectroscopy

In this article we report an experimental investigation of the far-infrared properties of several nonlinear crystals, LiNbO₃, LiTaO₃, ZnTe and CdTe. Using Terahertz Time-Domain Spectroscopy (THz-TDS) we have measured the complex frequency response, i.e. both index of refraction and absorption up to 3 THz (100 cm^–1) for the electro-optic crystals at room temperature. The single Lorentzian oscillator model is used to describe the aquired data. Additional resonance features are observed, especially in the II-VI compounds.     

Resonant Raman scattering from CdTe/ZnTe self-assembled quantum dot structures

We report resonant Raman scattering results of CdTe/ZnTe self-assembled quantum dot (QD) structures. Photoluminescence spectra reveal that the band gap energies of the CdTe QDs decrease with the increase of CdTe thickness from 2.0 to 3.5 monolayers, which indicates that the size of the QDs increases. When the CdTe/ZnTe QD structures are excited by non-resonant excitation, a longitudinal optical (LO) phonon response from the ZnTe barrier material is observed at 206 cm⁻¹. In contrast, when the CdTe/ZnTe QD structures are resonantly excited near the band gap energy of the QDs, additional phonon modes emerge at 167 and 200 cm⁻¹, while the ZnTe LO phonon response completely disappears. The 167 cm⁻¹ mode corresponds to the LO phonon of the CdTe QDs. A spatially resolved Raman scattering from the cleaved edge of the QD sample reveals that the 200 cm⁻¹ mode is strongly localized at the interface between the CdTe QDs and ZnTe cap layer. This phonon mode is attributed to the interface optical (IO) phonon. The analytically calculated value of the IO phonon energy using a dielectric continuum approach, assuming a spherical dot boundary, agrees well with the experimental value.     

Hydrogen-acceptor interaction in CdTe and ZnTe studied by photoluminescence

Photoluminescence studies on CdTe and Znte implanted with hydrogen and deuterium at 150keV in the fluence range 10¹⁴–10¹⁵ cm^-2 are reported. In CdTe the introduction of hydrogen (deutrium) makes the recombination lines of excitons bounds to residual acceptors to disappear. The donor bound excitons are not affected. In ZnTe the intensity of the principal acceptor-bound exciton (Cu) is much reduced. In both crystals a new sharp line (H) appears with an adjacent structure on the low energy side. Short time annealings in the temperature range 150–250°C lead to the disappearance of the sharp lines (H) and to the recovery of the initial spectrum. These results shows that hydrogen is able to interact with acceptors in CdTe and ZnTe leading to the formation of a new exciton trapping centre. Possible microscopic models for this isoelectronic-like complex are proposedl     

小型化外腔可调谐THz参量振荡器

基于LiNbO3晶体垂直表面输出技术,设计了一台小型化外腔THz参量振荡器。利用小型化灯1064 nm脉冲激光器泵浦MgO:LiNbO3,通过优化设计三波非共线相位匹配的光学参量振荡腔结构,实现THz垂直晶体表面输出,减少LiNbO3晶体对THz波的吸收,提高了THz波输出光束质量。当在泵浦光能量为128 mJ、重复频率为10 Hz时,获得THz波的调谐范围为0.69~3.01 THz,在1.6 THz处获得THz波最大平均功率为10.8 W,脉冲宽度为10 ns,对应THz波能量转换效率为8.4310-6。     

Manifestation of interaction effects in Raman spectra of CdTe quantum-dot layers with ZnTe narrow barriers

The Raman spectra of superlattices consisting of layers of CdTe self-assembled quantum dots separated by ZnTe narrow barriers with thicknesses of 10 and 5 monolayers are investigated. It is found that, apart from the bands previously observed at frequencies of ～120 and ～140 cm^?1 for samples with thicker barriers (25 and 12 monolayers), the Raman spectra exhibit a band at ～147 cm^?1 in the frequency range of CdTe vibrational modes. This band is attributed to a symmetric vibrational mode of a pair of quantum dots with oppositely directed oscillations of the dipole moments. It is this type of vibrational mode in the material surrounding the ZnTe quantum dot that accounts for the shift of the band at ～200 cm^?1 near the LO mode of ZnTe vibrations toward lower frequencies.     

Experimental study on terahertz radiation

In this letter, we describe a coherent subpicosecond terahertz (THz) spectroscopy system based on nonresonant optical rectification for the generation of THz radiation. We studied the two-photon absorption (TPA) of ZnTe induced by femtosecond laser pulses via THz generation, and its influence on the generation of THz radiation. Experimental results demonstrated that the intensity of pump beam against TPA must be traded off to get an optimum generation of THz radiation. As an example, we measured absorption spectrum of water vapor by time-domain spectroscopy (TDS) in the frequency range from 0.5 to 2.5 THzwith a high overall accuracy.     

Dependence of the interband transition and the activation energy on the ZnTe spacer thickness in CdTe/ZnTe double quantum dots

Photoluminescence (PL) measurements were carried out to investigate the interband transition and the activation energy in CdTe/ZnTe double quantum dots (QDs). While the excitonic peaks corresponding to the interband transition from the ground electronic subband to the ground heavy-hole (E₁-HH₁) in the CdTe/ZnTe double QDs shifted to higher energy with decreasing ZnTe spacer thickness from 30 to 10 nm due to transformation from CdTe QDs to Cd_xZn_1−xTe QDs, the peaks of the (E₁-HH₁) transitions shifted to lower energy with decreasing spacer thickness from 10 to 3 nm due to the tunneling effects of the electrons between CdTe double QDs. The decrease in the activation energy with decreasing ZnTe spacer thickness might originate from an increase in the number of defects in the ZnTe spacer. The present results can help improve the understanding of the interband transition and the activation energy in CdTe/ZnTe double QDs.     

晶向ZnTe单晶的太赫兹辐射及探测

通过Te熔剂方法生长出〈331〉晶向的ZnTe体单晶,利用X射线衍射和红外透射显微技术对材料进行了测试.在钛 宝石激光器的泵浦下,利用〈331〉晶向的ZnTe晶体辐射－探测到太赫兹波,激发频谱可达5 THz左右.〈331〉晶向的ZnTe晶体表现出良好的太赫兹辐射性能.     

Observation of ultra-broadband terahertz emission from ZnTe films grown by metaloganic vapor epitaxy

ZnTe films were grown on (0001) sapphire substrates by the metalorganic vapor phase epitaxy (MOVPE) method. Single crystalline (111) ZnTe epitaxial layers were confirmed by x-ray diffraction, reflection high-energy electron diffraction, and cathodoluminescence measurements. Emission of THz radiation with a spectral distribution up to 40 THz was clearly observed from the ZnTe film with a thickness of 10 μm. The results show that MOVPE is a promising growth method for obtaining high-quality ZnTe epitaxial films on sapphire substrates, which paves the way for obtaining thinner ZnTe films to provide a flatter frequency response in THz device applications.     

CdTe薄膜太阳电池背接触的研究

用近空间升华法制备了CdTe多晶薄膜，用硝酸-磷酸(NP)混合液对薄膜表面进行了腐蚀.经SEM观测，腐蚀后的CdTe薄膜晶界变宽，XRD测试发现，经NP腐蚀后，在CdTe薄膜表面生成了一层高电导的富Te层.在腐蚀后的CdTe薄膜上分别制备了Cu，Cu/ZnTe：Cu，ZnTe：Cu，ZnTe/ZnTe：Cu四种背接触层，比较了它们对太阳电池性能的影响.结果表明，用ZnTe/ZnTe：Cu复合层作为背接触层的效果较好，获得了面积为0.5cm²，转换效率为13.38%的CdTe多晶薄膜太 关键词： 硝磷酸腐蚀 背接触层 CdTe太阳电池     

Tunable source of terahertz radiation based on the difference-frequency generation in a GaP crystal

A tunable source of terahertz (1–4 THz) radiation with a line width of 12 GHz is created on the basis of difference- frequency generation in a GaP crystal. Radiation of a pulsed Nd:YAG laser with a wavelength of 1064 nm and a tunable optical parametric oscillator is used as the pump. The radiation is generated in 10-ns-duration pulses with a pulse repetition rate of 10 Hz. The pulsed radiation power is about 15 mW.     

反应离子刻蚀ZnTe的THz辐射和探测研究

运用电光采样技术揭示了反应离子刻蚀（RIE）ZnTe晶体表面THz辐射的光学整流产生机制， 观察到0.25 ps的THz场分布.通过比较刻蚀前后以及不同刻蚀条件下ZnTe样品在不同激发功 率下的THz辐射强度，发现由于反应离子刻蚀破坏了ZnTe样品表面的有序性，晶体的电光系 数随射频功率的增加而减小.借助于计算不同刻蚀条件下ZnTe晶体的频率响应函数，分析了 随射频功率增加ZnTe晶体响应频谱展宽的现象. 关键词： THz辐射 反应离子刻蚀 ZnTe     

Simulation of parametric oscillation in the submillimeter range at pumping of the ZnGeP2 crystal by a train of 100-ps high-power pulses

Analysis is given for a possibility of singly resonant parametric oscillation in the submillimeter range at synchronous pumping of the ZnGeP₂ crystal by a train of 100-ps second-harmonic pulses from the CO₂ laser with the radiation energy 1.0 J. The calculation shows that using the ZnGeP₂ crystal and the second harmonic of the CO₂ laser with the energy density 1.8 J cm⁻², one can get the peak submillimeter radiation power from 3.6 to 12 MW in the range from 95 to 300 μm (1.0–3.3 THz). The expected peak power values are larger than the experimental ones obtained by other nonlinear optics methods.     

Limitations of the tunability of dual-crystal optical parametric oscillators

We demonstrate what is believed to be the first continuous-wave dual-crystal optical parametric oscillator (D-OPO) for generation of waves with a frequency difference of some terahertz. It is based on two magnesium-doped periodically poled lithium niobate crystals pumped with near-infrared light at 1030 nm. By changing the temperature difference of the crystals we achieve a difference-frequency tuning. This ranges from an initially unexpected lower threshold of 1.3 THz up to higher frequencies. The linewidth of the considered signal waves is smaller than 5 GHz. Smaller difference frequencies were not achievable. Our model, describing the dual-crystal parametric gain, explains the observed lower tuning limit by showing that the assumption of independent oscillation conditions in a D-OPO is not necessarily valid.     

THz Generation by Optical Rectification and Competition with Other Nonlinear Processes

We present a study of the competition between tera-hertz （THz） generation by optical rectification in （110） ZnTe crystals, two-photon absorption, second harmonic generation and free-carrier absorption. The two-photon nonlinear absorption coefficient, second harmonic generation efficiency and free-carrier absorption coefficient in the THz range are measured independently. The incident pump field is shown to be depleted by two-photon absorption and the THz radiation is shown to be reduced, upon focusing, by free-carrier absorption. The reduction of the generated THz radiation upon tight focusing is explained, provided that one also takes into account diffraction effects from the sub-wavelength THz source.     

Noncascading THz-wave parametric oscillator synchronously pumped by mode-locked picosecond Ti:sapphire laser in doubly-resonant external cavity

A noncascading terahertz (THz) wave parametric oscillator synchronously pumped by a mode-locked picosecond Ti:sapphire laser whose average power was less than 1 W was demonstrated with a noncollinear phase-matching MgO:LiNbO₃ crystal in an external enhancement cavity doubly resonant for both pump (780 nm) and signal (781-784 nm) waves. In the external cavity, in which the pump wave enhanced so as to reduce the pumping threshold of parametric processes, the signal wave could also resonate and thus be enhanced simultaneously, resulting in a THz wave output at approximately 0.9 THz as the idler wave. The novel dual enhancement of pump and signal waves reduced the threshold pumping intensity to approximately 50 MW/cm², which was much lower than that of a conventional externally pumped THz wave parametric oscillator with a crystal.