不同抽运光强激发窄带隙半导体产生太赫兹辐射的研究

研究了窄带隙材料InAs和三种不同掺杂浓度的InN在不同抽运光强激发下产生太赫兹(THz)波的辐射特性.实验结果表明:在相同的抽运光强下,InN和InAs辐射的THz信号强度在同一量级,InAs较InN辐射效率要高一些.随着抽运光强的增大,这几种材料的发射光谱变得更宽,当抽运光增大到一定强度时,它们的发射光谱半极大值全宽(HMFW)趋于恒定.InN比InAs更容易在较低功率的抽运光作用下获得宽带太赫兹光谱.研究也表明,不同掺杂浓度对辐射THz波的强度及辐射效率有很大影响.这项研究对于探索半导体表面辐射太赫 关键词： InN InAs 太赫兹 抽运光强     

Transient-Pulse Nonlinear Spectroscopy with the Radiation of a Multimode THz Gas Laser

We report on transient-pulse nonlinear spectroscopy with the radiation of a multimode THz gas laser. The method is demonstrated for studying the nonlinear response of a current-carrying superlattice to THz radiation; the current through a superlattice can be suppressed by a strong THz field. The method makes use of the pulses of a high power multimode THz gas laser. By splitting the laser beam for selected laser modes into a main beam and a reference beam we monitored with a reference detector the transient power in the main beam. The simultaneous measurement of both the instantaneous response and the instantaneous power allows to obtain the power dependence of the response within a single laser pulse. The method is suitable to study the nonlinear response of matter to THz radiation fields in a large dynamic range.     

铁磁异质结构中的超快自旋流调制实现相干太赫兹辐射

利用飞秒激光脉冲在生长于二氧化硅衬底上的W/CoFeB/Pt和Ta/CoFeB/Pt两类铁磁/非磁性金属异质结构中实现高效、宽带的相干THz脉冲辐射.实验中, THz脉冲的相位随外加磁场的反转而反转,表明THz辐射与样品的磁有序密切相关.为了考察三层膜结构THz辐射的物理机制,分别研究了构成三层膜结构的双层异质结构(包括CoFeB/W, CoFeB/Pt和CoFeB/Ta)的THz辐射.实验结果都与逆自旋霍尔效应相符合, W/CoFeB/Pt和Ta/CoFeB/Pt三层膜结构所辐射的THz强度优于同等激发功率下的ZnTe (厚度0.5 mm)晶体.此外,还研究了两款异质结构和ZnTe的THz辐射强度与激发光脉冲能量密度的关系,发现Ta/CoFeB/Pt的饱和能量密度略大于W/CoFeB/Pt的饱和能量密度,表明自旋电子在Ta/CoFeB/Pt中的界面积累效应相对较小.     

基于锑化铟亚波长阵列结构的太赫兹聚焦器件

基于锑化铟材料在太赫兹波段的横向磁光效应,提出了一种金属-空气-锑化铟-金属非对称周期性亚波长线栅阵列结构的表面等离子体器件,研究了外加磁场和温度对不同频率透射波聚焦特性的影响.结果表明,在外加磁场强度B=0.6 T、温度T=172 K时,可实现0.8 THz透射光束的聚焦,焦点处能流密度透过率比没有外加磁场时增强28倍.对于不同频率入射波,通过主动调节磁场强度和温度,能实现从0.4—0.8 THz宽频带的聚焦,而且焦点处的透过率相比于无外加磁场时的普通狭缝聚焦透过率增强20倍以上,该器件是太赫兹波段理想的可调谐、宽频段、高透过率的聚焦器件.     

太赫兹液晶材料与器件研究进展

液晶是一种性能优异的可调控光电功能材料,基于液晶的太赫兹器件有着广泛的应用前景,但高性能太赫兹功能器件的研发仍处于初级阶段.本文综述了太赫兹领域液晶材料与器件的研究现状,探讨了液晶技术与太赫兹技术相结合的发展趋势.     

Generation of 1.5-kW, 1-THz coherent radiation from a gyrotron with a pulsed magnetic field

To cover a so-called terahertz gap in available sources of coherent electromagnetic radiation, the gyrotron with a pulsed solenoid producing up to a 40 T magnetic field has been designed, manufactured, and tested. At a 38.5 T magnetic field, the gyrotron generated coherent radiation at 1.022 THz frequency in 50 musec pulses. The microwave power and energy per pulse were about 1.5 kW and 75 mJ, respectively. Details of the gyrotron design, manufacturing, operation and measurements of output radiation are given.     

Terahertz time-domain spectrometer with module heads coupled to photonic crystal fiber

We constructed optical-fiber-based THz time-domain spectrometers (THz TDSs) with standard single-mode fibers (SSMFs) and large-mode-area photonic crystal fibers (LMA PCFs) and compared those to THz waves and spectra. The optical fibers are used for guiding optical pulses from ultra-fast lasers to a THz emitter and detector. The LMA-PCF-based THz TDS exhibits increased bandwidth from 1 to 2 THz and increased field amplitude by a factor of four compared with the SSMF-based THz TDS under the relatively higher excitation power of ultra-fast lasers. This improvement results from LMA PCFs that are suitable for high-power transmission without introducing nonlinear effects. We also fabricated compact THz emitter and detector module heads, which are connected with LMA PCFs. The LMA-PCF-based THz TDS had THz radiation power and bandwidth comparable with those obtained by a conventional THz TDS with a free-space optical arrangement. PACS 42.72.Ai; 42.65.-k; 42.81.DP     

Single-frequency terahertz source pumped by Q-switched fiber lasers based on difference-frequency generation in GaSe crystal

We demonstrate a unique terahertz (THz) source that is compact, utilizes recently developed all-fiber Q-switched lasers, and is based on difference-frequency generation in a GaSe crystal. A single piezo simultaneously Q switched the two fiber lasers by using stress-induced birefringence, to achieve the temporal overlap of pulses from the two fiber lasers. These correlated pulses then combine in the GaSe crystal to produce coherent and highly monochromatic THz pulses. The peak power for this THz source can reach 0.53 mW, corresponding to an average power of 0.43 microW and a conversion efficiency of 4.75 x 10(-7). The estimated linewidth of this THz source can be as narrow as approximately 35 MHz or 1.17 x 10(-3) cm(-1).     

Efficient generation of terahertz radiation by the method of optical rectification of terawatt laser pulses

Factors that lead to a decrease in the width and the mean frequency of the spectrum of terahertz (THz) radiation that were observed in recent experiments on optical rectification of powerful (∼0.5 TW) femtosecond laser pulses with a tilted amplitude front in a lithium niobate crystal are considered. A simple method is proposed that allows one to avoid the above transformation of the spectrum and to ensure a high efficiency of generation of THz radiation when laser pulses with a power of several TW are used.     

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.     

Dual-wavelength synchronously mode-locked Nd:CNGG laser

We have experimentally demonstrated a dual-wavelength synchronously mode-locked Nd:CNGG laser based on the semiconductor saturable absorber mirror technique. Mode locking was achieved simultaneously on two gain bands of the crystal that have a central wavelength separation of 2.4 nm. The fundamental mode-locked pulse train has a repetition rate of 88 MHz and pulse duration of 5 ps, with an average output power of approximately 90 mW. Autocorrelation measurements show that each of the synchronously mode-locked pulses consists of a train of quasi-periodic beat pulses with a 660 fs pulse width and a 0.63 THz repetition rate.     

Scaling submillimeter single-cycle transients toward megavolts per centimeter field strength via optical rectification in the organic crystal OH1

We present the generation of high-power single-cycle terahertz (THz) pulses in the organic salt crystal 2-[3-(4-hydroxystyryl)-5.5-dimethylcyclohex-2-enylidene]malononitrile or OH1. Broadband THz radiation with a central frequency of 1.5 THz (λ(c)=200 μm) and high electric field strength of 440 kV/cm is produced by optical rectification driven by the signal of a powerful femtosecond optical parametric amplifier. A 1.5% pump to THz energy conversion efficiency is reported, and pulse energy stability better than 1% RMS is achieved. An approach toward the realization of higher field strength is discussed.     

用于THz源的混合型波荡器数值模拟

波荡器是基于自由电子激光的小型THz源关键器件, 其可调节的周期性磁场结构与两端的光腔配合, 使得穿越的电子束产生带增益的相干辐射, 最终达到THz源所需要的功率. 同纯永磁结构相比, 混合型波荡器通过软铁材料调节由永磁块磁化方向性差异导致的磁场分布误差, 同时可提供更高的场强. 本文针对小型THz源需求, 对混合型波荡器进行了相关物理设计. 在解析方法分析的基础上, 采用OPERA3D/TOSCA有限元分析软件, 对波荡器进行了三维磁场数值模拟和积分场优化. 通过对波荡器端部结构的调整, 优化后模型的一次场积分(导向误差)小于0.01Gs﹒m, 电子轨迹偏移小于0.02mm.     

GaP波导型发射器产生频率可调谐太赫兹脉冲

报道了利用脉宽可调的光子晶体光纤飞秒激光放大器抽运矩形波导结构的GaP晶体太赫兹(THz) 发射器产生频率可调谐的超快THz脉冲.非线性晶体中光整流过程产生的THz辐射频率随抽运光脉冲宽度而 变化. GaP波导THz发射器可通过波导的几何尺寸来控制色散,以达到增加有效作用长度和提高输出功率的目的. 不同横截面尺寸的波导型发射器的THz辐射峰值频率随相位匹配条件的改变而改变,加以脉宽调节技术, 可以在大频谱范围获得频谱精细可调的THz脉冲.实验中在1 mm×0.7 mm的波导型THz发射器中获得了 频率可调谐的THz脉冲.提出实现THz辐射频率大范围调谐的GaP波导型阵列发射器的实施方案.     

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.     

Electrooptical sampling of ultra-short THz pulses by fs-laser pulses at 1060 nm

Electrooptical sampling of ultra-broadband terahertz (THz) radiation with the help of ultra-short 1060-nm pulses is reported for the first time. The THz pulses are generated by exciting a surface emitter (InAs) with a parabolic fiber laser amplifier delivering 100-fs pulses at a repetition rate of 75 MHz and an average power of 10 W. ZnTe and GaP crystals are used for detection and their velocity mismatch is compared at 800 nm and 1060 nm. PACS 42.55.Wd; 42.70.Nq; 42.72.Ai     

16.2-W average power from a diode-pumped femtosecond Yb:YAG thin disk laser

We demonstrate a power-scalable concept for high-power all-solid-state femtosecond lasers, based on passive mode locking of Yb:YAG thin disk lasers with semiconductor saturable-absorber mirrors. We obtained 16.2 W of average output power in pulses with 730-fs duration, 0.47-muJ pulse energy, and 560-kW peak power. This is to our knowledge the highest average power reported for a laser oscillator in the subpicosecond regime. Single-pass frequency doubling through a 5-mm-long lithium triborate crystal (LBO) yields 8-W average output power of 515-nm radiation.     

Review of recent efforts on efficient generation of monochromatic THz pulses based on difference-frequency generation

We review our recent efforts on power scaling of THz pulses generated from several nonlinear-optical crystals. By using a high-resistivity GaP crystal, we have significally increased the output peak power to as high as 722 W. By stacking three GaP wafers, we have further increased the highest output peak power to 2.36 kW. On the other hand, by using CO₂ laser pulses, we have obtained the average output power of 260 μW. We have also used these laser pulses to scale up the output power for the THz pulses to 29.8 μW by stacking GaAs wafers. Indeed, by stacking up to ten wafers, we have increased the output power by a factor of 160. Finally, by using ultrafast laser pulses, we have achieved record-high output powers for the THz pulses generated from multi-period periodically-poled LiNbO₃ crystals based on a backward configuration. The highest output power obtained by us so far is 10.7 μW.     

Origin of magnetic field enhancement in the generation of terahertz radiation from semiconductor surfaces

We present a theory of the magnetic field enhancement of terahertz (THz) emission from photogenerated carriers in the surface depletion region of a semiconductor. A combination of the Drude-Lorentz model for the carrier dynamics with an appropriate solution of the radiation problem is sufficient to explain the strong B -field enhancement in THz radiation that has been observed experimentally. The effect arises primarily from the increased radiation efficiency of transient currents flowing in the plane of the surface. The model provides quantitative agreement with experiment for the pronounced angular dependence of the enhancement and predicts the correct trend for the enhancement in a variety of materials.     

Generation and detection of ultrashort pulses of electromagnetic field in the terahertz range and their application for spectroscopy

We present experimental results on generation of pulsed terahertz radiation in a ZnTe crystal using nonlinear optical rectification of high-power femtosecond laser pulses. The use of terahertz pulses for spectroscopy is tested in the rotational lines of absorption of water vapors in the atmosphere. Panoramic spectra of absorption of a number of solvents and protein solutions are measured in the range from 0.2 to 2 THz.