Difference-Frequency Generation of THz Radiation via Parametric Three-Wave Interaction in CdTe and ZnTe Crystals

Difference-frequency generation of terahertz (THz) radiation in a collinear single-crystal scheme with distributed feedback implemented by means of parametric interaction of counterpropagating optical waves in a periodically inhomogeneous medium exhibiting quadratic nonlinearity is investigated. Using ZnTe and CdTe crystals optically pumped at λ = 0.77 and 1.06 μm, respectively, as an example, the possibility of obtaining parametric generation of THz radiation with an optical-to-THz conversion efficiency exceeding 10^–3 (with an extremely narrow line of oscillation of less than 4 GHz for ZnTe and less than 9 GHz for CdTe at 1 THz) is demonstrated in the undepleted-pump-intensity approximation. The obtained results can be used for construction of narrow-band lasers in the THz frequency range with an output power (in the quasicw regime of oscillation) of about 10⁵ W/cm².     

两注THz折叠波导行波结构的增益与传输特性

为了提高太赫兹行波管的输出功率,提出了多个传输信号进行功率合成的方法。首先,对D波段多注慢波结构进行设计及功分器的优化;然后,通过微铣削工艺完成了两注THz折叠波导结构的加工,加工精度满足实际设计要求;最后,利用CST软件对该结构的冷测与互作用特性进行了分析。仿真结果表明:该结构的S11值小于-20 dB,实际测试值在-20 dB左右,两者较吻合。冷测分析表明该结构具有22 GHz（16%）的冷带宽,3 dB增益带宽为12.5 GHz。在各电子注的电压、电流、峰值输入功率大小分别为15.79 kV, 12 mA, 10 mW时,单注结构获得了1.58 W的输出功率及22 dB的增益;而两路信号在互作用完成后,获得了2.91 W的合成功率输出,合成效率不低于90%。通过合成的方法可以有效提高THz行波管的输出功率。.     

Hot electron bolometric mixers: new terahertz technology

This paper presents an overview of recent results for NbN phonon-cooled hot electron bolometric (HEB) mixers. The noise temperature of the receivers based on both quasioptical and waveguide versions of HEB mixers has crossed the level of 1 K GHz⁻¹ at 430 GHz (410 K), 600–650 GHz (480 K), 750 GHz (600 K), 810 GHz (780 K) and is close to that level at 1.1 THz (1250 K) and 2.5 THz (4500 K). The gain bandwidth measured for quasioptical HEB mixer at 620 GHz reached 4 GHz and the noise temperature bandwidth was almost 8 GHz. Local oscillator power requirements are about 1 μW for mixers made by photolithography and about 100 nW for mixers made by e-beam lithography. A waveguide version of 800 GHz receiver was installed at the Submillimeter Telescope Observatory on Mt. Graham, AZ, to conduct astronomical observations of known submillimeter lines (CO, J=7→6, CI, J=2→1). It was proved that the receiver works as a practical instrument.     

基于太赫兹超材料的微流体折射率传感器

太赫兹生物医学是目前光谱研究领域的热点,其主要难点在于如何有效避免水分的干扰,进行液相环境下样本的灵敏分析与检测。超材料太赫兹传感器由于具有高灵敏、快速检测、痕量分析等优势,而成为太赫兹生物医学传感领域的重要研究方法。设计加工了一种基于单开口谐振环超材料的太赫兹液相传感芯片,为了有效克服水对太赫兹波的强烈吸收,利用微纳加工技术刻蚀深度为50 μm的流体通道。传感芯片整合了超材料基底与PDMS流道,在THz频段有两个位于0.771和2.129 THz的谐振峰。以水、无水乙醇作为常见化学溶剂进行传感实验,相对于空白传感器本身的THz时域谱而言,液体的加入导致时域峰的相位延迟和幅度减小。同时,由于水的折射率大于乙醇,THz透射频谱结果显示为水的频移改变量大于乙醇,且峰2大于等于峰1。上述结果表明,构建的超材料液相传感芯片是一个灵敏的折射率传感器,也证明了该传感器在测量液态样品方面的可行性。此外,利用该芯片研究了不同浓度的PBS溶液,发现水溶液中加入离子会导致谐振频率红移（以水为参考）,随着离子浓度增加,谐振频率改变量依次增加,10X PBS红移量最大,峰1为22.9 GHz,峰2为30.5 GHz。比较两个谐振峰的传感性能,峰2的传感能力更好,但是峰1对低浓度的离子溶液更加敏感。因此,构建的微流体传感器及检测体系作为一个灵敏的折射率传感器,可开发一个灵敏的无标记THz传感平台,为太赫兹生物医学研究提供新思路。     

高重复频率相干衍射辐射超宽谱太赫兹源

研究了一种新型相干衍射辐射（CDR）太赫兹源,该太赫兹源基于中国工程物理研究院自由电子激光相干强太赫兹源（FEL-THz）的加速器电子束束流,具有MHz量级重复频率。理论分析、数值计算和PIC模拟证明了此太赫兹源时间长度可达到ps量级,中心频率在100~400 GHz可调,截止频率位于1~2 THz,脉冲峰值功率可达10 kW量级,平均功率可达到W量级,并且功率随束流流强成平方正比关系。     

Study of broadband THz time-domain spectroscopy at different relative humidity levels

Two detection techniques of broadband terahertz(THz)time-domain spectroscopy-THz air-biased coherent detection(THz-ABCD;from 0.3 to 14 THz)and electro-optical(EO)detection(from 0.3 to 7 THz)-are both performed at several different relative humidity levels.The THz power exponentially decays with the increase in relative humidity.The dynamic range of the main pulse in the time domain linearly decreases as the relative humidity increases from 0%to 40%,and linear fittings show that the slopes are-0.017 and-0.019 for THz-ABCD and EO detection,respectively.Because of the multiple reflections caused by the crystal in the common EO detection,THz-ABCD has better spectral resolution(17 GHz)than that of EO detection(170 GHz).The spectrum of water vapor absorption measured by THz-ABCD is also compared with that measured by the Fourier transform infrared spectroscopy(FTIR).     

0.34 THz无线通信收发前端

描述了一种基于肖特基二极管技术的0.34 THz无线通信收发前端。该前端采用超外差结构,由0.34 THz谐波混频器、0.17 THz本振8倍频链和偏置电路组成。0.34 THz谐波混频器基于反向并联肖特基二极管,可以实现信号的上变频发射和下变频低噪声检测。0.17 THz本振8倍频链由三级二倍频及驱动放大链路组成,可将20~22.5 GHz信号倍频至0.16~0.18 THz,为混频器提供5~10 dBm左右的本振信号。实验测试结果表明:该前端用于信号发射时,在0.34 THz频点的饱和输出功率为-14.58 dBm;用于信号检测时,最低单边带(SSB)变频损耗为10.0 dB,3 dB中频带宽约30 GHz。限于测试条件,未能测试前端接收噪声温度,仿真得到的双边带噪声温度数值低于1000 K。在该前端基础上,完成了首次基于16QAM 数字调制体制的0.34 THz无线通信实验,传输速率达3 Gb/s。     

高速太赫兹探测器

太赫兹(terahertz,THz)技术在高速空间通信、外差探测、生物医学、无损检测和国家安全等领域具有广阔的应用前景.能响应1 GHz调制速率以上THz光的高速THz探测器是快速成像、THz高速空间通信、超快光谱学应用技术和THz外差探测等领域的核心器件.传统的THz热探测器难以实现高速工作,而基于半导体的THz探测器在理论上可实现高速工作.光导天线具有超快的响应速度,可实现常温和宽谱探测;肖特基势垒二极管混频器、超导-绝缘体-超导混频器和超导热电子混频器具有转换效率高、噪声低等优点,可用于高速THz空间外差和直接探测;基于高迁移率二维电子气的天线耦合场效应晶体管灵敏度高、阻抗低,可实现常温高速THz探测;THz量子阱探测器是一种基于子带间跃迁原理的单极器件,非常适合高频和高速探测应用,亚波长金属微腔耦合机理可显著提高器件的工作温度及光子吸收效率.本文对上述几种高速THz探测器进行了综述并分析了各种探测器的优缺点.     

0.22 THz二维电磁带隙波导和阵列天线设计

基于二维电磁带隙（EBG）结构,设计了工作在0.22 THz的直波导传输线和90弯波导传输线。数值模拟结果显示:直波导和弯波导的S21参数在0.22 THz处均大于10-5 dB,20 GHz模拟频带内的S21参数分别大于-0.001 dB和-0.003 5 dB。EBG结构很好地阻止了电磁波在电磁带隙内部的传播。基于EBG结构波导,设计了3 dB功分器和H面扇形喇叭阵列天线。数值模拟结果显示:功分器的S21参数在0.22 THz处为-3 dB,阵列天线方向性为17.1 dBi,具有较小的旁瓣和后瓣,E面和H面方向图均具有良好对称性。     

0.22THz宽带折叠波导行波管设计与实验

为了获得0.22THz宽带折叠波导行波管,对行波管的慢波结构和输入输出窗结构进行了宽带设计。通过理论分析和电磁仿真计算出合适的参数,使慢波结构在0.22THz工作点附近的色散曲线平坦,耦合阻抗变化小,模拟计算得到的慢波结构3dB带宽大于16GHz;通过对盒型窗结构及匹配段的优化计算,得到的输入输出结构在大于30GHz范围内S11参数小于-25dB。根据该设计进行了两轮制管和实验研究,得到了一支3dB瞬时带宽约8.8GHz,另一支3dB瞬时带宽大于12GHz的0.22THz折叠波导行波管,中心频率的峰值功率大于400mW。     

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.     

A flexible wideband bandpass terahertz filter using multi-layer metamaterials

We make a flexible wideband bandpass filter at terahertz (THz) frequencies using multi-layer metamaterials. A very flat response in the passband can be obtained since the Fabry–Perot reflection inside the rigid substrate is eliminated. The center frequency is about 0.89 THz with a 3 dB bandwidth of 0.69 THz for normal incidence. The sharp band-edge transitions are 53 and 70 dB/THz to the rejection bands, respectively. The measured average insertion loss is 1.4 dB with a ripple of 0.8 dB. Furthermore, the transmission feature is insensitive to the polarization of incident wave due to the symmetric structure of the unit cell of the metamaterials. Also, it has a small change as the increase of the curvature of the flexible substrate. This result manifests that the multi-layer metamaterials can provide an effective way to design wideband THz devices.     

Optical detection of terahertz radiation by using nonlinear parametric upconversion

We describe and demonstrate sensitive room-temperature detection of terahertz (THz) radiation by nonlinearly upconverting terahertz to the near-infrared regime, relying on telecommications components. THz radiation at 700 GHz is mixed with pump light at 1550 nm in a bulk GaAs crystal to generate an idler wave at 1555.6 nm, which is separated and detected by using a commercial p-i-n diode. The THz detector operates at room temperature and has an intrinsic THz-to-optical photon conversion efficiency of 0.001%.     

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.     

Microelectromechanical systems bimaterial terahertz sensor with integrated metamaterial absorber

This Letter describes the fabrication of a microelectromechanical systems (MEMS) bimaterial terahertz (THz) sensor operating at 3.8 THz. The incident THz radiation is absorbed by a metamaterial structure integrated with the bimaterial. The absorber was designed with a resonant frequency matching the quantum cascade laser illumination source while simultaneously providing structural support, desired thermomechanical properties and optical readout access. Measurement showed that the fabricated absorber has nearly 90% absorption at 3.8 THz. A responsivity of 0.1°/μW and a time constant of 14 ms were observed. The use of metamaterial absorbers allows for tuning the sensor response to the desired frequency to achieve high sensitivity for potential THz imaging applications.     

Dynamic optical arbitrary waveform generation and detection in InP photonic integrated circuits for Tb/s optical communications

This article presents recent results in the development of optical arbitrary waveform generation (OAWG) technologies based on optical frequency combs and indium phosphide devices. A novel spectral-slice dynamic-OAWG approach and waveform shapers with customized spectral multiplexers and modulators, enable continuous generation of high fidelity optical waveforms accessing bandwidths in excess of 1 THz. We show results for two types integrated waveform shapers, a 100 GHz electrically controlled device with 10 channels spaced at 10 GHz and a 1 THz optically controlled device with 100 channels spaced at 10 GHz. Additionally, we include results from a 640 GHz waveform measurement device with 16 channels and 40 GHz spacing.     

Frequency Lock of an Optically Pumped FIR Ring Laser at 803 and 1626 GHz

We report an automatic frequency control (AFC) for an optically pumped far infrared (FIR) ring laser applicable for high resolution THz sideband spectroscopy by mixing a fraction of the laser power and a harmonic of a phase-locked synthesizer on a planar Schottky diode. We achieve a relative frequency accuracy of about 0.5 kHz rms at 803 GHz ( ¹⁵ NH ₃ ) and about 1 kHz rms at 1626.6 GHz (CH ₂ F ₂ ) over hours of lock time. The absolute frequency accuracy is estimated to be about 5 kHz at 1626.6 GHz.     

Perfect metamaterial absorber based on a split-ring-cross resonator

In this paper, we present a polarization-insensitive metamaterial (MM) absorber which is composed of the dielectric substrate sandwiched with split-ring-cross resonator (SRCR) and continuous metal film. The MM absorber is not limited by the quarter-wavelength thickness and can achieve near-unity absorbance by properly assembling the sandwiched structure. Microwave experiments demonstrate the maximum absorptivity to be about 99% around 10.91 GHz for incident wave with different polarizations. The surface currents distributions of the resonance structure are discussed to look into the resonance mechanism. Importantly, our absorber is only 0.4 mm thick, and numerical simulations confirm that the MM absorber could achieve very high absorptivity at wide angles of incidence for both transverse electric (TE) wave and transverse magnetic (TM) wave. The sandwiched structure is also suitable for designing of a THz and even higher frequency MM absorber, and simulations demonstrate the absorption of 99% at 1.105 THz.     

Design and VNA-measurement of coplanar waveguide (CPW) on benzocyclobutene (BCB) at THz frequencies

The low permittivity and the low loss tangent of the benzocyclobutene polymer (BCB) offers to coplanar waveguides (CPW) a low dispersive propagation properties at THz frequency. These transmission lines have been designed, modeled with a three dimensional (3D) solver of Maxwell equations based on finite element method (FEM) from 20 to 1000 GHz at various characteristic impedances (Z_c). Their dispersion and losses (radiation, conduction and dielectric) have been investigated separately versus the waveguide size, the nature of the substrate (dielectric or semiconductor) to optimize the THz signal propagation. Monomode CPW on BCB numerically designed for various Z_c were realized and measured with vector network analyzer (VNA). S-parameters of CPW are de-embedded by optimization of the accesses’ model. A good agreement is found between experimental and numerical results with low attenuation constants of 2.7 dB/mm and 3.5 dB/mm at 400 GHz and 500 GHz, respectively.