Optically- and Electrically-Stimulated Terahertz Radiation Emission from Indium Nitride

Indium nitride is a novel narrow band gap semiconductor. The material is a potential strong source of terahertz frequency electromagnetic radiation with applications in time-domain terahertz spectroscopy and imaging systems. This article reviews recent experimental research on terahertz emission from the binary compound semiconductor indium nitride excited by near-infrared laser beams or microseconds electrical pulses. Advantages of indium nitride as terahertz radiation source material are discussed. It is demonstrated that different mechanisms contribute to the emission of terahertz radiation from indium nitride. The emission of up to 2.4 μW of THz radiation power is observed when InN is excited with near-infrared femtosecond laser pulses at an average power of 1 W.     

Kinetic inductive model of a millimeter-wave high-temperature superconducting optoelectronic mixer

We introduce and analyze an optoelectronic mixer (OEM) based on the kinetic inductive photoresponse in high-temperature superconducting (HTS) films. This device combines photodetection and optoelectronic mixing functions through a nonlinear change in the kinetic inductance of the HTS film when it is irradiated by an optically modulated microwave signal. A comprehensive theoretical analysis is presented using the two-temperature model to describe the nonbolometric (quantum) photoresponse and the kinetic inductance model for the electrical part. Upon the optical irradiation, the change in the electron temperature of the HTS film leads to a parametric change in the kinetic inductance of the photoexcited HTS bridge, which in the presence of a bias current produces a periodic voltage waveform. In order to obtain the temporal behavior and the frequency content of the output voltage in terms of the input local oscillator and modulation frequencies, the kinetic inductance model and Fourier series analysis have been used and their physical consequences have been discussed in detail. The merit characteristics of the kinetic inductive HTS-OEM, such as intrinsic and optical conversion gains and noise temperature, are evaluated and compared with other high-frequency mixers. This is followed by the numerical simulation of the proposed device.     

Optical characteristics of a light-focusing fiber guide and its applications

A lenslike glass fiber guide with a parabolic variation of refractive index has been developed. This optical guide named SELFOC® has the following characteristics: simultaneous transmission of laser beams modulated by wide-band signals through narrow space; optical image transmission; realization of a lens with tiny aperture or with extremely short focal length; and the possibility of being bent with a small radius of curvature without spoiling transmission characteristics. In the case of a typical fiber guide with length 1 meter and diameter 0.3 mm, transmission loss is about 0.2 dB and depolarization is about 20 dB at wavelength 0.63μ. The mode pattern of a laser beam after passing through the fiber guide is scarcely deformed. The fiber guide can be used as a transmission line or lens, in optical communication, optical data processing, and optical instruments.     

Calculation and Study of Graphene Conductivity Based on Terahertz Spectroscopy

Based on terahertz time-domain spectroscopy system and two-dimensional scanning control system, terahertz transmission and reflection intensity mapping images on a graphene film are obtained, respectively. Then, graphene conductivity mapping images in the frequency range 0.5 to 2.5 THz are acquired according to the calculation formula. The conductivity of graphene at some typical regions is fitted by Drude-Smith formula to quantitatively compare the transmission and reflection measurements. The results show that terahertz reflection spectroscopy has a higher signal-to-noise ratio with less interference of impurities on the back of substrates. The effect of a red laser excitation on the graphene conductivity by terahertz time-domain transmission spectroscopy is also studied. The results show that the graphene conductivity in the excitation region is enhanced while that in the adjacent area is weakened which indicates carriers transport in graphene under laser excitation. This paper can make great contribution to the study on graphene electrical and optical properties in the terahertz regime and help design graphene terahertz devices.     

太赫兹缝隙衍射的严格求解方法

基于严格电磁场理论和结构拓展思想,提出了太赫兹缝隙结构衍射的严格物理模型,得到了太赫兹缝隙衍射情形下入射、衍射和透射区域内电磁场基本模式的严格表达。数值计算表明太赫兹波在晶体(砷化镓)、非金属(FR-4)等材料缝隙的透射系数与太赫兹波频率的变化趋势大体相似,得出缝隙材料对太赫兹波的透射系数影响不明显;但是,缝隙的个数、材料的种类以及基底材料的厚度对太赫兹缝隙衍射最大透射系数的影响明显。本文方法也可用于研究太赫兹波亚波长区域内、微纳光学元件体内衍射传输等问题。     

共振声子太赫兹量子级联激光器研究

针对四阱有源区、一阱注入区及三阱有源设计,采用蒙特卡洛模拟方法研究了共振声子太赫兹量子级联激光器的性能差异。采用傅里叶变换光谱仪及远红外探测器测量了四阱共振声子太赫兹量子级联激光器的低温电光特性。详细讨论了提高太赫兹量子级联激光器发射功率的方案。     

Semi-Analytical Calculation of Terahertz Signal Generated from Photocurrent Radiation in Traveling-Wave Photonic Mixers

A semi-analytical method based on distributed source transmission line model is proposed to analyze a traveling-wave terahertz photomixer integrated with a coplanar stripline waveguide. Multilayer spectral domain method along with complex image technique have been applied to calculate the distributed voltage source element in the transmission line representation. To find the coupled terahertz signal along the coplanar stripline, the transmission line equations are solved. The results obtained from the proposed method are verified by the full wave analysis.     

亚波长线栅太赫兹偏振分束器的研究

为了提高工作在太赫兹波段偏振分束器的性能,采用激光诱导与化学镀铜的方法在聚酰亚胺衬底上制作了亚波长周期金属线栅太赫兹偏振分束器,并以返波振荡器作为太赫兹辐射源搭建了偏振分束特性测试系统。当入射波频率为340GHz、入射角为45°时,测得在入射波偏振在0°～180°变化过程中,该偏振分束器具有良好的偏振分束特性,透射和反射的消光比分别为27.3dB和10.5dB,插入损耗分别为0.13dB和0.32dB;用太赫兹时域光谱系统测得偏振分束器在0.2THz～1.5THz频域内透射消光比大于18dB。结果表明,测试结果与时域有限元方法模拟结果基本吻合。     

Study of atmospheric attenuation characteristics of terahertz wave based on line‐by‐line integration

Terahertz band communication promises new solution for satisfying the increasing demand for ultrahigh‐speed wireless communication. Channel models capturing the unique peculiarities of the terahertz (THz) band are required for communication systems designing. Extreme high molecular absorption is a distinctive phenomenon that has to be involved in terahertz communication models. Research in this field has mainly focused on the characteristics along the horizontal propagation path. In this paper, we developed a unified molecular absorption model along the slant propagation path of the THz wave, based on the line‐by‐line integration method developed by Van‐Vleck and Weisskopf and combining the molecular spectral line in the HITRAN database. Then, an in‐depth analysis on the THz channel characteristics is carried out by the developed propagation models. The attenuation characteristics of terahertz waves with frequencies in the range of 0.1 to 1 THz are analyzed by theoretical and mathematical modeling. The results show that the terahertz communication channel has a strong dependence on both the molecular composition of the medium and the transmission distance. The experimental results also indicate the strong absorption frequency points, weak absorption frequency points, and spectral windows.     

High-resolution emission spectra of pulsed terahertz quantum-cascade lasers

The spectra of pulsed terahertz quantum-cascade lasers were measured with high spectral resolution. The characteristic line width at half maximum was 0.01 cm⁻¹; it is controlled by laser temperature variations during the supply voltage pulse. It was shown that an increase in the laser temperature leads to a decrease in the emission frequency, which is caused by an increase in the effective refractive index of the active region. It was also found that a decrease in the supply voltage results in a decrease in the emission frequency, which is caused by a change in the energy of diagonal transitions between lasing levels.     

Continuous-Wave Frequency-Tunable Terahertz-Wave Generation From GaP

Continuous-wave (CW) single-frequency terahertz (THz) waves were generated using difference-frequency generation via excitation of phonon–polaritons in GaP. The two pump sources were an external cavity laser diode (LD) and an LD-pumped Nd : YAG laser. The power density of the latter beam was enhanced by using a ytterbium-doped fiber amplifier. The two incident beams were focused to near the wavelength of THz waves. This optical alignment enabled us to generate frequency-tunable CW THz waves in the 0.69–2.74 THz range. With a fixed angle between the pump beams, we obtained a frequency bandwidth as large as 600 GHz.     

共振声子THz QCL有源区结构设计

利用Airy函数代换与传输矩阵方法精确计算了有外加偏压下电子在共振声子太赫兹量子级联激光器有源区单个周期内的透射系数与波函数,得到了不同偏压下的电子波函数分布以及准束缚态能级位置与外加偏压的关系曲线.在仿真计算的基础上设计了一种共振声子太赫兹量子级联激光器的有源区结构.计算结果表明,对于设计的结构,当单个周期两端的外加...     

Nonlinear analysis of microwave superconductor devices usingfull-wave electromagnetic model

This paper presents a full electromagnetic wave analysis for modeling the nonlinearity in high temperature superconductor (HTS) microwave and millimeter-wave devices. The HTS nonlinear model is based on the Ginzburg-Landau theory. The electromagnetic fields associated with the currents on the superconducting structure are obtained using a three-dimensional full wave solution of Maxwell's equations. A three-dimensional finite-difference time-domain algorithm simultaneously solves the resulting equations. The entire solution is performed in time domain, which is a must for this type of nonlinearity analysis. The macroscopic parameters of the HTS, the super fluid penetration depth and the normal fluid conductivity, are calculated as functions of the applied magnetic field. The nonlinear propagation characteristics for HTS transmission line, including the effective dielectric constant and the attenuation constant, are calculated, As the power on the transmission line increases, the phase velocity decreases and the line losses increase. The nonlinearity effects on the current distributions inside the HTS, the electromagnetic field distributions, and the frequency spectrum are also analyzed     

Analysis and design of a continuous-wave terahertz photoconductive photomixer array source

An array of photoconductive photomixer/antenna elements as a continuous-wave terahertz source is proposed, and its radiation characteristic is studied. Employing photomixer/antenna elements in an array configuration increases available terahertz power, while each of the array elements consumes optical power less than its maximum sustainable power. A few microwatt terahertz power is achievable from a typical array structure. It is shown that the radiated beam can be steered by changing the angle between the two exciting laser beams.     

气体等离子体产生太赫兹波的特性研究

基于光电流模型,对不同偏振情况的双色飞秒激光脉冲聚焦产生的气体等离子体中辐射出的太赫兹波特性进行了研究。根据光电流理论,气体分子被电离释放出的自由电子在非对称的激光场的作用下运动形成电子电流,产生在太赫兹波段的辐射。研究结果表明,太赫兹辐射的偏振特性与强度和入射双色激光的偏振特性紧密相关,仅当双色脉冲均为线偏振时,辐射出的太赫兹波才为线偏振,且强度受到双色脉冲偏振方向的夹角的影响;而对于实验中经过倍频晶体后变成椭圆偏振的基频光,太赫兹强度与倍频晶体的具体放置情况有很大关系,并且产生的太赫兹为椭圆偏振。     

Compact cw Terahertz Spectrometer Pumped at 1.5 μm Wavelength

A compact and low-cost continuous wave terahertz spectrometer operating at an optical wavelength of 1.5 μm is presented. The spectrometer employs high power distributed feedback (DFB) laser diodes in integrated “butterfly” packages. No further optical amplification of the beating signal is required. An integrated photodiode antenna with an output power of 5 μW at 500 GHz is used as efficient terahertz emitter. Employing low-temperature grown (LT-) InGaAs/InAlAs photoconductive receivers as coherent detectors, SNR values of the terahertz power up to 75 dB are attained at an integration time of 300 ms. Accurate characterization of the thermal tuning behavior of the DFBs and precise thermal control yield an absolute accuracy of 1 GHz and a resolution of better than 5 MHz, without any on-line monitoring of the optical frequency. Due to the high frequency resolution no delay line is needed to vary the terahertz phase.     

He-Ne激光器远距离点亮问题的研究

为了实现He-Ne激光器的远距离点亮,从He-Ne激光器的电特性及电源负载特性入手,通过FFT变换分析,解释了冲击电压钝化的原因,并通过建立高压线分布参数模型,导出了电压衰减常数,从理论上提出使用屏蔽传输线并建立一个匹配网络,通过实验加以验证,成功解决了冲击高电压远距离传输的问题,从而实现了He-Ne激光器的远距离点亮,具有重要的工程应用价值.     

中物院太赫兹自由电子激光:设计、运行及升级

中物院太赫兹自由电子激光(CTFEL)装置是我国第一台基于超导加速器的高重复频率、高平均功率太赫兹自由电子激光装置。CTFEL利用光阴极直流高压电子枪和超导加速器产生约8 MeV电子束在波荡器中产生自发太赫兹(THz)辐射,并在光腔中受激放大获得饱和输出。得益于在0.7~4.2 THz频谱范围内连续可调以及平均功率大于10 W的特性,CTFEL为材料动力学、太赫兹成像、太赫兹生物学等领域提供了独特的研究平台。自2018年开放成为用户装置以来,每年提供不少于1000 h的稳定出光。未来CTFEL将在现有基础上升级成为红外太赫兹自由电子激光装置,实现太赫兹频率全覆盖以及最大功率大于100 W的目标,力争成为世界先进的长波长自由电子激光装置。     

FM subcarrier fiber optical transmission system design and itsapplication in next-generation wireless access

This paper presents a design method of optical frequency modulation (FM) subcarrier (with super carrier) transmission modem. The nonideal link characteristics, including laser chirping, fiber dispersion, voltage controlled oscillator (VCO) phase noise, relative intensity noise (RIN), and equivalent network model of laser diode of such a system that may bring about signal distortion are discussed first. We then propose a hierarchical methods to establish the system equivalent model. Finally this FM modem is applied to a GSM wireless system, in which multichannel signals are transmitted over fiber between radio port and basestation, and system performance is appraised by its dynamic range. It is found that optical FM subcarrier (with super carrier) transmission technique can improve the system dynamic range, compared with the intensity modulated direct detection (IMDD) method, and this is a economical and efficient method     

太赫兹波在Kolmogorov湍流大气中的水平传输特性研究

太赫兹的大气传输特性对太赫兹的空间应用非常重要.对太赫兹波在Kolmogorov湍流大气中的水平传输特性展开了研究.分析了太赫兹平面波在湍流大气中传输时其闪烁指数、饱和距离等参数随频率、空间传输距离和湍流强弱程度等因素的变化.以高斯波束为例,研究了太赫兹波束在湍流大气中传输时闪烁指数、束宽等参数随传输距离等物理量的演化规律.研究结果表明,在太赫兹波段,频率越高,闪烁指数越大,饱和距离就越小.总体上看,太赫兹波在大气中传输时受大气湍流的影响程度介于光波和微波之间;对于太赫兹大气通信和成像等短程应用,太赫兹波在湍流大气中传输时一般都处于弱起伏状态;但对于太赫兹雷达和遥感等应用,由于传输距离较远,可能会出现强起伏.