Terahertz Frequency-Domain Spectroscopy of Low-Pressure Acetonitrile Gas by a Photomixing Terahertz Synthesizer Referenced to Dual Optical Frequency Combs

A terahertz (THz) frequency synthesizer based on photomixing of two near-infrared lasers with a sub-THz to THz frequency offset is a powerful tool for spectroscopy of polar gas molecules due to its broad spectral coverage; however, its frequency accuracy and resolution are relatively low. To tune the output frequency continuously and widely while maintaining its traceability to a frequency standard, we developed a photomixing THz synthesizer phase-locked to dual optical frequency combs (OFCs). While the phase-locking to dual OFCs ensured continuous tuning within a spectral range of 120 GHz, in addition to the traceability to the frequency standard, use of a broadband uni-traveling carrier photodiode for photomixing enabled the generation of CW-THz radiation within a frequency range from 0.2 to 1.5 THz. We demonstrated THz frequency-domain spectroscopy of gas-phase acetonitrile CH₃CN and its isotope CH₃ ¹³CN in the frequency range of 0.600–0.720 THz using this THz synthesizer. Their rotational transitions were assigned with a frequency accuracy of 8.42?×?10^?8 and a frequency resolution of 520 kHz. Furthermore, the concentration of the CH₃CN gas at 20 Pa was determined to be (5.41?±?0.05)?×?10¹⁴ molecules/cm³ by curve fitting analysis of the measured absorbance spectrum, and the mixture ratio of the mixed CH₃CN/CH₃ ¹³CN gas was determined to be 1:2.26 with a gas concentration of 10¹⁴–10¹⁵ molecules/cm³. The developed THz synthesizer is highly promising for high-precision THz-FDS of low-pressure molecular gases and will enable the qualitative and quantitative analyses of multiple gases.     

双光频梳辅助的微波频率测量方法北大核心CSCD

为实现大频率范围内多微波频率的高精度即时测量,提出了一种基于双光频梳和法布里-珀罗滤波器(Fabry-Perot filter,FPF)的微波频率测量方案。将待测微波信号通过单边带调制加载在一路光频梳上进行频率复制,再将调制后的光信号通过一个法布里-珀罗滤波器进行信道分割。FPF的输出信号和另一路光频梳耦合后通过光解复用器实现信道化接收。通过设置合适的系统参数,可使得每个信道输出的拍频信号在同一窄带中频频段内。基于频率-光功率映射的原理,通过判断信号的存在性即可确定未知信号所处的频率段,实现信号频率的粗估计。通过对目标信道输出的微波信号进行采样、模数变换和信号处理可实现未知信号频率的高精度测量。通过实验仿真验证了所提方案的有效性,频率测量误差在±2.5 MHz内。     

Frequency Uncertainty for Optically Referenced Femtosecond Laser Frequency Combs

We present measurements and analysis of the currently known relative frequency uncertainty of femtosecond laser frequency combs (FLFCs) based on Kerr-lens mode-locked Ti:sapphire lasers. Broadband frequency combs generated directly from the laser oscillator, as well as octave-spanning combs generated with nonlinear optical fiber are compared. The relative frequency uncertainty introduced by an optically referenced FLFC is measured for both its optical and microwave outputs. We find that the relative frequency uncertainty of the optical and microwave outputs of the FLFC can be as low as 8times10^-20 and 1.7times10^-18, with a confidence level of 95%, respectively. Photo-detection of the optical pulse train introduces a small amount of excess noise, which degrades the stability and subsequent relative frequency uncertainty limit of the microwave output to 2.6times10^-17     

飞秒光学频率梳在精密测量中的应用

飞秒光学频率梳通过锁定飞秒锁模激光的重复频率和偏置频率至微波频率基准,在时域上得到重复频率稳定的飞秒脉冲激光,在频域上得到频率间隔稳定的激光频率梳。飞秒光学频率梳作为微波频率与光学频率的桥梁,可以实现对激光频率的直接精密计量,同时作为一种有别于传统连续波稳频激光的特殊激光光源,在激光频率标尺、绝对距离测量和精密光谱测量等光学精密测量领域都有着重要应用。综述了飞秒光学频率梳在若干光学精密测量应用中的研究进展、关键技术和研究动向,分析了其在未来光学测量中的重要作用。     

Polarization-independent Metamaterial Absorber for Terahertz Frequency

We present the design, fabrication, as well as the characterization of the large incident angle and polarization independent terahertz (THz) metamaterial absorber. The designed absorber consists of metallic cross structure and a metallic ground plane separated with a dielectric as a spacer, this structure shows a strong resonance at terahertz frequency. The influences of the structural parameters (including period, cross width and spacer thickness) on the absorption are discussed. An optimized structure is fabricated and characterized, and a good agreement between simulation and experimental result is obtained. The metamaterial absorbers are potential candidates as absorbing elements for terahertz imaging and other fields of terahertz technology.     

Advances in Polarizer Technology for Terahertz Frequency Applications

As investigations into potential applications of terahertz technology grow, there is an increasing need for improved terahertz optical components such as polarizers. To determine the optical properties of a sample accurately, the polarization properties of the light must also be known accurately. Many terahertz emitters will have both horizontal and vertical polarization components and often assumptions are made about device characteristics without measuring them-even the position of excitation beam on the photoconductive emitter can affect the resulting terahertz electric field and so the exact optical properties of a given device will vary depending on how they are configured. Polarizers operating at terahertz frequencies can be used to characterize the electric field accurately or remove unwanted components as long as the polarizer is of sufficiently high performance. In this paper we review the key properties of polarizers and look at recent advances in their design and development at terahertz frequencies.     

Double-Layer Frequency Selective Surface for Terahertz Bandpass Filter

A thin, low-cost, double-layer frequency-selective surface (FSS) bandpass filter has been designed, fabricated, and measured for operation in the terahertz band. The present FSS structure is composed of double-layer tin foil with a hexagonal lattice array of circular holes. A flat passband is observed with a 3-dB bandwidth of 0.20 THz from 0.81 to 1.01 THz, which is about 22% relative to the center frequency. This bandpass filter based on double-layer FSS structures achieved average insertion loss of 0.7-dB and 106.31-dB/THz band-edge steepness. We have found good agreement between theoretical calculations and experimental results.     

Real-Time Determination of Absolute Frequency in Continuous-Wave Terahertz Radiation with a Photocarrier Terahertz Frequency Comb Induced by an Unstabilized Femtosecond Laser

A practical method for the absolute frequency measurement of continuous-wave terahertz (CW-THz) radiation uses a photocarrier terahertz frequency comb (PC-THz comb) because of its ability to realize real-time, precise measurement without the need for cryogenic cooling. However, the requirement for precise stabilization of the repetition frequency (f _rep) and/or use of dual femtosecond lasers hinders its practical use. In this article, based on the fact that an equal interval between PC-THz comb modes is always maintained regardless of the fluctuation in f _rep, the PC-THz comb induced by an unstabilized laser was used to determine the absolute frequency f _THz of CW-THz radiation. Using an f _rep-free-running PC-THz comb, the f _THz of the frequency-fixed or frequency-fluctuated active frequency multiplier chain CW-THz source was determined at a measurement rate of 10 Hz with a relative accuracy of 8.2?×?10^?13 and a relative precision of 8.8?×?10^?12 to a rubidium frequency standard. Furthermore, f _THz was correctly determined even when fluctuating over a range of 20 GHz. The proposed method enables the use of any commercial femtosecond laser for the absolute frequency measurement of CW-THz radiation.     

基于SrTiO_3的频率可调谐太赫兹超材料吸波器（英文）

基于温度敏感材料钛酸锶(SrTiO3)提出了两款频率可调谐太赫兹(THz)超材料(MM)吸波器。由于SrTiO3材料的复值介电常数与外界温度相关,因此基于SrTiO3材料的太赫兹超材料吸波器的谐振频率可随外界温度变化。一款是基于十字金属谐振结构和SrTiO3介质层实现的,在200～400 K的温度范围内,其谐振频率可在1.62～2.44 THz的宽频带范围内实现主动调谐。另一款超材料吸波器通过在十字环金属谐振结构内填充SrTiO3材料来实现,而中间介电层仍然采用常见的聚二甲基硅氧烷(PDMS)材料。当外部温度从200 K变为400 K时,谐振频率从1.11 THz移至1.58 THz,频率偏移达到了470 GHz,实现了频率可调的太赫兹超材料吸波器。可调谐超材料吸波器的实现可进一步扩展超材料吸波器的应用领域,从而更好地适应如太赫兹成像、太赫兹检测、传感和隐身等各种应用。     

Nonohmic Contact Planar Varactor Frequency Upconverters for Terahertz Applications

The development of tunerless millimeter and sub-millimeter wavelength frequency upconverters with integrated planar varactor circuits is described in this paper. An upconverter operating at 200 GHz is implemented as a proof-of-principle demonstration, and designs using both ohmic and nonohmic contacts are tested. The nonohmic contact facilitates fabrication. Both the ohmic and nonohmic devices produced similar performance. Using the nonohmic cathode contact technique and an air-bridge process, 1.6-THz integrated upconverter circuits are fabricated, and the conversion loss is measured to be approximately 22 dB with 40 muW of output sideband power with a 10-GHz microwave pump. A phase-shift measurement for the 1.6-THz upconverter using a standing wave method is described and used to corroborate the results     

GaAs/AlN异构集成太赫兹倍频器芯片

针对太赫兹GaAs肖特基二极管倍频器芯片散热能力差导致输出功率低的问题,开展了GaAs/AlN异构集成太赫兹倍频器芯片研究。通过稳态热仿真发现,将肖特基二极管芯片衬底由GaAs替换为热导率更高的AlN可以降低结温。对芯片衬底替换工艺开展了研究,获得了GaAs/AlN异构集成太赫兹二极管。分别对基于GaAs衬底二极管和基于GaAs/AlN异构集成二极管的162 GHz倍频器开展功率性能测试对比。测试结果表明:装配GaAs衬底二极管的倍频器输入功率为200 mW时,输出功率最高为43.6 mW;而装配GaAs/AlN异构集成二极管的倍频器输入功率提高到316 mW,输出功率为72.4 mW。肖特基二极管由GaAs衬底替换为AlN衬底后耐受功率(输入功率)提高了约58%,倍频效率由21.8%提升至22.9%,输出功率也相应提升,验证了相比GaAs衬底肖特基二极管,GaAs/AlN异构集成太赫兹二极管的散热性能及耐受功率具有明显的优越性。     

Single Sampling Point Detection of Frequency Modulated Terahertz Waves

We present single sampling point detection of the amplitude and phase of frequency modulated continuous wave terahertz radiation. Employing a conventional photomixing setup with a precise wavelength control allows for rapid scan rates in the kHz regime. Two application examples demonstrate the potential of this approach for real time monitoring of samples’ thicknesses and transmittances.     

Terahertz Frequency Metrology for Spectroscopic Applications: a Review

We provide an overview on terahertz (THz) frequency metrology, starting from the nowadays available continuous wave THz sources, discussing their main features such as tunability, spectral purity, and frequency referencing to the primary frequency standards. A comparison on the achieved results in high-precision molecular spectroscopy is given and discussed, and finally, a special emphasis poses on the future developments of this upcoming field.     

太赫兹时域光谱法测定高电子迁移率晶体管的截止工作频率

随着高速半导体器件的发展,高电子迁移率晶体管(HEMT)的工作频率已经达到亚太赫兹波段,所以无法简单地通过传统电学方法进行检测。鉴于此,必须采用超快光学方法测定HEMT器件的截止工作频率。利用持续时间更短的飞秒脉冲激光瞬时关断处于饱和工作状态下的HEMT器件,并且采用太赫兹时域光谱技术测量在器件被关断后电流的变化情况(时间是皮秒量级),最后,利用所测量到的太赫兹波形(即源漏电流随时间变化的曲线)和截止工作频率的关系,直接推算出可达到亚太赫兹波段的HEMT器件的截止工作频率。     

A Fast Terahertz Spectrometer Based on Frequency Selective Surface Filters

We present a fast spectrometer working in the 0.7–4.8 THz range. Broadband radiation from a blackbody source is focused first on a rotating silicon wafer, whose surface was patterned with 18 metal band-pass filters, then on the sample under test and finally is detected by a superconducting microbolometer with microsecond time constant. The bolometer sensor is coupled to a spiral antenna whose frequency band matches the spectral range of the filters. The spectral resolution is set by the filters quality factor of about 3. A dynamic range of 100 and a S/N ratio of 20 are achieved by integrating for less than 10 second. The detector can operate up to 6 K in a closed-cycle cooler, hence making the present apparatus suitable for building up a simple terahertz video-rate spectrometer.     

基于频率选择表面的太赫兹双通带滤波器研究

目前双频带频率选择表面(Frequency Selective Surface,FSS)结构存在的通带选频性能有限。为了提高太赫兹频段通带滤波器件的性能,设计了一种基于频率选择表面的太赫兹双通带滤波器。该器件主要由三层正四边形的同心金属铜环组成,中间加载了介质衬底。金属层与衬底的厚度均为0.04λ,其中λ为低频波段的波长,单元尺寸为0.15λ。根据等效电路法分析了该结构产生双频通带的原因,并对设计的结构优化作了进一步的讨论。通过改变等效电路元件的参数等价地调整了原始结构的几何参数。仿真结果与理论计算基本吻合,优化后的器件可在2.74 THz和8.17 THz两个中心频率处实现通带滤波。低频处的-1 dB通带范围为2.24～3.25 THz,插入损耗为-0.05 dB;高频处的-1 dB通带范围为7.63～8.71 THz,插入损耗为-0.12 dB。滤波特性曲线的陡峭度好且相对带宽均超过10%。该结构尺寸小,频率选择性好,角度稳定性高,可用于太赫兹频段的宽频带滤波,对未来的太赫兹通信具有重要的应用价值。     

一种改进的THz-SAR高频振动误差补偿方法

将太赫兹波用于SAR成像可以解决常规SAR成像帧速低、慢动目标检测困难等问题。太赫兹合成孔径雷达(THz-SAR)与传统微波SAR成像最重要的区别在于运动补偿。因为THz-SAR的工作波长比传统微波SAR要短得多,平台的微小振动会影响成像质量,尤其是高频振动误差。平台的高频振动会在成像结果中引入成对回波,传统SAR成像算法无法实现成对回波的聚焦,也就无法准确估计振动参数,进而构造参考函数补偿高频振动带来的正弦调制相位。首先基于多普勒Keystone变换(DKT)的THz-SAR成像算法实现成对回波的聚焦成像;然后提出小波多分辨分析的方法估计高频振动频率,结合参数空间投影法完成振动幅相的估计,并实现高频振动误差的补偿;最后采用点目标的回波数据仿真验证了所提方法的有效性。     

基于高效棱镜耦合的氟化镁微腔孤子光频梳产生

微腔孤子光频梳在相干光通信、光学频率合成、激光雷达、微波光子学和量子光学等领域有着广阔的应用前景,高效棱镜耦合是晶体微腔孤子光频梳集成应用及系统封装的必然技术途径。文章研制开发了一种耦合效率和有载Q值分别达到71.56%和1.8×10⁹的氟化镁微腔-棱镜耦合系统,并且基于该高效棱镜耦合系统实现了氟化镁微腔孤子光频梳和15.99 GHz低相噪微波信号产生,拍频信号相位噪声水平约为-117 dBc/Hz@10 kHz,极大推动了低相噪微型光电振荡器的实际应用发展。     

克尔微腔光频梳理论分析模型及多波长光源应用探讨

基于高品质因子微谐振腔的参量四波混频实现光学频率梳是一种新的频率梳实现方法,拓展了传统固体及非线性光纤飞秒激光器等光频梳的应用范围,在精密频率标定、天文光谱校准、任意波形产生、光学存储和孤子传输、片上通信用光源等方面具有较高的优势。本文简要总结、评述了几种主要的光频梳动力学分析模型及数值方法,以及这些不同方法的内在联系。基于描述光频梳动态行为的非线性Lugiato-Lefever方程分析了可能存在的动力学过程,并据此对不同特点光频梳进行了分类。通过设计反馈结构理论上研究了正常色散微腔和反常色散微腔的光梳特点,探讨了作为片上光互连用多波长光源应满足的条件及可能的实现途径。