Terahertz Quantum Cascade Devices: From Intersubband Transition to Microcavity Laser

In this review, we report on the study of terahertz (THz) intersubband (ISB) transitions and on the optical devices based on them. We use time-resolved THz spectroscopy to examine ISB optical transitions in semiconductor quantum wells and quantum cascade lasers (QCLs). From these measurements, we obtain important information on the carrier relaxation, scattering mechanisms, and the gain. The waveguide losses are studied directly on the QCL devices and we show the main loss mechanism in the double-metal waveguides. Finally, we demonstrate THz-QCLs with low-mode volume optical cavity.     

Terahertz Quantum Well Photodetectors

The terahertz (THz) part of the electromagnetic spectrum promises a wide range of new and novel, some may be disruptive, applications. However, the development of technologies in the THz spectrum or the very far infrared region has been slow mainly because of the lack of convenient detectors and lasers. There are a few competing new approaches for better detectors, and here, we concentrate on one based on quantum wells. We report on the design and simulated performance of quantum-well photodetectors for the terahertz (1-10 THz). Quantum well, barrier, and doping parameters are optimized in terms of operating temperature, absorption, and detectivity. We also report on our experimental demonstration of GaAs/AlGaAs photodetectors with background limited infrared performance. These devices are suited for a variety of applications, especially in conjunction with the newly developed THz quantum cascade lasers. Examples include THz sensing and imaging and free space communication.     

Heterodyne Mixing of Terahertz Quantum Cascade Lasers Using a Planar Schottky Diode

Terahertz quantum cascade lasers (QCLs) have been used together with a monolithic planar Schottky diode receiver to study the heterodyne mixing between dual internal modes of a QCL and between a single mode of a QCL and a known molecular line from a molecular gas laser. Dual-mode mixing shows that the intrinsic linewidth of a free-running QCL is les30 kHz . Mixing against a molecular laser line gives a high precision measurement of a QCL's absolute frequency and can show transient turn-on behavior in a pulsed QCL.     

基于调频连续波的太赫兹透视测距技术

由于太赫兹波能够穿透非金属、非极性材料,太赫兹技术能够弥补激光技术的不足,实现对目标内部的透视测距。研究分析提出基于线性调频连续波的太赫兹透视测距技术,实现了多种材料的厚度以及多层介质材料的介电常数的非接触式测量,进一步拓展了太赫兹线性调频连续波的应用范围,为材料介电常数测量提供了新途径。以3种常见的材料作为检测目标,利用0.11～0.17 THz、0.17～0.22 THz、0.22～0.33 THz、0.33～0.50 THz 4个频段的太赫兹线性调频连续波实验验证方法的有效性,并证明厚度测量和介电常数测量的精度随信号带宽的增加而提高,误差最小均可达到1%以下。此外,以雷达天线罩陶瓷材料作为检测目标,应用太赫兹透视测距的原理,结合二维扫描架获取样品的全部信息,实现了对内部缺陷的透视成像与定位。     

Temperature Dependence of Thermal Conductivity and Boundary Resistance in THz Quantum Cascade Lasers

We measured the lattice temperature distribution, the cross-plane thermal conductivity , and the thermal boundary resistance (TBR) of the As quantum cascade lasers (QCLs) operating at 2.83 THz in the heat sink temperature range 45-300 K. This information was extracted from the analysis of microprobe band-to-band photoluminescence in QCLs operating in continuous wave. Both and TBR decrease monotonically at increasing temperature, the main influence on arising from the high density of interfaces.     

太赫兹光谱技术的应用进展

近年来太赫兹（THz）技术快速发展,在安全检测、航空航天、生命科学、化学等领域应用日趋广泛。由于许多炸药及其相关材料在THz波段具有特征吸收,许多非金属、非极性材料对THz波是透明的,且太赫兹波具有低能性,THz光谱技术在安检中具有巨大的应用潜力。本文介绍了太赫兹波的特性和国内外在太赫兹领域的研究进展,详细介绍了太赫兹波在爆炸物、毒品和包装材料检测中的应用,并讨论了在应用中存在的困难和面临的挑战。     

Partially coherent electron transport in terahertz quantum cascade lasers based on a Markovian master equation for the density matrix

We derive a Markovian master equation for the single-electron density matrix, applicable to quantum cascade lasers (QCLs). The equation conserves the positivity of the density matrix, includes off-diagonal elements (coherences) as well as in-plane dynamics, and accounts for electron scattering with phonons and impurities. We use the model to simulate a terahertz-frequency QCL, and compare the results with both experiment and simulation via nonequilibrium Green’s functions (NEGF). We obtain very good agreement with both experiment and NEGF when the QCL is biased for optimal lasing. For the considered device, we show that the magnitude of coherences can be a significant fraction of the diagonal matrix elements, which demonstrates their importance when describing THz QCLs. We show that the in-plane energy distribution can deviate far from a heated Maxwellian distribution, which suggests that the assumption of thermalized subbands in simplified density-matrix models is inadequate. We also show that the current density and subband occupations relax toward their steady-state values on very different time scales.     

Self-consistent thermal simulation of GaAs/Al0.45Ga0.55As quantum cascade lasers

This paper presents a self-consistent thermal model for quantum cascade lasers (QCLs) that takes into account the nonuniform heat generation distribution in the active region as well as the temperature dependences of the heat generation rate and thermal conductivity. The model extracts the heat generation rate from the electron-optical phonon scattering recorded during the ensemble Monte Carlo (EMC) simulation of electron transport in a single QCL stage at different temperatures. The extracted heat generation rate, in conjunction with temperature-dependent thermal conductivities, enables us to solve the nonlinear heat diffusion equation in a self-consistent manner. The model is used to investigate the cross-plane temperature distribution throughout a 9.4 μm infrared GaAs-based QCL. The nonlinear effects stemming from the temperature dependence of thermal conductivity and the heat generation rate are studied. Finally, the accuracy of using the equivalent uniform heat source with the total power obtained from experiments to model the thermal performance of QCLs is evaluated and discussed.     

Power Scaling on Efficient Generation of Ultrafast Terahertz Pulses

We have investigated power scaling for the efficient generation of the broadband terahertz (THz) pulses. These THz short pulses are converted from ultrafast laser pulses propagating in a class of semiconductor electrooptic materials. By measuring the dependence of the THz output on the pump beam in terms of incident angle, polarization, azimuthal angle, and pump intensity, we have precisely determined the contributions made by optical rectification, drift of carriers under a surface or external field, and photo-Dember effect. When a second-order nonlinear material is pumped below its bandgap, optical rectification is always the mechanism for the THz generation. Above the bandgap, however, the three mechanisms mentioned earlier often compete with one another, depending on the material characteristics and pump intensity. At a sufficiently high pump intensity, optical rectification usually becomes the dominant mechanism for a second-order nonlinear material. Our analysis indicates that second-order nonlinear coefficients can be resonantly enhanced when a material is pumped above its bandgap. In such a case, the THz output power and normalized conversion efficiency can be dramatically increased. We have also analyzed how the THz generation is affected by some competing processes such as two-photon absorption.     

Terahertz Science and Technology Trends

The recent progress in terahertz science and technology (THz-S&T) opens up a range of potential research opportunities. Historically, THz technologies were mainly used by the astronomy community for searching far-infrared radiation (cosmic background), and by the laser fusion community for the diagnostics of plasmas. Since the first demonstration of THz wave time-domain spectroscopy in the late 1980s, there has been a series of significant advances (particularly in recent years) as intense THz sources and more sensitive detectors provide new opportunities for understanding the basic science in the THz frequency range. THz radiation can penetrate through many nonpolar dielectric materials and can be used for nondestructive/noninvasive sensing and imaging of targets under nonpolar, nonmetallic covers or containers. An immediate application of THz wave technology is in nondestructive testing or inspection. Short-term applications (within three to five years) are expected in spectroscopic sensing and imaging for homeland security. Biomedical applications are expected in the long term (five to ten years). By comparing the publication record trend of THz-S&T related papers with the publication record of proxy fields (laser, microwave, Raman, and infrared), it is possible to anticipate that the number of publications in the THz-S&T arena will increase and also the impact in other research areas. We compare the publication pattern (number of papers versus time) with searching keywords in title or abstract. We found that all the publication trends share a common pattern with four periods defined by discovery, acceptance, adoption, and maturity. From this pattern trend, THz-S&T seems to be in an acceptance period. The unique properties of THz-S&T suggest that its applications will grow.     

Gain Measurements of THz Quantum Cascade Lasers using THz Time-Domain Spectroscopy

Terahertz (THz) time-domain spectroscopy is used to investigate the gain and losses of a THz quantum cascade laser (QCL) operating at 2.86 THz. This measurement technique allows access to the amplitude and phase spectra, allowing the direct determination of the gain. At the emission frequency of the QCL, a value of 6.5 cm^-1 is found. The gain can also be studied as a function of different operating conditions, even when no laser action is present. Effects such as gain clamping and spectral narrowing are also observed. Furthermore, temperature measurements illustrate the reduction of the gain as the temperature is increased.     

1 THz Modulation in InGaAsP Multiple Quantum Wells for 40 Gb/s Applications

The terahertz (THz) rate modulation of quantum well (QW) electrooptic modulators necessitates a new way of thinking about how the modulation field modulates light; specifically, an incident narrow-band (with respect to the modulation frequency) signal once modulated acquires frequency components separated from the input signal center frequency by multiples of the modulation frequency. In this paper, we discuss the design of the QWs comprising the modulator to maximize the output at such THz sidebands of the incident optical frequency in InGaAsP QW based devices. We present a theoretical treatment of the case in which the THz modulation frequency is out of resonance between any exci- tonic levels near those exploited for the optical modulation, thus enabling an adiabatic treatment of the modulated optical susceptibility. We show that THz sideband conversion efficiencies of ~1% may be possible.     

Development and Application of Terahertz Pulsed Imaging for Nondestructive Inspection of Pharmaceutical Tablet

Terahertz pulsed imaging (TPI) was evaluated for nondestructively characterizing the 3-D internal structures of pharmaceutical tablets. The structural information of a pharmaceutical tablet, such as coating thickness and interface uniformity, was obtained directly from the analysis of the time-domain terahertz waveform. The chemical map of a sample was obtained by using frequency-domain terahertz spectra, together with spectral matching techniques such as cosine correlation mapping. The axial spatial resolution achieved was 30 mum, limited by the confinement of terahertz pulses in time (pulse width); and the lateral spatial resolution was determined to be 150 mum at 90 cm^-1 (2.7 THz), limited by the confinement of terahertz pulses in space (focus size, which is diffraction limited and thus frequency dependent). In addition, the buried structure within a tablet was mapped using the TPI, and its chemical composition was successfully identified through spectral-time analysis of the recorded terahertz waveform. We also present a rigorous electromagnetic theory for simulating the terahertz propagation in a multilayered sample, to facilitate terahertz data analysis and interpretation. In conclusion, the TPI is a powerful tool for assaying the tablet coating layer thickness and interface uniformity, and for identifying polymorphs.     

Monte Carlo modeling of X-valley leakage in quantum cascade lasers

This paper presents the first comprehensive Monte Carlo simulation of GaAs/AlGaAs quantum cascade lasers (QCLs) that takes both Γ- and X-valley transport into account and investigates the effect of X-valley leakage on the QCL performance. Excellent agreement with experimental data is obtained for the GaAs/Al_0.45Ga_0.55As QCL at cryogenic and room temperatures. The model reveals two carrier-loss mechanisms into the X valley: coupling of the Γ continuum-like states with the X states in the same stage, and coupling between the Γ localized states in the simulated stage with the X states in the next stage. Simulation results demonstrate that the 45% Al QCL has small X-valley leakage at both 77 K and 300 K, due to the very good confinement of the Γ states, stemming from the high Al content.     

碳纤维复合材料不同深度缺陷的太赫兹检测研究

太赫兹(THz)无损检测技术具有非破坏性、非电离和非接触的优点,在航空航天领域纤维增强复合材料无损检测中得到了较快的发展和应用。在碳纤维复合材料层合板的4个不同深度(0.225、0.450、0.675、0.900 mm)插入聚四氟乙烯作为人工缺陷,采用太赫兹时域光谱和成像系统对其进行成像和光谱分析,探讨太赫兹波辐射下缺陷的成像效果和光谱特性。研究结果发现,在0.25~2.0 THz频率范围内,太赫兹反射成像可以成功检测出碳纤维复合材料中不同深度缺陷:随着缺陷深度的增加,太赫兹频域成像信号和光谱信号随缺陷深度线性增大,吸收系数成像信号和光谱信号随缺陷深度线性减小;随着频率的增加,缺陷的功率谱密度先增大后减小,吸收系数缓慢增大。该结果可以为碳纤维复合材料缺陷深度的可视化和定量化分析提供参考依据。     

太赫兹测试系统中锁相放大电路的设计

为了解决太赫兹时域光谱仪(TDS)所用锁相放大器方便携带和提取与太赫兹波电场强度相关微弱信号的问题,提出了一种基于互相关原理的锁相放大器设计方案。斩波器输出调制后的信号经前级放大滤波与通过移相电路的相干参考信号分别送入AD630的信号端和参考端,锁相后信号经低通滤波电路输出直流信号。测试结果表明:设计的锁相放大器具有良好线性度,可将信噪比为-60 d B的微伏级信号提取出来,能用于大背景噪声下太赫兹微弱信号的检测。     

Terahertz Sources Based on Intracavity Parametric Down-Conversion in Quasi-Phase-Matched Gallium Arsenide

We have efficiently generated tunable terahertz (THz) radiation using intracavity parametric down-conversion in gallium arsenide (GaAs). We used three types of microstructured GaAs to quasi-phase-match the interaction: optically contacted, orientation-patterned, and diffusion-bonded GaAs. The GaAs was placed in an optical parametric oscillator (OPO) cavity, and the THz wave was generated by difference-frequency mixing between the OPO signal and idler waves. The OPO used type-II phase-matched periodically poled lithium niobate as a gain medium and was synchronously pumped by a mode-locked laser at 1064 nm (7 ps and 200 nJ at 50 MHz). With center frequencies spanning 0.4-3.5 THz, 250-GHz bandwidth radiation was generated. We measured two orders of optical cascading generated by the mixing of optical and THz waves. In a doubly resonant oscillator (DRO) configuration, the efficiency increased by 21times over the singly resonant oscillator performance with an optical-to-THz efficiency of 10^-4 and average THz power of 1 mW. The GaAs stabilized the DRO by a thermooptic feedback mechanism that created a quasi- continuous-wave train of THz pulses.     

太赫兹波技术在药学上的应用研究

太赫兹(THz)辐射是一种新型的远红外相干辐射源,太赫兹时域光谱技术是利用太赫兹脉冲研究物质物理化学性质的一种新兴光谱技术,具有很多独特的性质。由于很多极性生物大分子的振动和旋转能级都处于在太赫兹波段内,因此利用太赫兹时域光谱技术和太赫兹成像技术研究药品的结构,分析药品的成分,对控制药品的质量稳定性具有重要的意义。综述了近些年来太赫兹技术在药品领域上的应用成果,探讨了存在的问题,并对发展前景进行了展望。     

A Dual Band Polarization-insensitive Tunable Absorber Based on Terahertz MEMS Metamaterial

We present the design, numerical simulations and characterization of a dynamically tunable dual band metamaterial absorber that shows two distinct absorption peaks in terahertz (THz) regime. The designed absorber consists of a metal ground plane fixed on substrate and a movable metamaterial suspended above the ground plane. Through applying a bias voltage to adjust the height between the suspended elements and metal ground plane, we can switch absorber's state between absorption and reflection. Moreover, the designed absorber is polarization-insensitive and can be operated over a wide range of incidence angles owing to the high degree symmetry of designed structure.