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.     

High‐brightness electron sources for a next‐generation light source based on an energy recovery linac

A high‐brightness electron source of ultrasmall emittance and high average current is one of the most important components for next‐generation light sources based on an energy‐recovery linac (ERL). Such a high‐brightness electron source can be realized by a DC photocathode gun driven by laser pulses tailored in the temporal and spatial dimensions. We propose a novel photocathode based on a quantum cascade laser (QCL). Since the ultrafast response of photoelectron emission from QCL is compatible with the tailored laser pulses, it is a candidate electron source for ERLs. © 2011 Wiley Periodicals, Inc. Electr Eng Jpn, 177(3): 46–53, 2011; Published online in Wiley Online Library ( wileyonlinelibrary.com ). DOI 10.1002/eej.21154     

Confined phonon scattering in multivalley Monte Carlo simulation of quantum cascade lasers

We present a detailed investigation of the effects that optical phonon confinement has on the electronic transport properties of GaAs-based multiple-quantum-well (MQW) quantum cascade laser (QCL) structures. Both confined and interface phonon modes are included based on the macroscopic dielectric continuum model. Interface phonon dispersions are obtained using the transfer matrix method with periodic boundary conditions. Scattering rates of both Γ- and X-valley electrons by all the interface and confined phonon modes are calculated and fully incorporated in the multivalley Monte Carlo simulation of a deep-active-well 6.7 μm GaAs-based MQW QCL. We find that the inclusion of phonon confinement enhances the electron-phonon scattering rates and output current to a relatively small extent with respect to the bulk phonon approximation.     

The role of temperature in quantum-cascade laser waveguides

The role of temperature on the properties of quantum-cascade laser (QCL) waveguides is investigated. One-dimensional waveguide parameters are obtained using a transfer-matrix technique and the complex dielectric constants of the waveguide layers are calculated using a semi-classical Drude–Lorentz model. To model the effect of temperature on the waveguide parameters, a temperature dependent electron mobility is incorporated within the Drude–Lorentz framework. It is shown that by including the effect of temperature, a significant improvement in the agreement with experiment of the waveguide loss and hence the laser threshold current density can be achieved.     

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.     

Non-equilibrium quantum transport theory: current and gain in quantum cascade lasers

We have developed a self-consistent non-equilibrium Green’s function theory (NEGF) for charge transport and optical gain in THz quantum cascade lasers (QCL) and present quantitative results for the I-V characteristics, optical gain, as well as the temperature dependence of the current density for a concrete GaAs/Al_.15Ga_.85As QCL structure. Phonon scattering, impurity, Hartree electron-electron and interface roughness scattering within the self-consistent Born approximation are taken into account. We show that the characteristic QCL device properties can be successfully modeled by taking into account a single period of the structure, provided the system is consistently treated as open quantum system. In order to support this finding, we have developed two different numerically efficient contact models and compare single-period results with a quasi-periodic NEGF calculation. Both approaches show good agreement with experiment as well as with one another.     

基于分布反馈半导体激光器的CO气体光声光谱检测特性

一氧化碳(CO)是变压器油中溶解的主要故障特征气体之一,能有效反映运行电力变压器中油纸绝缘的放电及老化;而光声光谱(photoacoustic spectroscopy,PAS)技术能够应用于变压器油中溶解气体的在线监测。基于光声光谱检测原理,利用分布反馈(distributed feedback,DFB)半导体激光器搭建了CO气体光声光谱检测平台,选择1.567m的CO分子谱线为研究对象,实验研究了DFB半导体激光器的辐射特性和CO气体的光声光谱响应特性,分析了光声信号与激光功率、气体浓度的关系。结果表明:在气体吸收未发生饱和效应的条件下,光声信号与激光功率、气体浓度均具有良好的线性关系。该研究结果为变压器油中气体光声光谱的在线监测奠定了基础。     

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.     

Frequency tripling of a Q-switched Cr:LiSAF laser to the UV region

Frequency tripling of a Q-switched, tunable Cr:LiSAF laser to the UV wavelength region is accomplished by a mixing scheme involving second harmonic generation in lithium triborate (LBO), followed by sum frequency generation in β-barium borate (BBO). The generated UV output is tunable between 260 nm and 320 nm     

线激光清除架空线路树障时温度和效率分析

架空线路树障会引起线路跳闸故障,是线路运行的重大隐患.采用高能激光清除架空输电线路下方树障是一种安全有效的清障方式.以6种典型树木类型为研究对象,建立有限元模型,分析点、线激光作用下6种树木的温度分布规律及灼烧效率并进行了点、线激光灼烧实验.结果表明,线激光照射时,树木表面温度从光斑中心向光斑边缘急剧降低,仅在灼烧中心...     

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.     

VUV laser in the Lyman band of molecular hydrogen pumped by fstitanium-sapphire laser pulses

We report lasing at 160 nm in the Lyman band of molecular hydrogen. The laser is pumped by 200 mJ/150 fs pulses from the ATLAS titanium-sapphire laser at our institute. The pump pulses are focused at an angle of incidence of 60° onto a 9-cm-long gold target to a line focus, generating traveling-wave excitation. With 80 mbar of hydrogen in the target chamber we measure an average gain of 1.1 cm^-1 and achieve a total gain-length product of 10. The evaluation of the far-field pattern shows that the beam originates from a region with an electron density of 5×10¹⁵ cm^-3. A simple model of the H₂ laser is presented which explains the main part of our observations and supports a pump mechanism of photoelectron pumping     

组合激光雷达系统在输电线路测量中的应用

针对传统人工测量输电线路方法精度与效率较低的问题,提出一种机载激光雷达系统与地面激光扫描仪协同工作的组合激光雷达系统进行输电线路测量。机载激光雷达系统负责输电线路走廊点云数据的采集,地面激光扫描仪负责电塔高精度点云数据的采集,通过计算两个坐标系之间的旋转矩阵与平移向量,实现两组点云数据的融合。在张北500KV高压输电线路工程中进行应用试验,试验结果表明,该系统可以高效、准确地实现输电线路走廊中危险点的检测以及电力塔的倾斜度与变形监测,具有一定的实用价值。     

Resonant-Phonon Terahertz Quantum-Cascade Lasers and Video-Rate Terahertz Imaging

We review the development of terahertz quantum-cascade lasers (QCLs) that can be uniquely qualified based on a resonant-phonon depopulation scheme. Record performances in terms of operating temperature and optical power output are reported. The best temperature performance is achieved in the metal-metal (MM) waveguides, which provide near-unity mode confinement and low waveguiding loss at terahertz (THz) frequencies even for cavities with subwavelength dimensions. A pulsed operation up to a heat-sink temperature of 169 K at v ~ 2.7 THz and a continuous-wave (CW) operation up to 117 K at v ~ 3 THz are demonstrated with a five-level design that has a two-well injector region. Some of the key temperature degradation mechanisms for this design are discussed. For operation at lower frequencies (v < 2 THz), a one-well injector design is developed that reduces intersubband absorption losses in the injector region. A QCL operating at v = 1.59 THz (lambda = 188.5 mum) up to a heat-sink temperature of 71 K in cw mode is demonstrated with that design. To obtain high-power output and low beam divergence from the MM waveguides, a lens-coupled scheme is demonstrated. A peak power output of 145 mW, a beam width of 4.8deg, and a maximum lasing temperature of 160 K are obtained from a 4.1 THz QCL in this configuration. In the latter part of the paper, we report on the demonstration of video-rate (20 frames/s) terahertz imaging with QCLs as the source for illumination and a 320 times 240 element room-temperature microbolometer focal plane array as the detector. The QCLs for the imaging system are processed into semiinsulating surface-plasmon waveguides, and are operated in a cryogen-free thermomechanical cooler in quasi-CW mode at a heat-sink temperature of ~30 K. Real-time imaging in transmission mode is demonstrated at a standoff distance of 25 m with a v ~ 4.9 THz QCL in this setup.     

旋转横流中侧边射流偏斜和扩散的实验研究

为研究旋转横流中侧边射流的偏斜和扩散特性,搭建了横向射流流动显示实验台架,采用可调同步激光器和CCD摄影技术获得了不同射流速度比率时(r=15、30、45、60、75)横向射流的流场结构,基于横流槽道特征尺寸和射流速度的雷诺数Re介于22 537和112 683之间.实验结果表明,旋转横流中侧边射流的射流速度比率较大(r>30)时,射流深度基本不变,异于均匀横流中侧边射流的气流偏斜特性.另外,并非越大的射流速度比率就能获得越大的射流扩展宽度,当r=60时,系统的射流扩展宽度最大,而且在较大的射流深度区域内维持较大的扩展宽度,有利于物质的混合扩散.     

融合轮廓特征的线激光点云的快速配准算法

点云配准是计算机三维视觉的研究热点。传统点云配准算法存在着配准时间长和配准成功率低的问题,针对上述问题,设计了融合轮廓特征的线激光点云配准算法。该算法通过搜索轮廓特征关键点,并将这些关键点用于配准迭代并计算配准结果,减少了迭代次数且对源点云和目标点云初始位置要求较低。实验对比了迭代最近点(ICP)算法、Fast ICP算法和改进的点云配准算法,实验结果表明改进的点云配准算法的配准效果明显改善,与ICP和Fast ICP算法相比,改进的点云配准算法在速度上分别提高了14倍和2倍,并且未出现配准失败的情况。     

航空三维激光扫描与成像技术在送电线路工程中的应用

航空三维激光扫描与摄影测量系统是当今世界上摄影测量与遥感领域最先进的对地观测系统。本文根据国外航空三维激光扫描与摄影测量技术在工程中的应用情况 ,结合超高压送电线路工程的特点 ,提出了将航空三维激光扫描与摄影测量技术应用于超高压送电线路工程的设想 ,就其应用原理、方法和结果进行了探讨。     

Development of a continuous-wave, deep-ultraviolet, and single-frequency coherent light source-challenges toward laser cooling of silicon

Laser cooling techniques of silicon may be a powerful tool to create a new silicon road map. In the first step, to realize the laser cooling of neutral silicon atoms highly efficient frequency conversions are conducted to obtain a deep-ultraviolet single-mode coherent light using two-stage external cavities. The 154-mW power at around 252 nm is obtained with a conversion efficiency of more than 8% by doubly resonant sum-frequency mixing of 373-nm light from the first-stage conversion and 780-nm light from a single-mode Ti:sapphire laser. This paper reviews a series of challenges for taking possession of the spatial design of nuclear spins of the family of stable isotopes with laser cooling of silicon.     

CW solid-state ultraviolet laser for optical disk masteringapplication

We have built a prototype laser head that emits 355-nm ultraviolet light in CW mode. The system is based on nonlinear sum-frequency mixing using a β-BaB₂O₄ (BBO) crystal in an external resonator in which two input emissions at 1064 and 532 nm are resonantly enhanced simultaneously. The output exhibits promising characteristics as the master recording laser source, in terms of beam shape and intensity stability. The result of reliability testing of the system in 1000-h real-time operation is also discussed