光泵调制掺杂阶梯量子阱THz激光器瞬态动力学的Monte Carlo模拟

采用系综Monte Carlo（EMC）方法首次对光泵量子阱THz激光器的载流子瞬态动力学进行了分析。提出的器件原型为三能级调制掺杂GaAs/GaxAl(1-x)As系列非对称阶梯量子阱,激射频率为6.1THz。模拟中包括了电子-电子、电子-光学声子和电子-声学声子等散射机制,采用调制掺杂以得到较高电子密度可以忽略电子-电离杂质散射。已报道的研究工作都是在量子阱中掺杂,而对于这种器件原型能否得到电子布居反转,报道的结果也是相互矛盾。器件原型在温度为77K,光泵强度达到一定值时可以得到电子布居反转,所得到的研究结果对相关的实验研究具有一定的指导意义。     

Backward Wave Oscillator Based THz Spectroscopy, Diatomic Formalism and Optically Pumped Lasers

In this paper the Backward Wave Oscillator (BWO) based spectroscopy technique and its application to molecular spectroscopy especially to diatomic molecules have been discussed. This is a new and electronically controlled technique with enormous capabilities most of which are yet to be exploited. This paper also presents a part of the culmination of the collective efforts in developing a cohesive and consolidated enunciation of the spectroscopic parameters and their relationship to effective molecular Hamiltonians for diatomic formalism, linear four- atomic formalism & quasi-linearity, Watson Hamiltonian and the complexities in symmetric and asymmetric top spectral structures and its relationship to optical pumping and / or interstellar space. All these considered together present a beautiful and consistent picture of molecular spectroscopy and THz Electromagnetic sources.     

Terahertz Semiconductor Quantum Well Devices

For eventually providing terahertz science with compact and convenient devices,terahertz (1～10THz) quantum-well photodetectors and quantum-cascade lasers are investigated. The design and projected detector performance are presented together with experimental results for several test devices,all working at photon energies below and around optical phonons. Background limited infrared performance (BLIP) operations are observed for all samples (three in total) ,designed for different wavelengths. BLIP temperatures of 17,13, and 12K are achieved for peak detection frequencies of 9.7THz(31μm) ,5.4THz(56μm) ,and 3.2THz(93μm) ,respectively. A set of THz quantum-cascade lasers with identical device parameters except for doping concentration is studied. The δ-doping density for each period varies from 3.2 × 1010 to 4. 8 × 1010cm-2. We observe that the lasing threshold current density increases monotonically with doping concentration. Moreover, the measurements for devices with different cavity lengths provide evidence that the free carrier absorption causes the waveguide loss also to increase monotonically. Interestingly the observed maximum lasing temperature is best at a doping density of 3.6 × 1010cm-2.     

Terahertz Semiconductor Quantum Well Devices

For eventually providing terahertz science with compact and convenient devices,terahertz (1～10THz) quantum-well photodetectors and quantum-cascade lasers are investigated. The design and projected detector performance are presented together with experimental results for several test devices,all working at photon energies below and around optical phonons. Background limited infrared performance (BLIP) operations are observed for all samples (three in total) ,designed for different wavelengths. BLIP temperatures of 17,13, and 12K are achieved for peak detection frequencies of 9.7THz(31μm) ,5.4THz(56μm) ,and 3.2THz(93μm) ,respectively. A set of THz quantum-cascade lasers with identical device parameters except for doping concentration is studied. The δ-doping density for each period varies from 3.2 × 1010 to 4. 8 × 1010cm-2. We observe that the lasing threshold current density increases monotonically with doping concentration. Moreover, the measurements for devices with different cavity lengths provide evidence that the free carrier absorption causes the waveguide loss also to increase monotonically. Interestingly the observed maximum lasing temperature is best at a doping density of 3.6 × 1010cm-2.     

Performance and Spectral Tuning of Optically Overtone Pumped Molecular Lasers

A laser model was developed to predict the performance of optically pumped higher overtone molecular lasers under pulsed and continuous wave (CW) excitations. This model takes into account up to 30 rotational levels in each of the eight vibrational states considered. Collision-induced relaxation among rotational levels, vibrational levels, energy transfer to translational degrees of freedom, and interactions with buffer gas, and the temperature dependence of these processes are included. Using parameters for a second overtone pulse pumped HBr laser, a complete lasing cascade can be expected at a certain pump pulse fluence resulting in maximum achievable efficiencies approaching 80%. Optimum operational pressure and temperature conditions are determined by the gas kinetic rates and line broadening effects. Frequency tuning and spectral narrowing of the laser output is possible with the insertion of intracavity filters without sacrificing laser output power owing to efficient energy redistribution between rotational levels mediated by appropriate buffer gases. CW lasing using waveguide (WG) like geometries is possible with efficiencies approaching 92% for first overtone pumped lasers. Heat conduction through the active gas enhanced by buffer gases and subsequent heat dissipation through the cooled WG walls are expected to handle the thermal load up to kW output power levels.     

光抽运石墨烯太赫兹负动态电导率的理论研究

石墨烯特殊的零带隙能带结构和载流子弛豫特性,在研究太赫兹辐射源相干放大领域引起广泛关注。考虑带内和带间跃迁对电导率的贡献,研究了光抽运单层和多层石墨烯中非平衡二维电子-空穴系统的动态电导率特性。结果表明,在足够强的光抽运下,石墨烯中的粒子数反转能够使得动态电导率的实部在太赫兹频段内出现负值,这使基于石墨烯的太赫兹放大或受激辐射源成为可能。同时,通过研究动量弛豫时间、温度、层数、光强对石墨烯的负动态电导率的影响表明,石墨烯多层结构的动态电导率最小值的绝对值更大,作为太赫兹激光器的激活介质更具优势。     

Directional Optically Pumped Laterally Coupled DFB Lasers With Circular Mirrors

In a previous work, we showed that an adequate choice of phase shifts introduced by defects in a laterally coupled distributed feedback laser (LC-DFB) structure can lead to improved performance. In this paper, we change that optimized structure by adding an in-plane back mirror to the structure, funneling light in only one direction into a single-mode waveguide. We show that a well designed circular grating considerably reduces lateral radiation losses in the back mirror, concentrating light in the front end of the device. We also examine the possibility of converting the in-plane emission at the front end of the device into vertical emission by placing a higher-order grating. Furthermore, we show that side mode suppression ratio (SMSR) of the device can be improved by incorporating a nanotaper.      

光抽运双光子激光的全量子理论

本文采用均匀加宽的四能级原子模型与光泵双光子激光相互作用,其系统具有双光子损耗机制。利用全量子理论推导各原子算符的朗之万方程,进而研究了稳态光子数、频率、线宽及激光运转过程。     

High Resolution Terahertz Spectroscopy with Quantum Cascade Lasers

High resolution terahertz (THz) spectroscopy is a powerful analytical tool for laboratory purposes as well as for remote sensing in astronomy, planetary research, and Earth observation. THz quantum cascade lasers (QCLs) are promising sources for implementation into THz spectrometers, in particular at frequencies above 3 THz, which is the least explored portion of the THz region. One application of QCLs in THz spectroscopy is in absorption spectrometers, where they can replace less powerful and somewhat cumbersome sources based on frequency mixing with gas lasers. Another one is using a QCL as local oscillator in a heterodyne spectrometer for remote sensing. This article will review the state-of-the art in high resolution THz spectroscopy with QCLs.     

太赫兹量子级联激光器光注入特性

基于三能级速率方程,研究了独立运行、主从注入和相互注入太赫兹量子级联激光器(THz-QCLs)的相对强度噪声和调制特性。在自由运行情况下,THz-QCL的自发辐射噪声在低频时表现出白噪声特性。与传统的半导体激光器不同,在低频区没有对应于弛豫振荡的共振峰。主从注入可以有效降低THz-QCLs噪声20 dB,提高THz-QCLs低噪声工作的效率。对于互注入THz-QCLs,即使在锁相区,噪声也明显高于自由运行情况。通过应用注入锁定方案,与直接调制方案相比,可以大大增加调制带宽。     

Numerical Simulation of ZnO-Based Terahertz Quantum Cascade Lasers

In this work, a particle-based Monte Carlo model is used to quantify the potential of terahertz sources based on the ZnO-based material system relative to existing devices based on GaAs/AlGaAs quantum wells. Specifically, two otherwise identical quantum cascade structures based on ZnO/MgZnO and GaAs/AlGaAs quantum wells are designed, and their non- equilibrium carrier distributions are then computed as a function of temperature. The simulation results show that, because of their larger optical phonon energy, ZnO/MgZnO quantum cascade laser structures exhibit weaker temperature dependence of the population inversion than in the case of similar structures made of GaAs/AlGaAs. In particular, as the temperature is increased from 10 K to 300 K, population inversion is found to decrease by a factor of 4.48 and 1.50 for the AlGaAs and MgZnO structure, respectively. Based on these results, the MgZnO devices are then predicted to be, in principle, capable of laser action without cryogenic cooling.     

基于D2O的光抽运腔式太赫兹波激光谱线竞争

从半经典密度矩阵理论出发,建立了光抽运太赫兹波谱线竞争的双三能级分子系统模型,理论推导了抽运信号吸收系数以及太赫兹波信号增益系数的数学表达式,采用迭代法数值计算了CO2激光-9P(32)抽运重水(D2O)气体分子腔式太赫兹激光66μm和116μm两条谱线之间的竞争。给出了抽运功率、工作气压及激光腔长等工作条件下谱线竞争的一般规律。     

MOCVD生长InGaAs/InGaAsP多量子阱光泵微碟激光器

用MOCVD方法生长了InGaAs/InGaAsP多量子阱微碟激光器外延片,用光刻、干法刻蚀和湿法刻蚀等现代化的微加工技术制备出直径9.5μm的InGaAs/InGaAsP微碟激光器,并详细介绍了整个制备工艺过程．在液氮温度下用氩离子激光器泵浦方式实现了低阈值光泵激射,测出单个微碟激光器的阈值光功率为150μW,激射波长约为1.6μm,品质因数Q=800,激射光谱线宽为2nm,同时指出微碟激光器激射线宽比F-P普通激光器宽很多是由于其品质因数很高造成的．     

半导体可饱和吸收镜锁模光抽运垂直外腔面发射半导体激光器

介绍了半导体可饱和吸收镜锁模光抽运垂直外腔面发射半导体激光器的原理、基本结构特点、应用及设计中的主要问题,对其研究进展及未来发展趋势作了分析和总结。     

Optical Properties of Active Regions in Terahertz Quantum Cascade Lasers

In this work, AlGaAs/GaAs superlattice, with layers’ sequence and compositions imitating the active and injector regions of a quantum cascade laser designed for emission in the terahertz spectral range, was investigated. Three independent absorption-like optical spectroscopy techniques were employed in order to study the band structure of the minibands formed within the conduction band. Photoreflectance measurements provided information about interband transitions in the investigated system. Common transmission spectra revealed, in the target range of intraband transitions, mainly a number of lines associated with the phonon-related processes, including two-phonon absorption. In contrast, differential transmittance realized by means of Fourier-transform spectroscopy was utilized to probe the confined states of the conduction band. The obtained energy separation between the second and third confined electron levels, expected to be predominantly contributing to the lasing, was found to be ~9 meV. The optical spectroscopy measurements were supported by numerical calculations performed in the effective mass approximation and XRD measurements for layers’ width verification. The calculated energy spacings are in a good agreement with the experimental values.     

p型量子阱太赫兹振荡器研究

采用非抛物平衡方程理论研究了直流偏置下 p型量子阱负有效质量 p+ pp+ 二极管电流的时空特性。在适当的掺杂和偏置条件下 ,由于高场畴的形成 ,二极管中将产生太赫兹 (THz,1 THz=1 0 1 2 Hz)电流自振荡。计算了自振荡频率对直流偏置的依赖性 ,提出了设计可调谐 THz振荡源的可能性     

Observation of Amplified Stimulated Terahertz Emission from Optically Pumped Heteroepitaxial Graphene-on-Silicon Materials

We experimentally observed the fast relaxation and relatively slow recombination dynamics of photogenerated electrons/holes in a heteroepitaxial graphene-on-Si material under pumping with a 1550-nm, 80-fs pulsed fiber laser and probing with the corresponding terahertz beam generated by and synchronized with the pumping laser. The time-resolved electric-nearfield intensity originating from the coherent terahertz photon emission is electrooptically sampled in total-reflection geometry. The Fourier spectrum fairly agrees the product of the negative dynamic conductivity and the expected THz photon spectrum reflecting the pumping photon spectrum. This phenomenon is interpreted as an amplified stimulated terahertz emission.     

量子阱半导体激光器概述

本文主要从量子理论中的电子波函数所满足的薛定锷方程出发,讨论了量子阱理论。进而介绍多量手阱激光器(MQWL)的重要特征之一——阈值电流密度随量子阱的数目变化的情况。     

Spectroscopic Study on Ultrafast Carrier Dynamics and Terahertz Amplified Stimulated Emission in Optically Pumped Graphene

This paper reviews recent advances in spectroscopic study on ultrafast carrier dynamics and terahertz (THz) stimulated emission in optically pumped graphene. The gapless and linear energy spectra of electrons and holes in graphene can lead to nontrivial features such as negative dynamic conductivity in the THz spectral range, which may lead to the development of new types of THz lasers. First, the non-equilibrium carrier relaxation/recombination dynamics is formulated to show how photoexcited carriers equilibrate their energy and temperature via carrier-carrier and carrier-phonon scatterings and in what photon energies and in what time duration the dynamic conductivity can take negative values as functions of temperature, pumping photon energy/intensity, and carrier relaxation rates. Second, we conduct time-domain spectroscopic studies using an optical pump and a terahertz probe with an optical probe technique at room temperature and show that graphene sheets amplify an incoming terahertz field. Two different types of samples are prepared for the measurement; one is an exfoliated monolayer graphene on SiO₂/Si substrate and the other is a heteroepitaxially grown non-Bernal stacked multilayer graphene on a 3C-SiC/Si epi-wafer.     

Spectroscopy of Methanol and Its Application to Optically Pumped THz FIR Lasers

In this paper, we have reviewed the details of experimental and theoretical aspects of the high-resolution molecular spectroscopy of methanol isotopomers. The methods of spectroscopic assignments and analysis have been discussed. The applications of the spectroscopic techniques, particularly of Fourier transform spectroscopy in the field of optically pumped Terra-Hertz Lasers (OPTL) and far infrared (OPFIR) lasers have been discussed. New assignments have been deduced for TEA-CO₂ laser pumped methanol and CW CO₂ pumped ¹³CD₃OD. Stark effect analyses and the determination of accurate dipole moment values have been discussed for CH₃OH, ¹³CH₃OH and CH₃ ¹⁸0H. The importance of high-resolution spectroscopy is also discussed in terms of astrophysical detection. Some recent ongoing astrophysical efforts are outlined. In our laboratory we are in the process of setting up a THz methanol laser for astrophysical applications and search for new coherent monochromatic laser sources.