偏置电压驱动下AlGaN/GaN二维电子气的宽谱太赫兹波发射

本文介绍了低温（6 K）下偏置电压加速AlGaN/GaN二维电子气产生的宽谱太赫兹波发射。根据“浅水波”等离子体波失稳理论和史密斯-珀塞尔理论,分别设计了两种器件结构。实验结果表明所观测到的太赫兹辐射与电子浓度无明显依赖关系,得到的而是自于晶格和热电子的黑体辐射。通过考虑湿度和衬底法布里-珀罗谐振腔作用,计算得到的热电子发射光谱与实验光谱良好吻合。研究结果表明通过电流驱动二维电子气产生太赫兹辐射的器件将不可避免地产生强烈的电子气加热和晶格加热问题,很难实现理论预言的等离子体波失稳与高效太赫兹发射。因此,需要设计更加精巧的器件结构才能实现高效的等离子体波激发和太赫兹发射。     

Lifetime Measurement of Excitons in Si by Terahertz Time-domain Spectroscopy with High Spectral Resolution

We have developed an optical pump and terahertz (THz) probe spectroscopy scheme to study the photoexcited dynamics in solids ranging from sub-microsecond to a millisecond regime. We applied the developed scheme to measure the lifetime of long-lived indirect excitons in Si through the observation of intra-exciton transitions, resolving the fine structure of excitons with high spectral resolution in a spectral range from 0.5 to 7 THz (2 to 29 meV). We also performed the lifetime measurement of the lowest energy spin-forbidden dark excitons under the magnetic field. Through the observation of intra-exciton transitions, otherwise inaccessible spin-forbidden dark excitons were directly probed by the THz time-domain spectroscopy. By comparing with the photoluminescence spectroscopy, we revealed that the lowest energy dark excitons are accumulated in the crystal, whereas the recombination dynamics is governed by the nonradiative decay process.     

New Class of Efficient Terahertz Generators: Effective Terahertz Spectral Filling by Complementary Tandem Configuration of Nonlinear Organic Crystals

Intense table‐top terahertz (THz) sources, which have progressed tremendously in the last decade, are becoming more important for advanced THz science to study light–matter interactions and subsequent applications. Nonlinear optical organic crystals exhibit great potential for intense broadband THz wave generation due to their large nonlinearities and advantageous phase‐matching characteristics. However, the phonon‐induced absorption of most organic crystals in the THz region leads to undesired modulation of the spectrum and limits the THz output efficiency. To overcome such drawbacks, phonon‐mode engineering by modification of molecular structures is suggested, but intrinsic limitations still remain. Here, an efficient alternative approach has been recently proposed for generating intense broadband THz waves based on a tandem configuration that combines two complementary nonlinear organic crystals. Such configuration compensates for the spectral gap of the generated THz waves mainly caused by phonon absorption and additionally enhances the optical‐to‐THz conversion efficiency. The proposed organic tandem generator indicates a substantial enhancement of the peak‐to‐peak THz electric field due to effective spectral filling at phonon absorption gaps. As a result, such tandem configuration provides a versatile platform to generate gapless broadband THz spectra with suppressed phonon absorption and contributes to advancing the development of intense broadband coherent THz sources.     

Josephson Plasmon Resonance in Tl2Ba2CaCu2O8 High-Temperature Superconductor Tunable Terahertz Metamaterials

The Josephson plasmon resonance (JPR) offers a valuable probe to investigate the superconductivity in layered cuprate superconductors. However, the coupling between free space radiation and JPR in high-temperature superconductor (HTS) film remains challenging because the excitation of JPR demands the c-axis oriented electric field. The subwavelength resonators in metamaterials can enhance the localized electric field, which can be utilized to resolve this difficulty. Here, a tunable terahertz (THz) metamaterial made from Tl₂Ba₂CaCu₂O₈ (Tl-2212) HTS film is developed. The spectral response of Tl-2212 metamaterial has a tunable property at temperatures up to 90 K. The resonant excitation of Josephson plasmon in the metamaterial is observed. Simulation results indicate that the scattering of subwavelength resonators can provide the component of the z-axis electric field for the resonant excitation. The coupling between JPR and resonance modes of metamaterials is observed and explained using coupled mode theory. The temperature dependence of JPR frequency shows accordance with the experimental results of the pure film. This work provides an avenue to excite the JPR and probe superconducting condensate in the layered superconductor. The development of Josephson plasmonic metamaterials may contribute to tunable and nonlinear THz devices.     

Ion-Bolometric Effect in Grain Boundaries Enabled High Photovoltage Response for NIR to Terahertz Photodetection

The excellent performance of bolometers in the infrared and terahertz regions has attracted great attention. Understanding the transport process of charged particles is an efficient approach to determine detector performance. However, the lack of studies on the fine-scale spatial motion of microscopic particles in bolometers has prevented a full understanding of the physical process. Herein, using micro-nano-scale optoelectronic performance correlation measurements, it is described how prevalent defect states at the grain boundaries (GBs) decrease current responses. Ions at the GBs of the polycrystalline perovskite bolometer contribute to the photocurrent via thermal excitons. In addition, the built-in electric field established by ion migration fluctuates periodically with the strength of the light-heating process due to the interaction between the bolometric effect and the Coulomb force. Additionally, the first ion-bolometric detector is demonstrated with a significant photovoltage response to infrared and THz waves (75.3 kV W⁻¹ at 1064 nm and 2.3 kV W⁻¹ at 0.22 THz). An examination of the THz images shows the potential for large-area THz imaging applications. The ion-bolometric effect combines the broad spectral characteristics of the bolometer effect with the temperature sensitivity due to ion migration and provides a unique perspective on detector technology.     

Photoluminescence of silicon nanocrystals under the effect of an electric field

The effect of an electric field on photoluminescence (PL) of silicon nanocrystals formed in silicon dioxide by ion implantation with subsequent annealing has been studied. Application of an electric field leads to an increase in PL intensity by ～10% at low temperatures and an electric field strength of 12 kV/cm and to its decrease at temperatures above 20 K. The increase in exciton PL intensity in an electric field is inconsistent with the model of recombination of quantum-confined excitons in nanocrystals. The effect can be described in terms of a model of recombination of self-trapped excitons formed at the interface between a Si nanocrystal and SiO₂.     

Electro-optical trap for dipolar excitons

An electro-optical trap for spatially indirect dipolar excitons is implemented in a GaAs/AlAs Schottky diode with a 400-?-wide GaAs single quantum well. In the presence of bias voltage applied to the gate, the trap for excitons appears upon annular illumination of the structure by a continuous-wave or pulsed laser generating hot electron-hole pairs in the quantum well. A barrier for excitons collected inside the illuminated ring appears due to screening of the applied electric field by nonequilibrium charge carriers within the excitation region. Excitons are collected inside the ring due to the ambipolar drift of carriers and the dipole-dipole repulsion of excitons in the optically pumped region. For dipolar excitons thus accumulated in the middle of the ring-shaped electro-optical trap, significant narrowing of the luminescence line is observed with increasing excitation density, which indicates the collective behavior of the excitons.     

Reliability investigations on LTG-GaAs photomixers for THz generation based on optical heterodyning

Optical heterodyne conversion, or photomixing, is a frequency-agile technique that generates continuous-wave radiation at THz frequencies using thin films of low-temperature-grown (LTG) GaAs at high electrical bias. Very high electric fields (>300 kV/cm) and thermal excitation by focused laser illumination require reliability assessments prior to implementation in THz systems. Pulsed electrical stress and DC measurements under optical illumination are used to characterise the reliability of photomixers for THz generation based on optical heterodyning. An improvement in terms of reliability is observed by replacing Ti/Au by Pt metallisation.     

Electro-optically Induced and Manipulated Terahertz Waves from Fe-doped InGaAs Surfaces

We demonstrate the presence of dual simultaneous nonlinear mechanisms: field-induced optical rectification (FIOR) and field-induced surge current (FISC) for the generation of terahertz (THz) pulses from p-type and n-type Fe:In_0.53Ga_0.47As surfaces upon excitation with femtosecond laser pulses centered at 800 nm wavelength. Experimental investigations of the dependence of the generated THz waves on the incident angular optical polarization, optical irradiance, and the direction and magnitude of applied electric DC fields give confirming results to the proposed THz generation mechanisms. Applying external DC electric fields in the plane of the incident optical field shows efficient capability in manipulating the direction and phase of the generated THz waves, and controlling the refractive index of Fe:In_0.53Ga_0.47As material in the THz range, in addition to enhancing the emitted THz power up to two orders of magnitude. The fast and reliable response of Fe:In_0.53Ga_0.47As to the changes in the direction and magnitude of the optical and electrical fields suggests its use in amplitude and phase modulators, and ultrafast optoelectronic systems.     

Autosolitons in an electron-hole plasma/excitons system in silicon at 4.2K

Results are presented of the experimental discovery and study of auto-oscillations in an electron-hole plasma/excitons system in silicon under conditions of impact ionization of the excitons in a constant electric field. It is shown that the current auto-oscillations are due to disruption of the uniform density distribution of the electron-hole plasma and its stratification, where the latter is caused by the formation of strongly nonequilibrium structures, namely, autosolitons. The microplasma breakdown of the reverse-biased Schottky barrier is the cause of the spontaneous excitation of autosolitons. Fiz. Tekh. Poluprovodn. 33, 1183–1186 (October 1999)     

Influence of Disassociation Probability on External Quantum Efficiency in Organic Electrophosphorescent Devices

An analytical model is presented to calculate the disassociation probability and the external quantum efficiency at high field in doped organic electrophosphorescence（EPH） devices. The charge recombination process and the triplet（T）-triplet（T） annihilation processes are taken into account in this model. The influences of applied voltage and the thickness of the device on the disassociation probability, and of current density and the thickness of the device on the external quantum efficiency are studied thoroughly by including and ignoring the disassociation of excitons. It is found that the dissociation probability of excitons will come close to 1 at high electric field, and the external EPH quantum efficiency is almost the same at low electric field. There is a large discrepancy of the external EPH quantum efficiency at high electric field for including or ignoring the disassociation of excitons.     

半导体THz辐射的Monte Carlo模拟

本文介绍了作者开发的基于面向对象语言C++和统一建模语言UML的半导体输运及THz辐射的蒙特卡罗模拟软件,并用该软件模拟了在强超短脉冲激光(光生载流子密度10¹⁹cm^-3)及强电场 (100kV/cm) 作用下GaAs的THz时域波形和相应的半导体表面局域场.通过分析THz时域波形,我们发现强外加电场下的载流子速度过冲、载流子屏蔽(或器件反应过冲)是形成THz时域波形双极结构的原因.功率谱的分析表明增加外加电场有益于提高THz的低频成份的辐射,但对高频部分(＞6THz)影响不大.     

Dissociation of electron–hole pairs interacting with triplet excitons in amorphous molecular semiconductors

The photoconductivity of films of amorphous molecular semiconductors increases upon simultaneous photogeneration of singlet electron–hole pairs (EHPs) and triplet excitons and decreases upon photogeneration of triplet EHPs and triplet excitons. An increase in external electric field strength leads to a decrease in the effect of triplet excitons on the film photoconductivity caused by EHP dissociation. It is concluded that under increasing external electric field strength the mobility of charge carriers increases and the velocity of EHP dissociation becomes comparable with the velocity of spin conversion of EHPs interacting with triplet excitons. © 1997 John Wiley & Sons, Ltd.     

Spontaneous oscillations and chaos in silicon induced by excitonic impact ionization

We have observed spontaneous photocurrent oscillations leading to chaos in boron doped Czochralski silicon under optical excitation at liquid helium temperatures. The silicon samples have been annealed at 450°C for more than 100 hours to contain thermal donor complexes in a concentration of about 10¹⁶/cm³. The oscillations start at a certain critical electric field, where the impact ionization of the thermal donor bound excitons by hot free carriers set in. The current-voltage characteristic shows a highly nonlinear behaviour with a negative differential resistance. Near the breakdown region, which is close to 150 V/cm at 2 K, the current oscillates between a few mA to some hundred mA in pulses of less than a microsecond at a frequency of around 10 kHz, critically dependent on the external parameters like the applied voltage, temperature, laser excitation intensity or the specific sample used. The physical mechanism causing these oscillations is suggested to be a critical balance between the impact ionization- and capture- rates of the thermal donor bound excitons.     

飞秒激光作用ZnSe晶体产生和探测THz辐射的实验

实验研究了ZnSe单晶的光学整流THz产生,借助电光取样技术得到THz脉冲的时域波形和FFT频谱分布,观察到约113fs的THz辐射场分布,及相应约5.8THz的频谱分布,辐射峰位于3THz左右.对比研究了不同表面的ZnSe晶体的THz辐射峰值强度随激发光功率的变化,通过轻微烧蚀模型定性解释了高激发功率下THz信号的饱和机制.     

Optical Properties of AlGaAs/GaAs Resonant Bragg Structure at the Second Quantum State

Photoluminescence, optical reflectance and electro-reflectance spectroscopies were employed to study an AlGaAs/GaAs multiple-quantum-well based resonant Bragg structure, which was designed to match optical Bragg resonance with the exciton-polariton resonance at the second quantum state in the GaAs quantum wells. The structure with 60 periods of AlGaAs/GaAs quantum wells was grown on a semi-insulating substrate by molecular beam epitaxy. Broad and enhanced optical and electro-reflectance features were observed when the Bragg resonance was tuned to the second quantum state of the GaAs quantum well excitons manifesting an enhancement of the light-matter interaction under double-resonance conditions. By applying an alternating electric field, we revealed electro-reflectance features related to the x(e2-hh2) and x(e2-hh1) excitons. The excitonic transition x(e2-hh1), which is prohibited at zero electric field, was allowed by a DC bias due to brake of symmetry and increased overlap of the electron and hole wave functions caused by electric field.     

基于柱坐标系抛物方程的太赫兹目标RCS计算

针对太赫兹(THz)波段目标雷达散射截面(RCS)的计算问题,提出柱坐标系抛物方程模型的计算方法。基于柱坐标系中的电场通解式,利用三角函数的正交性分解各模式的激励系数,将抛物方程方法拓展到柱坐标系,得到柱坐标系中抛物方程的分步傅里叶求解形式。在此基础上,将目标等效为一系列的面元或线元,然后通过边界条件和场的迭代递推方法求解抛物方程,进而获得这一系列面元在传播方向某一截面上的散射场。数值算例表明,该方法能用于电大尺寸目标的RCS计算,相比于传统的抛物方程方法,克服了散射角度的限制,计算误差更小。     

基于场效应晶体管的太赫兹探测器中天线设计的一种有效方法

在场效应晶体管太赫兹探测器中,合理的天线设计可以增强晶体管和太赫兹波之间的耦合效率,从而提高太赫兹探测器的响应度.提出一种基于晶体管栅极边缘沟道电场的仿真来设计平面天线的方法.这种方法尤其适用于太赫兹波段晶体管输入阻抗不容易得到的情况.通过流片完成的基于氮化镓高电子迁移率晶体管的太赫兹探测器的响应度测试证实了这种方法的有效性.集成碟形天线和双偶极子天线的太赫兹探测器最大响应度分别在170.7 GHz(1568.4 V/W)和124.3 GHz(1047.2 V/W)频点处测得,这个测试结果接近基于晶体管栅极边缘沟道电场的仿真结果.     

Metal-Coated <100>-Cut GaAs Coupled to Tapered Parallel-Plate Waveguide for Cherenkov-Phase-Matched Terahertz Detection: Influence of Crystal Thickness

The influence of crystal thickness of metal-coated <100>-cut GaAs (M-G-M) on Cherenkov-phase-matched terahertz (THz) pulse detection was studied. The M-G-M detectors were utilized in conjunction with a metallic tapered parallel-plate waveguide (TPPWG). Polarization-sensitive measurements were carried out to exemplify the efficacy of GaAs in detecting transverse magnetic (TM)- and transverse electric (TE)-polarized THz waves. The reduction of GaAs’ thickness increased the THz amplitude spectra of the detected TM-polarized THz electro-optic (EO) signal due to enhanced electric field associated with a more tightly-focused and well-concentrated THz radiation on the thinner M-G-M. The higher-fluence THz beam coupled to the thinner M-G-M improved the integrated intensity of the detected THz amplitude spectrum. This trend was not observed for TE-polarized THz waves, wherein the integrated intensities were almost comparable. Nevertheless, good agreement of spectral line shapes of the superposed TM- and TE-polarized THz-EO signals with that of elliptically polarized THz-EO signal demonstrates excellent polarization-resolved detection capabilities of M-G-M via Cherenkov-phase-matched EO sampling technique.     

Nonlinear THz-Nano Metasurfaces

Extreme terahertz (THz) science and technologies, the next disruptive frontier in nonlinear optics, provide multifaceted capabilities for exploring strong light-matter interactions in a variety of physical systems. However, current techniques involve the need for an extremely high-field free space THz source that is difficult to generate and has limited investigations to a rather weak and linear regime of light-matter interactions. Therefore, new approaches are being sought for the tight confinement of THz waves that can induce nonlinear effects. Here, a nonlinear “tera-nano” metasurface is demonstrated exhibiting extremely large THz nonlinearity and sensitive self-modulation of resonances at moderate incident THz field strengths. A record deep-subwavelength (≈λ/33 000) confinement of strongly enhanced (^≈3200) THz field in a nano-gap (15 nm) exhibits remarkable THz field-tailored nonlinearity. Further, ultrafast injection of photocarriers reveals a competition between nonlinear THz field-induced intervalley scattering and optically driven interband excitations. The results on “tera-nano” metasurfaces enable a novel platform to realize enhanced nonlinear nano/micro composites for field-sensitive extreme THz nonlinear applications without the need for intense THz light sources.