Terahertz field-induced modulations of intersubband absorptions in quantum wells

Nonlinear optical properties of intersubband electrons in a 3-level quantum well under intense terahertz field are investigated by using a density matrix approach. The results show that the terahertz fields with different frequencies cause the distinct modulations of the intersubband absorptions. The terahertz-induced sideband and Autler--Towns splitting in the absorption spectrum are obtained, respectively for the terahertz-photon energy below and close to the transition energy between the ground and first excited state.     

Optical detection of terahertz radiation by using nonlinear parametric upconversion

We describe and demonstrate sensitive room-temperature detection of terahertz (THz) radiation by nonlinearly upconverting terahertz to the near-infrared regime, relying on telecommications components. THz radiation at 700 GHz is mixed with pump light at 1550 nm in a bulk GaAs crystal to generate an idler wave at 1555.6 nm, which is separated and detected by using a commercial p-i-n diode. The THz detector operates at room temperature and has an intrinsic THz-to-optical photon conversion efficiency of 0.001%.     

量子阱在沿其平面方向偏振太赫兹波驱动下子带间光吸收

利用半导体布洛赫方程,讨论了量子阱在沿其平面方向偏振的太赫兹场驱动下的光学吸收谱。研究结果揭示了半导体量子阱在沿平面方向偏振的强太赫兹场驱动下吸收谱的一些新奇效应,如主吸收的太赫兹边带、动态弗兰兹-凯尔迪什效应。太赫兹场的频率及其相位对探测场吸收谱的影响也很显著。     

intersubband pumping of an : InP symmetric double quantum well terahertz laser

The feasibility of intersubband optical excitation of a terahertz (1–10 THz) or far-infrared (30–300 ) emitter/laser by a laser diode is investigated by means of envelope function/effective mass approximation and subband carrier lifetime calculations. The material system is employed in order to supply the necessary conduction band offset and the basic design is that of a 6-level symmetric double quantum well. This can simultaneously satisfy the criteria of an 800 meV intersubband absorption and a far-infrared emission. It is shown that these device designs can satisfy a necessary criterion for population inversion at room temperature. A scheme for improving the population ratio based on a 9-level triple quantum well is discussed.     

黑砷磷室温太赫兹探测器

由于太赫兹波与众多物质之间存在着丰富的相互作用,太赫兹技术在众多领域均有应用需求。因此,基于独特物理机制和优异材料特性的高灵敏度、便携式太赫兹探测器的研制刻不容缓。黑砷磷是一种新型二维材料,其带隙和输运特性随化学组分可调,在光电探测领域被广泛关注。目前基于黑砷磷的研究集中在红外探测方面,而对于太赫兹探测的应用未见报道。本文介绍了一种基于黑砷磷的天线耦合太赫兹探测器。实验结果表明,在探测过程中存在两种不同的探测机制,并且两者之间存在竞争关系。通过改变黑砷磷的化学组分可以定制不同的探测机制,使其达到最优响应性能。在平衡材料带隙和载流子迁移率的情况下,探测器实现了室温下对0.37 THz电磁波的灵敏探测,其电压响应度和噪声等效功率分别为28.23 V/W和0.53 nW/Hz1/2。     

Terahertz wave polarization rotation in bianisotropic metamaterials

Utilizing a polarization sensitive terahertz detection method where the detector is rotated by either 0° or 90° to measure the electric field E_{p, s} (t) of each polarization component, we have characterized the properties of split ring resonators. The strong polarization dependence of the bianisotropic-circular-current-driven and linear-polarization-induced resonances is in excellent agreement with the simulation when the p-polarized terahertz transmission is measured. However, these electromagnetic responses vanish when the s-polarized terahertz transmission is measured. There is only a transmission minimum at 1.64 THz and the terahertz polarization rotation angle of about 90° is observed. The polarized terahertz transmission amplitudes and spectra detected at orthogonal orientations show that these behaviours are probably attributed to the birefringent effect of the sample.     

Integration of a field-effect-transistor terahertz detector with a diagonal horn antenna

Efficient coupling of terahertz electromagnetic wave with the active region in a terahertz detector is required to enhance the optical sensitivity. In this work, we demonstrate direct integration of a field-effect-transistor(FET) terahertz detector chip at the waveguide port of a horn antenna. Although the integration without a proper backshot is rather preliminary, the noise-equivalent power is greatly reduced from 2.7 nW/Hz~(1/2) for the bare detector chip to 76 pW/Hz~(1/2) at340 GHz. The enhancement factor of about 30 is confirmed by simulations revealing the effective increase in the energy flux density seen by the detector. The simulation further confirms the frequency response of the horn antenna and the onchip antennas. A design with the detector chip fully embedded within a waveguide cavity could be made to further enhance the coupling efficiency.     

High-temperature all-optical intersubband quantum well Terahertz switch

In this article an asymmetric intersubband quantum well structure as a high temperature terahertz (THz) optical switch is proposed. In our proposed structure the incoming low power energy photon (THz control signal) causes an optical switching. In this structure we introduce an optical terahertz switch based on coherent population trapping (CPT) phenomena. In the presence of electromagnetic THz field, quantum interference between the terahertz control field and short-wavelength probe field under appropriate condition, the medium becomes transparent (zero absorption) for the probe field. So the absorption and refraction characteristic of optical probe field can be modified with THz radiation. Therefore this idea is suitable for all – optical terahertz switching.     

Enhanced terahertz bistability in a nonlinear Fabry-Perot resonator with n-type quantum wells

The intrinsic bistable terahertz response of intersubband plasmons in wide n-type delta-doped quantum wells is predicted to be enhanced by a resonant Fabry-Perot cavity. With a simple low-Q resonator, the threshold for bistability is decreased by a factor of 2-3 compared with that for bare multiple quantum wells.     

Si/SiGe量子级联激光器研究进展

Si／SiGe量子级联激光器是一种新型的带内跃迁的红外光源，突破了Si基材料间接带隙特性对光跃迁的限制。Si/SiGe量子级联激光器的开发将为实现太赫兹有源器件的硅基集成产生深远影响。文章介绍了Si/SiGe量子级联激光器的工作原理，以及这类激光器在能带设计、材料生长和波导制作方面的最新进展。     

高精度太赫兹探测器响应度定标系统

为了保障太赫兹探测器测试的准确度和可信度,对太赫兹探测器响应度定标溯源技术进行研究,针对常用太赫兹探测器黑体辐射测试技术存在对环境、设备要求高和搭建难度大等问题,以中国计量科学研究院的热电型太赫兹探测器作为计量标准,提出一种校准方案,设计搭建了一套太赫兹探测器响应度定标系统。为提高定标准确度,实验测试了定标光学系统的光束质量,并合理设置光阑孔径以满足定标要求。在0.1 THz频点处对2个响应度未知的场效应自混频太赫兹探测器进行响应度定标。结果表明,定标的相对扩展不确定度为6.80% (k=2),验证了定标系统的可行性,系统可实现太赫兹探测器功率响应度溯源到国家激光功率标准,保障太赫兹功率测量的准确可靠。     

Monolithically integrated near-infrared and mid-infrared detector array for spectral imaging

A multi-band focal plane array sensitive in near-infrared (near-IR) and mid-wavelength infrared (MWIR) is been developed by monolithically integrating a near-infrared (1–1.5 μm) p–i–n photodiode with a mid-infrared (3–5 μm) QWIP. This multiband detector involves both intersubband and interband transitions in III–V semiconductor layer structures. Each detector stack absorbs photons within the specified wavelength band, while allowing the transmission of photons in other spectral bands, thus efficiently permitting multiband detection. Monolithically grown material characterization data and individual detector test results ensure the high quality of material suitable for near-infrared/QWIP dual-band focal plane array.     

A monolithic resonant terahertz sensor element comprising a metamaterial absorber and micro‐bolometer

In this article a monolithic resonant terahertz sensor element with a noise equivalent power superior to that of typical commercial room temperature single pixel terahertz detectors and capable of close to real time read‐out rates is presented. The detector is constructed via the integration of a metamaterial absorber and a micro‐bolometer sensor. An absorption magnitude of 57% at 2.5 THz, a minimum NEP of and a thermal time constant of 68 ms for the sensor are measured. As a demonstration of detector capability, it is employed in a practical Nipkow terahertz imaging system. The monolithic resonant terahertz detector is readily scaled to focal plane array formats by adding standard read‐out and addressing circuitry enabling compact, low‐cost terahertz imaging.     

Improved measurement of polarization state in terahertz polarization spectroscopy

A calibration scheme is presented for improved polarization state measurement of terahertz pulses. In this scheme the polarization response of a two-contact terahertz photoconductive detector is accurately measured, and is used to correct for the impact of the nonidealities of the detector. Experimental results show excellent subdegree angular accuracy and at least 60% error reduction with this scheme.     

低温生长砷化镓的超快光抽运-太赫兹探测光谱

本文采用光抽运-太赫兹探测技术系统研究了低温生长砷化镓(LT-GaAs)中光生载流子的超快动力学过程.光激发LT-GaAs薄层电导率峰值随抽运光强增加而增加,最后达到饱和,其饱和功率为54μJ/cm~2.当载流子浓度增大时,电子间的库仑相互作用将部分屏蔽缺陷对电子的俘获概率,从而导致LT-GaAs的快速载流子俘获时间随抽运光强增加而变长.光激发薄层电导率的色散关系可以用Cole-Cole Drude模型很好地拟合,结果表明LT-GaAs内部载流子的散射时间随抽运光强增加和延迟时间(产生光和抽运光)变长而增加,主要来源于电子-电子散射以及电子-杂质缺陷散射共同贡献,其中电子-杂质缺陷散射的强度与光激发薄层载流子浓度密切相关,并可由散射时间分布函数α来描述.通过对光激发载流子动力学、光激发薄层电导率以及迁移率变化的研究,我们提出适当增加缺陷浓度,可以进一步降低载流子迁移率和寿命,为研制和设计优良的THz发射器提供了实验依据.     

Intersubband optical absorption in a quantum well under a tilted magnetic field

By using an appropriate coordinate transform we have calculated the intersubband optical absorption in the single square well under a tilted magnetic field. In this study the dependence of the intersubband transitions on the magnetic field strength and the direction of the magnetic field (tilt angle) is discussed. We show that intersubband optical absorption is sensitive to the tilt angle. This behaviour in the intersubband optical absorption gives a new degree of freedom in regions of interest device applications.     

Strong correlation of electronic and lattice excitations in GaAs/AlGaAs semiconductor quantum wells revealed by two-dimensional terahertz spectroscopy

Coulomb-mediated interactions between intersubband excitations of electrons in GaAs/AlGaAs double quantum wells and longitudinal optical phonons are studied by two-dimensional spectroscopy in the terahertz frequency range. The multitude of diagonal and off-diagonal peaks in the 2D spectrum gives evidence of strong polaronic signatures in the nonlinear response. A quantitative theoretical analysis reveals a dipole coupling of electrons to the polar lattice that is much stronger than in bulk GaAs, due to a dynamic localization of the electron wave function by scattering processes.     

Long wavelength infrared (LWIR) photodetection in a modulation doped thyristor

A modulation doped thyristor concept is described for LWIR photodetection based upon intersubband bound to continuum absorption. The intersubband absorption generates photocurrent from undoped quantum wells to modulation doped layers (MDL). Due to the lower dark current compared to conventional quantum well infrared photodetectors (QWIPs), the thyristor infrared detector operates with little or no cooling and with similar or better performance than QWIPs at low temperatures. The operating characteristics of absorption coefficient, quantum efficiency, responsivity, detectivity, infrared gain, and dark current are determined as a function of thyristor voltage and input power level in the range of 1 μW/cm².     

High-sensitive terahertz detection by parametric up-conversion using nanosecond pulsed laser

A high-sensitive terahertz detector operating at room temperature was demonstrated based on parametric upconversion.A nanosecond 1064-nm Nd:YAG laser was used to pump the parametric up-conversion detector and the upconversion from terahertz wave to NIR laser was realized in a lithium niobate crystal.The minimum detectable terahertz energy of 9 p J was realized with the detection dynamic range of 54 d B,which was three orders of magnitude higher than that of commercial Golay cell.The detectable terahertz frequency range of the detection system was 0.90 Thz–1.83 THz.Besides,the effects of pump energy and effective gain length on the detection sensitivity were studied in experiment.The results showed that higher pump energy and longer effective gain length are helpful for improving the detection sensitivity of parametric up-conversion detector.     

Terahertz nonlinear optics with strained p-type quantum wells

Valence-subband nonparabolicity is shown theoretically to lead to nonlinearities associated with terahertz third-harmonic generation in strained p -type quantum wells. For strained InAs quantum wells it is found that the corresponding value of chi((3)) can be as large as ~10(-12)(m/V)(2). The predicted values of chi((3)) are in the range of those associated with intersubband transitions in the mid-infrared region of the spectrum.