基于半导体等离子体频率光学调控的太赫兹波调制系统

介绍了一种太赫兹波光学调制系统,该系统首先通过刀片与半导体之间的狭缝实现太赫兹波和太赫兹表面等离子体波之间的耦合,然后通过改变照射到本征半导体表面的光强用以调控半导体表面的等离子体频率,使得半导体表面的等离子体频率在有光照和无光照条件下分别大于和小于其上传输的太赫兹表面等离子体波的频率,从而实现对在半导体表面传输的太赫兹表面等离子体波以及由其耦合出的太赫兹波的强度调控。该调制方法与传统方法相比具有调控频带宽、速度快、成本低、常温工作等优点,可用于太赫兹波通讯。仿真和实验结果进一步验证了该调制系统应用的可行性。     

Tunable,Room Temperature CMOS-Compatible THz Emitters Based on Nonlinear Mixing in Microdisk Resonators

We propose and investigate in detail a novel tunable, compact, room temperature terahertz (THz) emitter using individual microdisk resonators for both optical and THz waves with the capability of radiating THz field in 0.5–10 THz range with tuning frequency resolution of 0.05 THz. Enhanced THz generation is achieved by employing a nonlinear optical disk resonator with a high value of second-order nonlinearity (χ ⁽²⁾) in order to facilitate the difference-frequency generation (DFG) via nonlinear mixing with the choice of two appropriate input infrared optical waves. Efficient coupling of infrared waves from bus to the nonlinear disk is ensured by satisfying critical coupling condition. Phase matching condition for efficient DFG process is also met by employing modal phase matching technique. Our simulations show that THz output power can be reached up to milliwatt (mW) level with high optical to THz conversion efficiency. The proposed source is Silicon on Insulator (SoI) technology compatible enabling the monolithic integration with Si complementary metal-oxide-semiconductor (CMOS) electronics including plasmonic THz detectors.     

Silicon Carbide Terahertz Emitting Devices

In recent years, terahertz (THz) sources between 0.1 THz and 10 THz have attracted much attention for imaging and sensing applications. THz emission from radiative transitions in impurity states has been demonstrated in Si and Ge devices by either electrical or optical pumping. Compared to Si as the material for THz emission, the wide-band-gap material SiC exhibits several advantages such as a higher dopant ionization energy, which allows a higher device operating temperature. Combining with its superior material qualities such as high breakdown field and high thermal conductivity, SiC is a promising material for high-temperature and high-power THz emitting devices. This article describes recent progress in using SiC materials to increase the operating temperature and output power of dopant-based THz sources.     

Monte Carlo Simulation of THz-Pulse Generation from Bulk GaAs Surface

An ensemble Monte Carlo simulation method has been used to study THz-pulse generation from semiconductor GaAs surface excited by a femtosecond laser pulse and biased in high electrical field near 100kV/cm. Electron transport is simulated using the three-valley conduction band model and taking into account acoustic, optical (polar and nonpolar) and intervalley phonon scattering mechanisms. Our simulations show that the THz temporal waveforms have a close relationship with the biased external field and the THz radiation is saturated with the increase of the biased field, these findings have not been exploited by the existing theoretical analyses. Power spectra show that the higher biased electrical field benefits the frequency extension for the THz radiation.     

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.     

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.     

High‐Order Shift Current Induced Terahertz Emission from Inorganic Cesium Bromine Lead Perovskite Engendered by Two‐Photon Absorption

High‐order nonlinear optical phenomena are interesting from a fundamental point of view as they reveal intrinsic symmetries of the materials. Potentially they can also be used for practical optoelectronic applications. High‐order shift current is one of these phenomena, and yet it has never been detected in experiments, primarily due to the difficulty in conventional contact detection. In this work, the shift current due to the two‐photon absorption (TPA) from all‐inorganic perovskite CsPbBr₃ is first observed by a contactless and nondestructive terahertz (THz) emission method. The results reveal that the THz emission is dominated by a high‐order shift current (fourth‐order nonlinear optical effect) with the below‐bandgap photon energy of femtosecond laser excitation. The high‐order shift current origins from the broken inversion symmetry induced by the dynamic stereochemical activity of the Pb²⁺ lone pair. A microscopic TPA‐assisted nonlinear optical model is presented to describe the photophysical process of the THz emission. The model matches well with the quadratic pump fluence and angular dependence of the THz emission. This work can not only open a new venue for the all‐inorganic perovskite‐based nonlinear optoelectronics and THz devices, but also afford a THz technology for the high‐order nonlinear effect analysis.     

快速热处理对磁控溅射VO2薄膜光电特性的影响

采用反应磁控溅射法制备二氧化钒(VO2)薄膜,并对其进行快速热处理(RTP)。主要研究500℃快速热处理10、15、20s工艺条件下VO2薄膜结晶状况和光电性能的变化。在20℃～80℃温区内,应用四探针薄膜电阻测试方法和太赫兹时域频谱技术(THz-TDS)测量了各样品的电学相变特性和光学相变特性。结果表明,经过快速热处理的样品电学相变幅度均达到了2个数量级以上;THz波的透射率在半导体-金属相变前后的最大变化达到了57.9%。同时发现,热处理500℃,10s时VO2的电学和光学相变幅度相对要大,当热处理时间达到15s左右时薄膜的相变幅度变化不再明显。快速热处理时间的长短对热致相变温度点的影响较小,但热致电学相变和光学相变的相变温度点不同:光学相变的温度为60℃左右,电学相变温度则在56℃附近。     

Sub-THz Photonic-Transmitters Based on Separated-Transport-Recombination Photodiodes and a Micromachined Slot Antenna

We demonstrate a novel photonic transmitter, which is composed of a low-temperature-grown GaAs (LTG-GaAs)-based separated-transport-recombination photodiode and a micromachined slot antenna. Under femtosecond optical pulse illumination, this device radiates strong electrical pulses (4.5-mW peak power) without the use of a Si-lens. It can be observed in the Fourier transform infrared spectrometer spectrum of radiated pulses that a significant resonance, with a peak power of approximately 300 muW, occurs at 500 GHz, which corresponds to the designed resonant frequency of the slot antenna. The saturation problem related to the output terahertz power that occurs with the traditional LTG-GaAs-based photonic-transmitters when operated under high external applied electrical fields (>50 kV/cm) has been eliminated by the use of our device     

THz wireless communication system based on wavelet transform

With a very wide frequency band not allocated at present, THz waves have many optimal characteristics such as high transmission rate, large capacity, and high security. The research of THz communication technology has become a hotspot in wireless communication. For THz wireless communications, it is crucial to study advanced electrical signal processing techniques. In this paper, in view of the shortcomings of traditional orthogonal frequency division multiplexing (OFDM) technology, we propose wavelet transform orthogonal multicarrier modulation method in THz system. In addition, we study THz channel coding technology to ensure that the THz wireless communication baseband system has better bit error rate (BER) performance and low computational complexity. Based on above, a THz wireless communication baseband system is conceived.     

Far-Infrared Spectrum Analysis Using Plasmon Modes in a Quantum-Well Transistor

Excitation of resonant plasmon modes by far-infrared (FIR) radiation in a quantum-well transistor is used to analyze the spectral content of FIR illumination at frequencies between 0.58 and 0.99 THz. A split grating gate design that allows localized pinch-off of the transistor channel greatly enhances FIR response and allows completely electrical tuning of the plasmon resonance, enabling broadband FIR spectrum analysis without moving parts. A voltage ramp applied to the gate can generate a spectrum at video rate.     

Tunable,Grating-Gated,Graphene-On-Polyimide Terahertz Modulators

An electrically switchable graphene terahertz (THz) modulator with a tunable-by-design optical bandwidth is presented and it is exploited to compensate the cavity dispersion of a quantum cascade laser (QCL). Electrostatic gating is achieved by a metal grating used as a gate electrode, with an HfO₂/AlO_x gate dielectric on top. This is patterned on a polyimide layer, which acts as a quarter wave resonance cavity, coupled with an Au reflector underneath. The authors achieve 90% modulation depth of the intensity, combined with a 20 kHz electrical bandwidth in the 1.9–2.7 THz range. The modulator is then integrated with a multimode THz QCL. By adjusting the modulator operational bandwidth, the authors demonstrate that the graphene modulator can partially compensate the QCL cavity dispersion, resulting in an integrated laser behaving as a stable frequency comb over 35% of the operational range, with 98 equidistant optical modes and a spectral coverage ~1.2 THz. This paves the way for applications in the terahertz, such as tunable transformation-optics devices, active photonic components, adaptive and quantum optics, and metrological tools for spectroscopy at THz frequencies.     

基于级联马赫-曾德尔调制器的太赫兹通信系统

为了有效解决太赫兹通信系统中信号难以调制,影响通信系统性能的问题,提出了一种基于级联马赫-曾德尔调制器的太赫兹通信系统。将要传递的伪随机不归零码与频率为10GHz的射频本振信号混频后调制到级联马赫-曾德尔调制器上,通过调节两个调制器的偏置电压,使其分别偏置在最大传输点和最小传输点上,得到的光载波信号经过光放大器放大,结合高非线性光纤的四波混频效应,利用相移布喇格光栅进行模式选择,经过光电转换后的太赫兹信号通过基带数据恢复,可以得出该太赫兹通信系统传输的误比特率。结果表明,基于级联马赫-曾德尔调制器结构可以将太赫兹信号的产生与调制结合到一起。该研究对太赫兹通信系统的实用化有一定借鉴意义。     

Cobalt-Based Ferrites Characterization Using Two Different Terahertz Time-Domain Spectrometers

We report an experimental study of the electrical properties of manganese cobalt (MnCo) and nickel cobalt (NiCo) ferrites in the terahertz (THz) frequency band. The study is motivated by the potential of MnCo, NiCo, and other magnetic ceramics for the fabrication of active and passive devices for THz wave communications. Using two different terahertz (THz) time-domain spectroscopy systems, we characterized the optical constants of cobalt ferrites doped with manganese and nickel in the technologically important 0.2–1 THz frequency band. The MnCo and NiCo ferrites investigated in our study exhibit a lower refractive index and absorption coefficient in the 0.2–1 THz frequency band than commercial strontium ferrite. We observed that using different valency ion oxide leads to a sudden change of the refractive index as a function of sample stoichiometries. Our experimental results provide evidence that microwave ferrite technology can be extended to the THz frequency band.     

Real-time imaging using a 4.3-THz quantum cascade laser and a 320 /spl times/ 240 microbolometer focal-plane array

We report the use of a /spl sim/50-mW peak power 4.3-THz quantum cascade laser (QCL) as an illumination source for real-time imaging with a 320 /spl times/ 240 element room-temperature microbolometer focal-plane array detector. The QCL is modulated synchronously with the focal-plane array for differential imaging. Signal-to-noise ratios of /spl sim/340 are achieved at a 20-frame/s acquisition rate, and the optical noise equivalent power of the detector array at 4.3 THz is estimated to be /spl sim/320 pW//spl radic/Hz. Both reflection and transmission mode imaging are demonstrated.     

Graphene Microstrip Patch Ultrawide Band Antennas for THz Communications

Future generation local communication systems will need to employ THz frequency bands capable of transferring sizable amounts of data. Current THz technology via electrical excitation is limited by the upper limits of device cutoff frequencies and by the lower limits of optical transitions in quantum confined structures. Current metallic THz antennas require high power to overcome scattering losses and tend to have low antenna efficiency. It is shown here via calculation and simulation that graphene can sustain electromagnetic propagation at THz frequencies via engineering the intra‐ and interband contributions to the dynamical conductivity to produce a variable surface impedance microstrip antenna with a several hundred GHz bandwidth. The optimization of a circular graphene microstrip patch antenna on silicon with an optimized return loss of ?26 dB, a ?10 dB bandwidth of 504 GHz, and an antenna efficiency of ?3.4 dB operating at a frequency of 2 THz is reported. An improved antenna efficiency of ?0.36 dB can be found at 3.5 THz but is accompanied by a lower bandwidth of about 200 GHz. Such large bandwidths and antenna efficiencies offer significant hope for graphene‐based flexible directional antennas that can be employed for future THz local device‐to‐device communications.     

Wide-span optical frequency comb generator for accurate opticalfrequency difference measurement

An optical frequency comb (OFC) generator was realized for accurate optical frequency difference measurement of 1.5 μm wavelength semiconductor lasers by using a high frequency LiNbO₃ electrooptic phase modulator which was installed in a Fabry-Perot cavity. It was confirmed that the span of the OFC was wider than 4 THz. By using semiconductor lasers whose spectrum linewidths were narrowed to 1 kHz and a sensitive optical balanced-mixer-receiver for measuring beat signal between the sideband of the comb and the laser, we demonstrated a frequency difference measurement up to 0.5 THz with a signal-to-noise ratio higher than 61 dB, and a heterodyne optical phase locking with a heterodyne frequency of 0.5 THz in which the residual phase error variance was less than 0.01 rad². The maximum measurable frequency difference, which was defined as the sideband frequency with the signal-to-noise ratio of 0 dB, was estimated to be 4 THz     

THz Photoconductive Antennas Made From Ion-Bombarded Semiconductors

We review the most important developments in the technology of THz photoconductive antennas made from ion-bombarded semiconductors. We describe the structural, optical and electrical properties of various ion-bombarded semiconductors and discuss the nature of the defects introduced by the ion bombardment technique and their impact on the characteristics of THz photoconductive antennas. Finally, we present the performances achieved by photoconductive antennas based on ion-bombarded semiconductors for optical excitation at various wavelength.     

A 1550-nm millimeter-wave photodetector with a bandwidth-efficiencyproduct of 2.4 THz

The monolithic integration of an optical preamplifier and a waveguide p-i-n photodiode has resulted in a 1550-nm photodetector having an external quantum efficiency of 72 (a responsivity of 89 A/W) and an electrical bandwidth of 33.5 GHz. These figures combine to give a bandwidth efficiency product of 2.4 THz, which is approximately one hundred times higher than for any other type of millimeter-wave photodetector published to date. In addition, the preamplifier performance is supplemented by wide optical bandwidth (60 nm), low polarization sensitivity (1 dB), near traveling wave amplification (1-dB maximum gain ripple), and high saturated output power (+11 dBm). The principal advance over work reported earlier is the traveling wave operation of the monolithic optical preamplifier     

Reliability and micro-structural properties of GaAs Schottky diodes for submillimeter-wave applications

Whisker contacted GaAs Schottky barrier diodes are the standard devices for mixing and multiplier applications in the THz frequency range. This is mainly due to their minimum parasitics and mature technology. But with the decreasing size of the anode contact, which is required for operation at high frequencies (up to approx. 3 THz), the reliability and the micro-structural understanding of the Schottky barrier becomes increasingly important. This contribution presents new results concerning the reliability of Schottky diodes and the physical properties of small-area Schottky junctions, especially at low current densities. For these purposes a number of different Schottky diodes have been fabricated with different epilayer doping concentrations and anode diameters. Measured I/V characteristics show that the diode current deviates considerably from the ideal thermionic current behavior with decreasing diode diameter. This deviation shows an exponential dependence on the diode voltage and is a function of the doping concentration of the active layer. For a given doping concentration in the epi-layer and decreasing anode diameter, this phenomenon shifts the minimum of the ideality factor towards higher current densities. An explanation is given in terms of a difference of the cyrstallinity of the polycrystalline platinum films on the GaAs for decreasing SiO₂ aperture size in connection with a reduced Pt mobility in the electrolyte. The reliability of Schottky barrier diodes under thermal and electrical stress has been investigated on different THz Schottky diode structures. The results show that the barrier height and the ideality factor of the fabricated structures are not affected by thermal stress. Electrical stress induced by large forward currents up to a current density of 10 kA/mm² even leads to a slight increase of the barrier height and a reduction of the series resistance.