低温生长砷化镓光电导天线产生太赫兹波

宽频太赫兹(THz)技术在国防、科研等领域有着广阔的应用前景,光电导天线是产生宽频THz波的重要手段.分析了低温生长和高温退火对光电导天线材料载流子寿命和电阻率的影响.在生长温度为230℃和250℃,退火温度为475℃的低温生长砷化镓(LTG-GaAs)上制备了领结(BowTie)和偶极子(Dipole)两种电极结构的小孔径光电导天线.实验给出,在250℃生长的LTG-GaAs上制备的光电导天线产生的太赫兹波辐射强度和频谱宽度较好,谱宽达到了3.6 THz,BowTie天线的辐射强度优于Dipole天线.两种形状的光电导天线皆可在10 V的偏置电压下产生太赫兹波辐射.     

GaAs参量振荡产生太赫兹波的腔相位匹配研究

基于腔相位匹配的方法,研究了GaAs微片状晶体构成的光学微腔光参量振荡产生太赫兹波的条件与参数设计.计算了腔相位匹配下GaAs晶体的最优化腔长,通过调节GaAs晶体的温度研究了太赫兹波的输出情况,模拟了不同波长下最低的能量阈值.结果表明,在完全相位匹配很难实现的情况下,采用腔相位匹配能很容易地实现大范围的太赫兹波调谐输出.结果为小型化光学太赫兹源的实验与理论研究提供了参考.     

Widely Frequency-Tunable Terahertz Wave Generation and Spectroscopic Application

The THz-wave generator based on the resonance of phonon-polaritons in GaP enabled THz spectral measurements of organic molecules like DNA/RNA-related molecules in wide frequency range (0.6–5.8 THz). High spectral purity of the generated THz-wave enables even the detection of defect structures of organic compounds, and will give a new tool for molecular sciences. We demonstrated THz imaging of a liver cancer. The THz-wave generator can be made small-size, light-weight system.     

Plasmon Excitation and Plasmonic Detection of Terahertz Radiation in the Grating-Gate Field-Effect-Transistor Structures

Physics of plasma oscillations and basic principles of plasmonic detection of terahertz radiation in the grating-gate transistor structures with two-dimensional electron channels are considered. It is shown that the grating-gate-transistor plasmonic detectors can be efficiently coupled to terahertz radiation. Plasmonic detection response considerably increases if the electron density in the grating-gate transistor structure is spatially modulated.     

Terahertz Luminescence and Electrical Characteristics of SiC Structures with Natural Superlattice in Strong Electric Fields

Recently, the intense terahertz electroluminescence from monopolar n⁺⁺–n^?–n⁺ structures of 8H-SiC natural superlattice at helium temperatures due to Bloch oscillations was found out. In the present work, we compare the THz emission and electrical characteristics of monopolar n⁺⁺–n^?–n⁺ and bipolar n⁺⁺–π–n⁺ 8H-SiC structures at 7 K. The bipolar n⁺⁺–π–n⁺ 8H-SiC structures were analogous to those on which the negative differential conductivity effect was observed earlier for three polytypes (4H, 6H, and 8H) at T = 300 K. The obtained results allow one to draw a conclusion about common nature of the negative differential conductivity and THz emission effects in the natural superlattice of SiC caused by Bloch oscillations. These results give the proof of fundamental importance supporting the objectivity of postulates of the F. Bloch – C. Zener – G. N. Wannier theory     

Terahertz Photonics: Optoelectronic Techniques for Generation and Detection of Terahertz Waves

Optoelectronic techniques for generation and detection of terahertz (THz) signals have been reviewed. The operation principles of THz photomixer sources and THz parametric sources have been studied and recent developments in these areas have been presented. The performances of developed THz optoelectronic sources have been discussed and compared.      

The Interaction of Laser and Photoconductor in a Continuous-Wave Terahertz Photomixer

The interaction of laser and photoconductor in an optical heterodyne conversion scheme is studied in detail. A dc biased photoconductor excited by two continuous-wave (CW) laser beams with a difference in their central frequencies falling in the terahertz spectrum is considered as the core element in all photoconductive photomixing structures. For this configuration the continuity equations for the electron and hole densities are solved in their general form along with the appropriate boundary conditions to find photocurrent distribution inside the photoconductor. It is shown that in a CW terahertz photomixing scheme the resulting photocurrent contains a dc component and a terahertz component. It is also shown that the amplitude and the phase of the terahertz component of the photocurrent are functions of the applied bias, physical parameters of the photoconductor, parameters of the lasers, and photomixer configuration. The dependency of the photocurrent on all of these parameters is explored in detail for a typical photomixer made of low-temperature-grown GaAs photoconductor.     

Analysis of Linewidth Tunable Terahertz Wave Generation in Periodically Poled Lithium Niobate

A new scheme of optical rectification of femtosecond laser pulses in a periodically poled lithium niobate crystal, which generates high energy and linewidth tunable multicycle terahertz (THz) pulses, is analyzed. The developed simple theoretical model allows investigating the generated THz spectrum and its dependence on spot size of the pump beam. It is shown that the transformation of THz radiation from narrowband to broadband is possible by simply reducing the pump beam size. The temporal waveform and energy of the multicycle THz pulses were calculated as well. It is shown that THz energy is inversely proportional to the pump beam size r _y , whereas energy spectral density is independent of r _y . The efficiency of optical-to-THz energy conversion for pump pulse energy of 1?mJ is estimated to be 0.8?×?10^?4. The possibility of tuning the generation frequency by changing the direction of the pump beam propagation is analyzed.     

Numerical Comparisons of Terahertz Radiation from GaAs and GaAs: As+ Substrates

In this paper, we have numerically analyzed the ultrafast change of local fields on the surfaces of a large-aperture photoconducting (LA-PC) antenna with GaAs and GaAs: As+ substrates. We find that the ultrafast screening of photogenerated carriers to the externally applied electric field has different effects on the saturation of THz radiation as the function of the laser fluence in the near and far field, respectively. Both screening effect of photocarriers and radiation effect are important in forming the saturation phenomena in the case of near field. But in far field, only the radiation effect is important.     

脉冲MOS结构少子产生寿命的统一表征

本文提出了一种采用脉冲ＭＯＳ结构测量少子产生寿命的统一表征谱方法，此方法基于任何一种收敛弛豫过程均可以转换成一种衰减的指数函数的思想，应用关以样原理获得脉冲ＭＯＳ结构瞬态电容差值谱，从谱图中我们可以直接得到关于少子产生寿命信息。本文综合了众多脉冲ＭＯＳ结构测量少子产生寿命的物理模型，分析了不同模型之间的精细差别。     

Automatic Generation of Wave Digital Structures for Numerically Integrating Linear Symmetric Hyperbolic PDEs

The numerical integration of partial differential equations (PDEs) resulting from passive physical problems can be performed by simulation of the actual system with multidimensional (MD) passive wave digital filters. Due to the principle of action at proximity, physical systems are usually massively parallel and only locally connected. Beyond these properties, the wave digital filters are well-known for their excellent numerical stability behavior and their high robustness. The new result of this paper is a synthesis procedure for automatic generation of the algorithms, based on MD wave digital filters for the numerical integration of PDEs describing linear hyperbolic passive systems. The proposed procedure permits a fully automated software development, based on the given partial differential equation.     

Double-resonance Semiconductor Superlattice Parametric Oscillator for Generation of Subterahertz Radiation

We report on a subterahertz superlattice parametric oscillator that operated simultaneously at two different harmonic frequencies of a microwave pump field. A pump field (frequency near 100 GHz) was coupled to a GaAs/AlAs superlattice in a resonator for the third and the fifth harmonic. The pump field produced a third harmonic field and this together with the pump field created a fifth harmonic field. A theoretical analysis indicates that the nonlinearity, which is based on the dynamics of miniband electrons, should allow for the upconversion of pump radiation of higher frequency into the terahertz frequency range.     

Structural,electrical and optical characterization of singlecrystal ErAs layers grown on GaAs by MBE

A detailed study is presented of the structural, electrical, and optical properties of ErAs films grown on GaAs by molecular beam epitaxy (MBE). ErAs layers 1500Å thick were grown successfully over a relatively wide range of substrate temperatures (420-580° C), although overgrowth of GaAs on ErAs was found to be difficult. In-situ reflection highenergy electron diffraction (RHEED), x-ray diffraction, and Rutherford backscattering (RBS) measurements all indicate single crystal growth. Analysis of X-ray rocking curves reveals that, over the range of substrate temperatures studied, strain due to the lattice mismatch between ErAs and GaAs is completely inelastically relieved in the 1500Å thick ErAs layers. Variable-temperature Hall measurements reveal metallic behaviour in all samples, with no pronounced dependence on substrate temperature. Spectrally narrow (0.6 meV) intra 4f-shell transitions of Er³⁺ (4f¹¹), at 1.54 μm, have been observed in ErAs epitaxial layers both in absorption (by Fourier transform infra-red spectroscopy, FTIR) and in emission (by cathodoluminescence). The crystal-field splittings observed in the FTIR spectra are consistent with the cubic(O _h)symmetry expected for the Er lattice site in unstrained ErAs, in good agreement with the x-ray analyses.     

Nonlinear Terahertz Generation: Chiral and Achiral Meta-Atom Coupling

Generation and manipulation of terahertz (THz) waves are of vital importance for advancing THz technology. Nonlinear metasurfaces allow effective integration of both processes into a single compact device. However, such existing THz devices commonly rely on utilizing a single meta-atom, which has fixed THz generation properties and thus limits the range of achievable functionalities. Here, it is demonstrate how coupling between different meta-atoms within the unit-cell can be used as a degree of freedom for controlling nonlinear THz generation, where achiral coupling provides full control over the amplitude of the generated THz field, while chiral coupling makes the THz generation sensitive to the handedness of the pump polarization. In particular, chiral coupling allows the realization of multiplexed pump-handedness-selective nonlinear metasurfaces, which is illustrated experimentally by selectively generating THz beams with different orbital angular momentum with a single nonlinear metasurface. This approach provides opportunities for developing various integrated nonlinear THz devices.     

Znse/GaAs超晶格的电子态和芯态激子

本文用半经验紧束缚法计算了ZnSe/GaAs(001)超晶格的能带结构,研究了其能隙与有效质量随层厚的变化.计算了(ZnSe)_5/(GaAs)_5超晶格中与杂质有关的芯态激子,其结果能说明相应异质结中束缚在Ga上的激子峰.本文还提出了该材料中导带底存在界面态.     

Terahertz/Millimeter Wave Characterizations of Soils for Mine Detection: Transmission and Scattering

The detection of anti-personnel mines depends on the soil transmission and scattering at a given wavelength. Transmission spectra were measured over the range 90–4200 GHz for 19 soil samples that span a number of soil orders that have extensive worldwide distribution using a vector network analyzer (90–140 GHz) and a Fourier spectrometer (120–4200 GHz). Transmission drops to nearly zero for wavelengths shorter than the characteristic particle size of the sample as a consequence of scattering. This interpretation is supported by a fit to a standard scattering model with physically reasonable fitting parameters. Transmission spectra were also measured for various liquids (90–600 GHz) for possible index matching. These liquids were mixed with the soil sample and were found to reduce scattering and increase transmission through the soil at higher frequencies. This work is relevant to mine detection using terahertz and millimeter wave radiation for high resolution images through the soil.     

产生太赫兹辐射源的Nd:YAG双波长准连续激光器

产生太赫兹波辐射的方法可分为电子学和光子学两大类.在光子学领域,非线性光学差频方法是获取高功率、低成本、便携式、室温运转太赫兹波的主要方法之一.实验研究了激光二极管(LD)端面抽运Nd:YAG1319 nm/1338 nm双波长准连续线偏振运转激光器,理论计算了输出双波长在非线性晶体DAST(4-N,N-dimethylamino-4'-N-'methyl-stilbazolium tosylate)中差频产生太赫兹辐射的平均功率.在重复频率50 kHz时,双波长激光平均输出功率达到2.22 W,斜率效率12.72%,线偏振度0.983,脉冲宽度71.91 ns.M2因子仅为1.165,不稳定性小于0.487%.根据非线性差频理论,计算出可在1 mm厚DAST晶体中获得4.71 mw的3.23 THz高相干性太赫兹波辐射.这两条非常接近的谱线为进一步通过非线性光学差频方法获得高相干性太赫兹波提供了理论基础.     

Superconducting Structures for Study and Phase Synchronization of Integrated Terahertz Oscillators

Journal of Communications Technology and Electronics - Superconducting integrated circuits based on high-quality Nb–AlOx–Nb and Nb–AlN–NbN tunnel junctions, including a...     

Generation and Detection of Terahertz Waves Using Low‐Temperature‐Grown GaAs with an Annealing Process

In this letter, we present low‐temperature grown GaAs (LTG‐GaAs)‐based photoconductive antennas for the generation and detection of terahertz (THz) waves. The growth of LTG‐GaAs and the annealing temperatures are systematically discussed based on the material characteristics and the properties of THz emission and detection. The optimum annealing temperature depends on the growth temperature, which turns out to be 540°C to 580°C for the initial excess arsenic density of to .