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.     

Hybrid Computational Simulation and Study of Terahertz Pulsed Photoconductive Antennas

A photoconductive antenna (PCA) has been numerically investigated in the terahertz (THz) frequency band based on a hybrid simulation method. This hybrid method utilizes an optoelectronic solver, Silvaco TCAD, and a full-wave electromagnetic solver, CST. The optoelectronic solver is used to find the accurate THz photocurrent by considering realistic material parameters. Performance of photoconductive antennas and temporal behavior of the excited photocurrent for various active region geometries such as bare-gap electrode, interdigitated electrodes, and tip-to-tip rectangular electrodes are investigated. Moreover, investigations have been done on the center of the laser illumination on the substrate, substrate carrier lifetime, and diffusion photocurrent associated with the carriers temperature, to achieve efficient and accurate photocurrent. Finally, using the full-wave electromagnetic solver and the calculated photocurrent obtained from the optoelectronic solver, electromagnetic radiation of the antenna and its associated detected THz signal are calculated and compared with a measurement reference for verification.     

Photoconductive LT-GaAs Terahertz Antennas: Correlation Between Surface Quality and Emission Strength

We investigate the influence of the surface properties of a low-temperature-grown GaAs photoconductive antenna on the terahertz (THz) emission strength, using a specially designed THz time-domain spectroscopy system. The system allows us to excite six different positions along the 10 μm gap of a coplanar stripline antenna with a length of 10 mm without changing the alignment of the optical or THz beam path. A comparison to the surface roughness and the grain size which are extracted from an atomic force and a scanning electron microscope is given.     

Scalable Microstructured Photoconductive Terahertz Emitters

The development of scalable emitters for pulsed broadband terahertz (THz) radiation is reviewed. Their large active area in the 1 – 100 mm² range allows for using the full power of state-of-the-art femtosecond lasers for excitation of charge carriers. Large fields for acceleration of the photogenerated carriers are achieved at moderate voltages by interdigitated electrodes. This results in efficient emission of single-cycle THz waves. THz field amplitudes in the range of 300 V/cm and 17 kV/cm are reached for excitation with 10 nJ pulses from Ti:sapphire oscillators and for excitation with 5 μJ pulses from amplified lasers, respectively. The corresponding efficiencies for conversion of near-infrared to THz radiation are 2.5 × 10^-4 (oscillator excitation) and 2 × 10^-3 (amplifier excitation). In this article the principle of operation of scalable emitters is explained and different technical realizations are described. We demonstrate that the scalable concept provides freedom for designing optimized antenna patterns for different polarization modes. In particular emitters for linearly, radially and azimuthally polarized radiation are discussed. The success story of photoconductive THz emitters is closely linked to the development of mode-locked Ti:sapphire lasers. GaAs is an ideal photoconductive material for THz emitters excited with Ti:sapphire lasers, which are widely used in research laboratories. For many applications, especially in industrial environments, however, fiber-based lasers are strongly preferred due to their lower cost, compactness and extremely stable operation. Designing photoconductive emitters on InGaAs materials, which have a low enough energy gap for excitation with fiber lasers, is challenging due to the electrical properties of the materials. We discuss why the challenges are even larger for microstructured THz emitters as compared to conventional photoconductive antennas and present first results of emitters suitable for excitation with ytterbium-based fiber lasers. Furthermore an alternative concept, namely the lateral photo-Dember emitter, is presented. Due to the strong THz output scalable emitters are well suited for THz systems with fast data acquisition. Here the application of scalable emitters in THz spectrometers without mechanical delay stages, providing THz spectra with 1 GHz spectral resolution and a signal-to-noise ratio of 37 dB within 1 s, is presented. Finally a few highlight experiments with radiation from scalable THz emitters are reviewed. This includes a brief discussion of near-field microscopy experiments as well as an overview over gain studies of quantum-cascade lasers.     

Noise Analysis of Photoconductive Terahertz Detectors

The noise performance of photoconductive terahertz detectors is analyzed and the tradeoff between low-noise and high-responsivity operation of photoconductive detectors is investigated as a function of device parameters and operational settings. The analysis is conducted on two general photoconductive detector architectures, symmetrically pumped and asymmetrically pumped photoconductive detector architectures. The results indicate that the highest signal-to-noise ratios are offered by the symmetrically pumped and asymmetrically pumped detector architectures for the photoconductive detectors based on short-carrier lifetime and long-carrier lifetime semiconductors, respectively.     

Principles of Impedance Matching in Photoconductive Antennas

The principles of impedance matching in photoconductive antennas in comparison with conventional antennas are described. Because of the optical nature of the input signal in photoconductive antennas and the dependence of photoconductor conductance on the optical pump power, the optimum photoconductor impedance is not necessarily determined by the complex conjugate of antenna impedance. Using the equivalent circuit model of photoconductive antennas, the photoconductor impedance optimization criteria are evaluated according to the photoconductive antenna structure and operational settings.     

偏振可调的太赫兹光电导天线设计北大核心

提出了一种偏振可调的太赫兹光电导天线,其结构由四个弧形金属电极、低温砷化镓衬底和氮化硅抗反射涂层构成。通过对光电导天线的工作原理分析,可以发现弧形金属电极结构决定天线的辐射偏振,且衬底的载流子迁移率、载流子寿命和衬底材料对激光的吸收直接影响天线的辐射特性。利用COMSOL软件对该光电导天线进行建模仿真,其结果表明该光电导天线可以在45°方向倍增地辐射线偏振太赫兹波,且辐射强度相较于常规光电导天线提高了30%,辐射带宽高达10 THz。所设计太赫兹光电导天线具有偏振可调、结构简单和易于加工等特点,在太赫兹光谱检测领域具有广阔的应用前景。     

Photoconductive Terahertz antenna with radial symmetry

It has been demonstrated recently that it is possible to guide broadband Terahertz pulses utilising a novel coaxial waveguide. This technique is hindered by the difficulty in exciting the radially polarised fundamental mode of the waveguide. To optimise mode matching, a novel radially symmetric photoconductive Terahertz antenna is proposed. Using finite element method simulations and analytical calculations, it is shown that this antenna can generate the desired radial polarisation.     

光电导太赫兹天线效率参数的讨论

光电导天线是产生太赫兹波最常用的器件之一,但是目前它面临的主要问题是天线效率很低,难以获得高功率的太赫兹波.为了提高其输出功率和效率,介绍了计算光电导天线效率的公式,讨论了影响光电导天线光-太赫兹转换效率的参数:与激光源相关的参数如激光强度、脉冲宽度、激励光斑尺寸及位置;与光电导天线相关的参数如偏压电场、天线材料和几何结构、天线暗电阻率、基底材料等.这对光电导太赫兹天线的分析与设计也是大有裨益的.     

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.     

光电导太赫兹天线及其研究进展

太赫兹科学技术是一门新学科,其关键技术是太赫兹波的产生。光电导天线是目前产生宽频带太赫兹波最重要的方法之一。光电导天线的几何结构对提高天线在太赫兹频段的效率、方向性、带宽等都非常重要。因此有必要对光电导太赫兹天线的各种几何结构和国内外研究进展做较为全面的评述,对其中存在的问题和解决思路进行分析,对未来研究发展的方向进行探讨。     

低温生长砷化镓光电导天线产生太赫兹波的辐射特性

研究了低温生长砷化镓光电导天线(LT-GaAs PCA)产生太赫兹(THz)波的辐射特性。利用太赫兹时域光谱(TDS)技术测量了光电导发射极在飞秒激光作用下辐射的太赫兹脉冲,得到了时域发射光谱,并通过快速傅里叶变换(FFT)得到相应的频域光谱。结果表明,低温砷化镓光电导天线产生的太赫兹波信号比飞秒激光激发半导体表面产生的太赫兹波信号具有更高的强度和信噪比;太赫兹波信号与光电导天线的偏置电压成线性关系;随着抽运激光功率的增强,太赫兹波信号增大并出现饱和。     

Efficient Terahertz Photoconductive Emitters with Improved Electrode Structures

We present the design,fabrication,and characterization of two new types of terahertz photoconductive emitters.One has an asymmetric four-contact electrode structure and the other has an arc-shaped electrode structure,which are all modified from a traditional strip line antenna.Numerical simulations and real experiments confirm the good performance of the proposed antennas.An amplitude increase of about 40%is experimentally observed for the terahertz signals generated from the new structures.The special electrode structure and its induced local bias field enhancement are responsible for this radiation efficiency improvement.Our work demonstrates the feasibility of developing highly efficient terahertz photoconductive emitters by optimizing the electrode structure.     

基于新电极的太赫兹光电导天线设计

提出了分别采用凹形电极和新叉指形电极的两种光电导天线,其电极产生的静电场电场强度是中间小而周围大,而激光光斑的光强是中间大而周围小,因此提高了衬底的抗击穿能力和光电流。相比于采用传统带状线电极的光电导天线,采用凹形电极的光电导天线产生的光电流提升了62%;相比于采用传统叉指形电极的光电导天线,采用新叉指形电极的光电导天线产生的光电流提升了41%。所设计的两种光电导天线都具有结构简单、便于加工且易与其他提高辐射功率的技术相结合的优点,其在成像、生物传感、通信和光电探测等领域有着重要的潜在应用价值。     

太赫兹光导天线阵列功率空间合成规律研究

采用天线瞬态辐射理论和数值仿真方法,分别研究了阵元个数、阵元间距、激励延迟等物理参量对光导天线阵列功率空间合成效率的影响。通过深入分析和对比文献报道的实验数据,发现在不考虑非线性饱和效应和电磁屏蔽的条件下,对于轴向排列的偶极子光导阵列,导致功率合成效率不高的重要原因主要源自:偶极子的方向性辐射、电磁能量空间耗散和非同步光激励。     

小孔径蝴蝶型光电导天线太赫兹辐射源的研究

研究了5种小孔径光电导天线的太赫兹发射特性,并且对它们所发射的太赫兹波进行了对比,为研制高效率的太赫兹波发射源提供了参考依据。利用太赫兹时域光谱技术测量了光电导天线发射的太赫兹（THz）脉冲,得到了时域发射光谱,并通过快速傅里叶变换得到相应的频域光谱。结果表明,太赫兹信号强度随偏置电压的增大而增强;随着泵浦激光功率的增大而增强并出现饱和现象。偏置电压与泵浦激光功率相同时,我们对比5种光电导天线产生的太赫兹信号,从中找到了一种发射效率较高的小孔径光电导天线,并且研究了电极形状、电极间距对光电导天线发射效率的     

小孔径螺旋型光电导天线的太赫兹辐射特性研究

研究了小孔径螺旋型光电导天线的太赫兹辐射特性.利用太赫兹时域光谱技术测量了螺旋型光导天线辐射的太赫兹波谱,得到了其时域发射光谱.通过快速傅里叶变换得到相应的频域光谱,同时对两种不同孔径螺旋天线的太赫兹辐射特性进行了比较.实验结果表明,太赫兹信号强度会随偏置电压的增大而增强;在偏置电压和泵浦光功率相同的情况下,较小孔径的...     

改进光电导太赫兹天线输出功率与方向的研究

光电导天线被广泛用于太赫兹波的产生,但是产生的太赫兹功率较低,方向性差。为了改进光电导太赫兹天线输出功率与方向,研究了不同基底厚度对光电导天线辐射功率的影响,其仿真结果表明:辐射功率受基底厚度影响明显,大部分辐射功率进入基底的一侧而不是自由空间,厚度薄的基底更能辐射出较大的功率。为了提高光电导天线的方向性,设计出一个加载在光电导天线输出端的锥形喇叭天线结构,仿真得到该喇叭天线的方向性为20.7dBi,这表明可以通过加载锥形喇叭,在期望的方向上获得较高的太赫兹波功率。     

Impact of the Metal Adhesion Layer on the Radiation Power of Plasmonic Photoconductive Terahertz Sources

The use of plasmonic contact electrodes in a photoconductive terahertz source offers high optical-to-terahertz conversion efficiencies. The high efficiency is because plasmonic contact electrodes concentrate a large portion of the incident optical pump beam in close proximity to the contact electrodes. By reducing the average transport path length of the photo-generated carriers from the contact electrodes, a larger number of the photocarriers drift to the terahertz radiating elements of the photoconductive source within a sub-picosecond time scale. Therefore, higher terahertz radiation power levels are achieved compared to a similar photoconductive source without plasmonic contact electrodes. Au is a preferred metal for plasmonic contact electrodes because of the strong plasmonic enhancement factors it offers at near-infrared optical wavelengths. However, it requires an adhesion layer to stick well to most III–V semiconductor substrates used in photoconductive terahertz sources. In this paper, we analyze the impact of the Au adhesion layer on the performance of plasmonic photoconductive sources fabricated on a GaAs substrate. Our analysis suggests that Cr is the most promising adhesion layer for plasmonic contact electrodes. We show that the use of a Cr adhesion layer instead of Ti, which is used in previously demonstrated plasmonic photoconductive sources, offers up to an 80% enhancement in the generated terahertz powers. We report record-high terahertz power emissions of up to 6.7 mW from plasmonic photoconductive sources with Cr/Au contacts.