Terahertz generation and detection of LT-GaAs thin film photoconductive antennas excited by lasers of different wavelengths

A new method of generating and detecting terahertz waves is proposed. Low-temperature-grown gallium arsenide (LT-GaAs) thin films are prepared by etching a sacrificial layer (AlAs) in a four-layer epitaxial structure constituted with LT-GaAs, AlAs, GaAs, and semi-insulating gallium arsenide (SI-GaAs). The thin films are then transferred to clean silicon for fabricating the LT-GaAs thin film antennas. The quality and transmission characteristics of the films are analyzed by an 800-nm asynchronous ultrafast time domain spectroscopy system, and the degree of bonding between the film and silicon wafer is determined. Two LT-GaAs thin film antennas for generating and detecting the terahertz waves are tested with a 1550-nm femtosecond laser. The terahertz signal is successfully detected, proving the feasibility of this home-made LT-GaAs photoconductive antennas. This work lays a foundation for studying the mechanism of terahertz wave generation in GaAs photoconductive antennas below the semiconductor band gap.     

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.     

Anomalous behaviours of terahertz reflected waves transmitted from GaAs induced by optical pumping

Femtosecond pump-terahertz probe studies of carrier dynamics in semi-insulating GaAs have been investigated in detail for various pump powers. It is observed that, at high pump powers, the reflection peaks flip to the opposite polarity and dramatically enhance as the pump arrival time approaches the reflected wave of the terahertz pulse. The abnormal polarity-flip and enhancement can be interpreted by the pump-induced enhancement in the photoconductivity of GaAs and half-wave loss. Moreover, the carrier relaxation processes and surface states filling in GaAs are also studied in these measurements.     

Ultrafast response of multi-energy proton-bombarded GaAs photoconductors

We have investigated the ultrafast optical and optoelectronic characteristics of multi-energy proton-bombarded GaAs (GaAs:H⁺) material and devices in some detail. Photo-excited carrier lifetimes of GaAs:H⁺ were observed to be as low as 350 ± 50 fs. Photoconductive switches (PCS) fabricated on GaAs:H⁺ were found to exhibit lower dark currents (15 nA at a bias of 10 V) and higher breakdown voltage (> 100 kV/cm) than PCS's fabricated on semi-insulating (S.I.) GaAs. The temporal response of the GaAs:H⁺ PCS was about 2 ps at full-wave half minimum. Optically excited terahertz (THz) radiation from GaAs:H⁺ was reported for the first time to our knowledge. The temporal response and spectral bandwidth of the emitted THz radiation were 0.7 ps and 1.25 THz, respectively. The field strength of the THz signal was about 20 mV/cm. From the THz data, we are able to deduce that the effective carrier mobility of GaAs:H⁺ was less than 1 cm²/V-sec.     

Transient surface photoconductivity of GaAs emitter studied by terahertz pump-emission spectroscopy

This paper investigates the ultrafast carrier dynamics and surface photoconductivity of unbiased semi-insulating GaAs in detail by using a terahertz pump-emission technique. Based on theoretical modelling, it finds that transient photoconductivity plays a very important role in the temporal waveform of terahertz radiation pulse. Anomalous enhancement in both terahertz radiation and transient photoconductivity is observed after the excitation of pump pulse and we attribute these phenomena to carrier capture in the EL2 centers. Moreover, the pump power- and temperature-dependent measurements are also performed to verify this trapping model.     

光温双控太赫兹波调制特性研究

基于多频带金属开口谐振环结构,利用GaAs材料的光敏特性和VO₂薄膜的热致相变特性,设计了一种既能实现光控又能实现温控的太赫兹(Terahertz, THz)波调制器,研究了光强和薄膜温度对 THz波调制特性的影响。结果表明,随着光强的增加,谐振频率均出现蓝移且谐振强度减小,当光强达到0.2 μJ·cm-2时,第二个谐振点(0.52 THz)蓝移了0.14 THz,透射幅度增加达50%;随着VO₂温度增加至相变温度以上,THz波透射幅度急剧减小,在0.63 THz处透射幅度减小达45.5%;当光强和温度同时控制时,随着光强和温度的增加,谐振点频率蓝移且谐振点处的THz波透射幅度增加,但在温度超过相变温度后,则温度控制起主导作用。设计的THz波调制器能通过光控和温控实现对THz波的明显调制效果,可为实现多功能的THz波功能器件的设计及应用提供参考。     

Photocarriers dynamics in silicon wafer studied with optical-pump terahertz-probe spectroscopy

Optical pump-terahertz probe spectroscopy is employed to investigate the optical characteristics of silicon wafer. The wafer surface undergoes a phase transition from insulator to metal for terahertz wave with increasing pump fluence. The real part of the pump-induced conductivity shows strong frequency dependence, which can be well described with Drude–Smith model. Our results also demonstrate that the photoexcited Si layer acts as a broadband terahertz pulse antireflection coating with proper pump fluence. In addition, it is observed that the terahertz pulse apparently arrives at the detector earlier when silicon is optically excited.     

Enhancement of terahertz photoconductive antenna operation by optical nanoantennas

Photoconductive antennas are promising sources of terahertz radiation that is widely used for spectroscopy, characterization, and imaging of biological objects, deep space studies, scanning of surfaces, and detection of potentially hazardous substances. These antennas are compact and allow for generation of both ultrabroadband pulses and tunable continuous wave terahertz signals at room temperatures, with no need for high‐power optical sources. However, such antennas have relatively low energy conversion efficiency of femtosecond laser pulses or two close pump wavelengths (photomixers) into the pulsed and continuous terahertz radiation, correspondingly. Recently, an approach to solving this problem that involves known methods of nanophotonics applied to terahertz photoconductive antennas and photomixers has been proposed. This approach comprises the use of optical nanoantennas for enhancing the absorption of pump laser radiation in the antenna gap, reducing the lifetime of photoexcited carriers, and improving the antenna thermal efficiency. This Review is intended to systematize the main results obtained by researchers in this promising field of hybrid optical‐to‐terahertz photoconductive antennas and photomixers. We summarize the main results on hybrid THz antennas, compare the approaches to their implementation, and offer further perspectives of their development including an application of all‐dielectric nanoantennas instead of plasmonic ones.

     

Measurements of residual stress in semi-insulating GaAs by Cr-related luminescence lines

We have measured systematically the Cr-related zero-phonon lines in the 0.839 eV region in a series of plastically-bent semi-insulating GaAs:Cr with compressive or tensile stress along various bending axes. As a result, it has been found that the residual stress in semi-insulating GaAs:Cr wafers can be sensitively characterized from a splitting and energy shift of the 0.839 eV Cr-related luminescence lines in the low-temperature photoluminescence spectra. Furthermore, we have applied this method to the characterization of the interface stress of OMVPE-grown ZnSe/GaAs:Cr heterostructure and found that anomalous stress exists at the ZnSe/GaAs interface, which is inconsistent with stress predicted by the lattice mismatch of the heterojunctions.     

Generation and characterization of terahertz pulse trains from biased, large-aperture photoconductors

The saturation properties of terahertz emission from biased, large-aperture photoconductors excited by trains of amplified femtosecond optical pulses are presented. A direct comparison is made of the multiple-pulse saturation properties of terahertz emission from semi-insulating GaAs and low-temperature-grown GaAs emitters with different carrier lifetimes. When the carrier lifetime is less than or comparable with the interpulse spacing, a significant enhancement of the narrow-band terahertz output is observed. The enhancement is not observed for emitters with long carrier lifetimes, consistent with the results of a previously derived saturation theory [Opt. Lett. 18, 1340 (1993)].     

Measurement of nonlinear absorption coefficients in GaAs, InP and Si by an optical pump THz probe technique

An optical pump terahertz (THz) probe method for measuring carrier mobility and multiphoton absorption coefficients in semiconductors is demonstrated. A THz probe pulse is used to detect the transient photoconductivity generated by an optical pump pulse. The change in transmission coefficient at THz frequencies due to a pump pulse with photon energy greater than the band gap energy is used to determine the sum of electron and hole mobilities. The weak nonlinear absorption of a pump pulse with photon energy less than the band gap energy produces an approximately uniform free carrier distribution. The THz transmission coefficient vs. pump fluence, and the mobility, are used in a bulk photoconductivity model to determine the multiphoton absorption coefficients. For GaAs, InP and Si we find two photon absorption coefficients at 1305 nm of 42.5 ± 11, 70 ± 18 and 3.3 ± 0.9 cm/GW, respectively. For GaAs and InP we determine three photon absorption coefficients at 2144 nm of 0.19 ± 0.07 and 0.22 ± 0.08 cm³/GW².     

Ultrafast photoconductors from low-temperature MOCVD-grown GaAs and InGaAs epitaxial layers

GaAs and InGaAs epitaxial layers were grown by Metal-Organic Chemical Vapor Deposition at Low substrate Temperatures (LT-MOCVD). The layers, grown at temperatures below 430° C were semi-insulating. Transmission electron microscopy images reveal uniformly distributed 3–10nm size clusters which consist, most probably, of zinc. Photodetectors fabricated from those layers feature nonlinear photocurrent versus optical power dependence. The nonlinearities are explained in terms of electric field redistribution. The nonlinear photoconductive correlation measurements show that excess carrier lifetimes are in the range of 20–50 ps.Deceased     

Semi-insulating GaAs: A possible substrate for a field-assisted positron moderator

Positron lifetime spectroscopy measurements have been carried out for semi-insulating GaAs with applied electric fields in the samples directed towards, and away from the positron injecting contact. The lifetime spectra have been decomposed into two components, the longer of which (400 ps) is characteristic of open volume defects at the metal-semiconductor interface through which positrons are injected. The interesting feature of these experiments is the large increase in the intensity of this interface component as the field is directed towards the contact. We show that this increase is caused by a significant fraction of implanted positrons drifting under the influence of a strong electric field produced by a layer of space-charge formed adjacent to the positron injecting contact. The general trend of the intensity variation is well explained by the proposed model. Experiments involving the application of an ac bias to the samples strengthen the suggestion that the space charge region is largely formed from ionized EL2 donors. The results of the present work indicate that semi-insulating GaAs possesses properties that make it a suitable material for the fabrication of a high efficiency (10%) room-temperature field-assisted positron moderator. The extraction of positrons from the GaAs substrate into the vacuum through a thin metalization is discussed based upon available positron affinities for the GaAs and various elemental metals. These data suggest that a few monolayers of a strongly electronegative metal such as Au or Pd may allow vacuum emission through quantum tunneling.     

光电导天线辐射阻抗特性模拟分析(英文)

针对连续太赫兹光电导天线辐射功率较低的缺点,利用有限积分方法对三种常用的光电导天线,包括偶极天线、蝶形天线和螺旋天线,进行数值模拟并分析比较其辐射阻抗特性.仿真结果表明,偶极天线的辐射阻抗与偶极长度、宽度、电极间隙以及传输线宽度有关,且在其谐振频率存在峰值阻抗,适用于特定频率的太赫兹波辐射.蝶形天线和螺旋天线在所研究的太赫兹波段具有近似稳定的辐射阻抗,广泛应用于宽带领域.对带有交叉电极的电极间隙进行计算,结果表明由交叉电极引入的附加电容降低了天线的高频阻抗.     

Efficient photoconductive terahertz source using line excitation

We report significant improvement in terahertz (THz) power and efficiency using photoconductive sources by use of a spatially extended line source excitation and the trap enhanced field effect that occurs in sources made on semi-insulating GaAs. The combination of high electric fields and reduced screening effects allows 10 microW of THz power to be generated with 14 mW of absorbed optical power, demonstrating nearly 0.1% optical-to-THz conversion efficiency.     

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.      

Integrated terahertz source based on three-wave mixing of whispering-gallery modes

We propose an integrated terahertz emitter operating at room temperature between 2.4 and 6 THz. Based on difference-frequency generation in a triply resonant Au/AlAs/GaAs/AlAs/Au microcylinder, this nonlinear source is pumped by two near-IR whispering-gallery modes that are excited by InAs quantum dots embedded in the resonator. In the vertical direction, these pump modes are due to total internal reflection at GaAs/AlAs interfaces, while the terahertz mode is confined between the metallic layers. This parametric source offers potential advantages with respect to existing terahertz sources for spectroscopic applications, such as room-temperature operation and electrical pumping.     

Influences of As flux on the lattice constants, magnetic and transport properties of (Ga, Mn)As epilayers

A series of (Ga, Mn)As epilayers have been prepared on semi-insulating GaAs (001) substrates at 230  ^°C by molecular-beam epitaxy under fixed temperatures of Ga and Mn cells and varied temperatures of the As cell. By systematically studying the lattice constants, magnetic and magneto-transport properties in a self-consistent manner, we find that the concentration of As antisites monotonically increases with increasing As flux, while the concentration of interstitial Mn defects decreases with it. Such a trend sensitively affects the properties of (Ga, Mn)As epilayers.     

Effects of silicon negative ion implantation in semi-insulating gallium arsenide

ABSTRACT

In the present work, effects of silicon negative ion implantation into semi-insulating gallium arsenide (GaAs) samples with fluences varying between 1?×?10¹⁵ and 4?×?10¹⁷?ions?cm^?2 at 100?keV have been described. Atomic force microscopic images obtained from samples implanted with fluence up to 1?×?10¹⁷?ion?cm^?2 showed the formation of GaAs clusters on the surface of the sample. The shape, size and density of these clusters were found to depend on ion fluence. Whereas sample implanted at higher fluence of 4?×?10¹⁷?ions?cm^?2 showed bump of arbitrary shapes due to cumulative effect of multiple silicon ion impact with GaAs on the same place. GXRD study revealed formation of silicon crystallites in the gallium arsenide sample after implantation. The silicon crystallite size estimated from the full width at half maxima of silicon (111) XRD peak using Debye-Scherrer formula was found to vary between 1.72 and 1.87?nm with respect to ion fluence. Hall measurement revealed the formation of n-type layer in gallium arsenide samples. The current–voltage measurement of the sample implanted with different fluences exhibited the diode like behavior.     

Modified slot antennas on high-permittivity substrates for the millimeter-wave range

We consider the possibility of manufacturing a planar antenna on the dielectric layer with high permittivity, which is intended for a monolithic planar detector of the millimeter-wave range. The previous design of the slot antenna on a foil-cladded dielectric with low permittivity (ε = 2.2) is analyzed. The simulation results show that a transition from the substrate with low dielectric permittivity to a semi-insulating GaAs substrate (ε ≈ 13) with the directivity kept at a level of about ∼10 at the resonant frequency is possible for antennas with external sizes comparable with the wavelength. The parameters of the planar detectors on semi-insulating GaAs substrates in the 3-mm wavelength range (94–97 GHz) were measured. Directional patterns of the planar antennas at the resonant frequencies reasonably coincide with the calculated data. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 51, No. 10, pp. 864–871, October 2008.