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.     

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.     

Far-infrared conductivity of CuS nanoparticles measured by terahertz time-domain spectroscopy

This paper reports that terahertz time-domain spectroscopy is used to measure the optical properties of CuS nanoparticles in composite samples. The complex conductivity of pure CuS nanoparticles is extracted by applying the Bruggeman effective medium theory. The experimental data are consistent with the Drude--Smith model of conductivity in the range of 0.2--1.5~THz. The results demonstrate that carriers become localized with a backscattering behaviour in small-size nanostructures. In addition, the time constant for the carrier scattering is obtained and is only 64.3~{fs} due to increased electron interaction with interfaces and grain boundaries.     

Picosecond transient photoconductivity in functionalized pentacene molecular crystals probed by terahertz pulse spectroscopy

We have measured transient photoconductivity in functionalized pentacene molecular crystals using ultrafast optical pump-terahertz probe techniques. The single crystal samples were excited using 800 nm, 100 fs pulses, and the change in transmission of time-delayed, subpicosecond terahertz pulses was used to probe the photoconducting state over a temperature range from 10 to 300 K. A subpicosecond rise in photoconductivity is observed, suggesting that mobile carriers are a primary photoexcitation. At times longer than 4 ps, a power-law decay is observed consistent with dispersive transport.     

硅中浅杂质的激光感应光电导谱

首次采用光子能量小于硅中浅受主杂质电离能的可调谐远红外激光器作为激发源,获得了硅中浅受主杂质的光电导谱.可调谐半导体远红外激光器的调谐范围为380～500cm~(-1),光子流密度约10~(18)/cm~2·sec,用双光子跃迁对光电导谱进行了解释.对于Si:Al样品,光电导谱中的双峰分别相应于2P~1和2P~2中间态的双光子共振跃迁.也观察到了双光子透明的反共振现象.     

Defects in ion-implanted crystalline silicon probed by femtosecond laser spectroscopy

Summary Radiation damage is generated in a controlled manner by MeV ion implantation of Si⁺ and He⁺ ions in c-Si and studied by ultrafast laser pulses on a subpicosecond time scale. In Si⁺-implanted samples the amorphization of the sample is achieved at sufficiently high doses, while He implants only produce a very low level of damage. Defects are investigated after implantation by measuringex situ the change of reflectivity caused by a high density of electron-hole plasma generated by femtosecond laser pulses. The plasma decay time decreases as a function of the implantation dose in both Si- and He-implanted samples, reaching a minimum value of ≈1 ps. It is observed that the saturation of the decay time is not related to the amorphization of the sample, but rather to the formation of simple defects produced during ion implantation.     

Carrier dynamics and dispersive terahertz Bloch gain in semiconductor superlattices

We have directly determined the spectral shape of the complex conductivities of Bloch oscillating electrons by using time-domain terahertz (THz)-electrooptic sampling technique and presented an experimental evidence for a dispersive Bloch gain in superlattices. This unique dispersive gain without population inversion arises from a non-classical nature of Bloch oscillations; i.e., the phase of the Bloch oscillation is shifted by π/2 from that of the semi-classical charged harmonic oscillation when driven by the same AC field. By increasing the bias electric field, the gain bandwidth reached in our particular sample.     

Polarization-sensitive correlation spectroscopy of Faraday-effect femtosecond dynamics

Femtosecond dynamics of the Faraday effect is experimentally studied in thin magnetic films by polarization-sensitive correlation spectroscopy.     

掺铜及掺铝多孔硅的光致发光及其发光机理

分别将特定杂质铜和铝引入多孔硅后，观察到了杂质铜和铝所引起的附加发光带：对于没有掺铜的多孔硅，其光致发光谱只有一个发光带；而掺过铜的多孔硅，其光致发光谱出现两个发光带，其中能量较低的发光带随主发光带而变化。在掺铝多孔硅的光致发光谱中，则出现4个与铝杂质能级有关发光带。我们认为上述与杂质有关的发光带是由截流子在杂质深能级上复合所致。     

Stimulated terahertz stokes emission of silicon crystals doped with antimony donors

Stimulated Stokes emission has been observed from silicon crystals doped by antimony donors when optically excited by radiation from a tunable infrared free electron laser. The photon energy of the emission is equal to the pump photon energy reduced by the energy of the intervalley transverse acoustic (TA) g phonon in silicon (approximately 2.92 THz). The emission frequency covers the range of 4.6-5.8 THz. The laser process occurs due to a resonant coupling of the 1s(E) and 1s(A1) donor states (separation approximately 2.97 THz) via the g-TA phonon, which conserves momentum and energy within a single impurity center.     

Probing solvation dynamics with femtosecond vibrational spectroscopy

We explain why solvent reorganization can induce both red- and blue-shifting of vibrational transitions of a particular probe molecule upon excitation to the S₁ electronic state. We observe with femtosecond vibrational spectroscopy, after hydrogen-bond cleavage dynamics, an additional blue shift of the carbonyl stretch of coumarin 102 of 7 cm^-1 when dissolved in chloroform. Received: 28 June 2000 / Published online: 7 August 2000     

Mixing of low-frequency Raman-active phonons studied by femtosecond pump–probe spectroscopy

We report an optical study of the superconducting transition in the YBa₂Cu₃O_7−x films on MgO substrate carried out by femtosecond pump–probe spectroscopy. In both the normal and the superconducting state, we have observed the transient time-resolved reflectivity caused by the oscillations associated with two A_1g metal-ion modes. Temporal interference between the modes is modified below the phase transition, which also appeared in an altered relative intensity of the modes in the Fourier transformed spectra.     

Energy-gap dynamics of superconducting NbN thin films studied by time-resolved terahertz spectroscopy

Using time-domain terahertz spectroscopy we performed direct studies of the photoinduced suppression and recovery of the superconducting gap in a conventional BCS superconductor NbN. Both processes are found to be strongly temperature and excitation density dependent. The analysis of the data with the established phenomenological Rothwarf-Taylor model enabled us to determine the bare quasiparticle recombination rate, the Cooper pair-breaking rate and the electron-phonon coupling constant, λ=1.1±0.1, which is in excellent agreement with theoretical estimates.     

飞秒脉冲烧蚀单晶硅的超快动力学

利用基于Pump-probe系统的超快时间分辨阴影图的方法,研究了空气中飞秒激光烧蚀单晶硅的动力学过程。实验采用脉宽为50 fs、平均能量密度约35 J/cm2的单脉冲激光烧蚀单晶硅,获取飞秒激光烧蚀单晶硅过程中等离子体和冲击波的形成和发展过程的时间分辨阴影图。实验结果表明：飞秒激光烧蚀单晶硅导致其表面物质喷发的过程是不连续的,分为明显的两次喷发过程。这表明飞秒激光与单晶硅作用的过程中,在不同的时间段可能由不同的机制主导,在前期可能是多光子电离为主,在后期可能是由多光子效应和雪崩效应共同作用。研究还发现,延迟时间较长时,冲击波形状发生畸变。     

Applications of time-resolved terahertz spectroscopy in ultrafast carrier dynamics

Three time-resolved terahertz (THz) spectroscopy methods (optical-pump/THz-probe spectroscopy,THzpump/THz-probe spectroscopy,and THz-pump/optical-probe spectroscopy) are reviewed.These are used to characterize ultrafast dynamics in photo-or THz-excited semiconductors,superconductors,nanomaterials,and other materials.In particular,the optical-pump/THz-probe spectroscopy is utilized to investigate carrier dynamics and the related intervalley scattering phenomena in semiconductors.The recent development of intense pulsed THz sources is expected to affect the research in nonlinear THz responses of various materials.     

Nonlinear polarizability and plasmon resonances in terahertz photoconductivity

The nonlinear polarization of an electron plasma in an incident terahertz (THz) field is examined in an iterated shielded potential approximation. It is found to exhibit resonant behavior when the incident terahertz frequency matches the plasma frequency, with concomitant resonant increase of the dielectric function. Such resonant behavior substantially reduces the effectiveness of the screened impurity scattering potentials, admitting resonant increase of conduction when the THz frequency matches the plasma frequency.