Ultrafast spin dynamics including spin-orbit interaction in semiconductors

This Letter presents a theoretical investigation of ultrafast spin-dependent carrier dynamics in semiconductors due to strong spin-orbit coupling using holes in bulk GaAs as a model system. By computing the microscopic carrier dynamics in the anisotropic hole-band structure including spin-orbit coupling, we obtain spin-relaxation times in quantitative agreement with measured hole-spin relaxation times [Phys. Rev. Lett. 89, 146601 (2002)10.1103/PhysRevLett.89.146601]. We show that different optical techniques for the measurement of hole-spin dynamics yield different results, in contrast to the case of electron-spin dynamics.     

长寿命吸收过程对超快动力学过程测量的影响

使用飞秒时间分辨抽运-探测透射光谱技术,实验研究了GaAs体材料中光激发载流子的超快弛豫动力学的波长依赖.在相同的光激发载流子浓度和抽运/探测比时,发现760 nm和780 nm两中心波长处的瞬态透射变化延迟扫描信号出现负的和振荡的信号.与模拟计算结果对比,判定该实验瞬态信号是错误的.分析探测器输出波形,发现是由于反相波形导致的,而引起反相波形的原因在于样品中存在长寿命的吸收过程.指出通过提高探测器上的抽运/探测比能够矫正反相波形,从而获得正确的瞬态透射变化动力学.提高探测器上的抽运/探测比与目前的应尽量减小抽运光对探测器的散射贡献的观点是对立的.文章的研究结果对应用抽运-探测时间分辨光谱技术正确地测量超快瞬态动力学过程具有重要的参考价值. 关键词： 时间分辨抽运-探测透射光谱 饱和吸收 吸收增强 GaAs体材料     

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.     

自旋极化度对GaAs量子阱中吸收饱和效应与载流子复合动力学的影响研究

本文研究了(001) GaAs量子阱薄膜中重空穴激子近共振抽运-探测的载流子自旋弛豫动力学, 发现载流子的自旋极化对传统的线偏振光吸收饱和效应和载流子复合动力学都有影响. 进一步的抽运流依赖的自旋弛豫和复合动力学研究表明, 自旋极化对线偏振光的吸收饱和效应的影响随抽运流降低而变弱. 在低激发流时, 自旋极化对线偏振吸收饱和效应的影响才可忽略. 然而, 又显现出自旋极化对复合动力学的影响. 分析表明复合动力学的自旋极化依赖性起源于重空穴激子形成浓度的自旋极化依赖性. 复合动力学的自旋极化依赖性表明自旋弛豫时间计算中所涉及的复合时间应该使用自旋极化载流子的复合时间. 基于二维质量作用定律的激子浓度计算表明, 库仑屏蔽效应对激子形成的影响在较低激发载流子浓度下可以忽略.     

Ultrafast carrier dynamics of Cu2O thin film induced by two-photon excitation

Cuprous oxide (Cu₂O) has attracted plenty of attention for potential nonlinear photonic applications due to its superior third-order nonlinear optical property such as two-photon absorption. In this paper, we investigated the two-photon excitation induced carrier dynamics of a Cu₂O thin film prepared by radio-frequency magnetron sputtering, using the femtosecond transient absorption experiments. Biexponential dynamics including an ultrafast carrier scattering (< 1 ps) followed by a carrier recombination (> 50 ps) were observed. The time constant of carrier scattering under two-photon excitation is larger than that under one-photon excitation, due to the different transition selection rules and smaller absorption coefficient of the two-photon excitation.     

Control of condensation and evaporation of electron–hole liquid in diamond by femtosecond laser pulses

We demonstrate the ultrafast control of condensation and evaporation of an electron–hole liquid in diamond prepared by chemical vapor deposition. The electron–hole liquid dynamics was measured using a three‐pulse pump and probe experiment. The transient transmission changes in the presence of electron–hole drops were assigned to the increase in the Drude scattering rate in the model of free carrier absorption due to a high carrier density. The fast (～1 ps) liquid evaporation was induced by infrared femtosecond pulses and its dynamics was investigated under different photon energies and fluences. The value of the electron–hole liquid binding energy per electron–hole pair 80 ± 40 meV determined from the measured transient transmission signal agrees well with previously published values. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Picosecond timescale carrier dynamics of InAs quantum dots: The role of a continuum background

We have investigated the ultrafast carrier dynamics in Molecular Beam Epitaxy (MBE)-grown InAs/InGaAs/GaAs quantum dots emitting at 1.3 μm by means of time resolved photoluminescence upconversion measurements with a time resolution of about 200 fs. The detection energies scan the spectral region from the energy of the quantum dot excitonic transition up to the barrier layer absorption edge. We found, under high excitation intensity, that the intrinsic electronic states are populated mainly by carriers directly captured from the barrier.     

Probing dopant incorporation in InAs/GaAs QDIPs by polarization-dependent Fourier transform infrared spectroscopy

In order to improve spectral response tunability of quantum-dot infrared photodetectors (QDIPs), it is critical to understand how dopants are incorporated into quantum dots (QDs). In this letter, polarization-dependent Fourier transform infrared spectroscopy is used to measure intraband absorption in InAs/GaAs QDs. Through the investigation of device heterostructures with varying modulation-doped carrier concentrations, we have correlated the charge filling process of energy levels in high-density QD ensembles with IR absorbance spectra. In addition, we have observed the IR signature of a transition originating in deep-level defect centers arising from Si-doped GaAs.     

双光子激发ZnSe自由载流子超快动力学研究

采用Z扫描和抽运-探测实验技术, 在波长为532 nm、脉冲宽度为41 fs的条件下测得ZnSe晶体的双光子吸收系数, 并获得了不同激发光强下的自由载流子吸收截面、电子-空穴带间复合时间和电子-声子耦合时间. 研究发现, 随着激发光强的增大, 自由载流子吸收截面减小, 复合时间变短. 当激发光强增大导致载流子浓度大于10¹⁸ cm^-3时, 抽运-探测信号出现明显改变, 原因归结为强光场激发导致样品在短时间内带隙变窄和电子-空穴等离子体的形成.     

Optical pump-probe scanning tunneling microscopy for probing ultrafast dynamics on the nanoscale

The development of a method for exploring the ultrafast transient dynamics in small organized structures with high spatial resolution is expected to be a basis for further advances in current science and technology. Recently, we have developed a new microscopy technique by combining scanning tunneling microscopy (STM) with ultrashort-pulse laser technology, which enables the visualization of ultrafast carrier dynamics even on the single-atomic level. A nonequilibrium carrier distribution is generated using ultrashort laser pulses and its relaxation processes are probed by STM using the optical pump-probe method realized in STM by the pulse-picking technique. In this paper, the fundamentals of the new microscopy technique are overviewed.     

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.     

Modification of carrier dynamics in CdSe nanocrystals by excess Cd in deposition bath

Photoexcited carrier dynamics in thin CdSe nanocrystalline films prepared by chemical bath deposition are strongly dependent on the deposition parameters. In this paper, we show how an increase in concentration of CdSO₄ in deposition bath affects the photoexcited carrier dynamics in nanocrystals. We used ultrafast absorption and photoluminescence laser spectroscopy to compare the carrier dynamics in samples prepared with and without increased CdSO₄ concentration. We have found that in the Cd-rich samples the spectral dependences of both photoluminescence and absorption dynamics are considerably less pronounced and the dynamics are slower. The observed effects were explained by the suppression of surface mediated carrier relaxation due to the passivation of individual nanocrystals by cadmium hydroxide and/or by sulphatocomplexes of Cd²⁺.     

Generation of THz transients by photoexcited single-crystal GaAs meso-structures

We report a sub-picosecond photoresponse and THz transient generation of GaAs single-crystal mesoscopic platelets excited by femtosecond optical pulses. Our structures were fabricated by a top-down technique, by patterning an epitaxial, 500-nm-thick GaAs film grown on top of an AlAs sacrificial layer and then transferring the resulting etched away 10 × 20-μm² platelets onto an MgO substrate using a micropipette. The freestanding GaAs devices, incorporated into an Au coplanar strip line, exhibited extremely low dark currents and ~0.4 % detection efficiency at 10 V bias. The all-optical, pump–probe carrier dynamics analysis showed that, for 800-nm-wavelength excitation, the intrinsic relaxation of photocarriers featured a 310-fs-wide transient with a 290 fs fall time. We have also carried out a femtosecond, time-resolved electro-optic characterization of our devices and recorded along the transmission line the electrical transients as short as ~600 fs, when the platelet was excited by a train of 100-fs-wide, 800-nm-wavelength optical laser pulses. The platelets have been also demonstrated to be very efficient generators of free-space propagating THz transients with the spectral bandwidth exceeding 2 THz. The presented performance of the epitaxial, freestanding GaAs meso-structured photodevices makes them uniquely suitable for THz-frequency optoelectronic applications, ranging from ultrafast photodetectors to THz-bandwidth optical-to-electrical transducers and photomixers.     

Fast emission dynamics in droplet epitaxy GaAs ring-disk nanostructures integrated on Si

The emission dynamics in GaAs/AlGaAs coupled ring-disk (CRD) quantum structures fabricated on silicon substrates is presented. The CRD structures are self-assembled via droplet epitaxy, a growth technique which, due to its low thermal budget, is compatible with the monolithic integration of III-V devices on Si based electronic circuits. Continuous wave, time resolved photoluminescence and theoretical calculations in the effective mass approximations are presented for the assessment of the electronic and carrier properties of the CRDs. The CRDs show a fast carrier dynamics which is expected to be suitable for ultrafast optical switching applications integrated on silicon.     

Effects of two-photon absorption on carrier dynamics in quantum-dot optical amplifiers

We investigate ultrafast gain-dynamics in a quantum-dot (QD) semiconductor optical amplifier where two-photon absorption (TPA) occurs in the bulk region of the waveguide during propagation of a sub-picosecond optical pulse, and gain bleaching occurs in the QD active region. Our calculation reveals that TPA provides optical pumping which is quasi-synchronous in time and space with QD carrier depletion, leading to enhancement of carrier capture into the QDs on an ultrafast timescale. The TPA-induced optical pumping, which is qualitatively different from electrical pumping, changes qualitatively carrier recovery dynamics, reducing pattern effects when a train of optical pulses is injected at the ultra-high speed. PACS 85.60.Bt; 42.79.-e; 07.60.-j     

Time‐resolved ultrafast carrier dynamics in as‐grown nanocrystalline silicon films: the effect of film thickness and grain boundaries

In this letter, we have studied transient photoinduced absorption in as‐grown nanocrystalline silicon films with thickness varied from 5 to 30 nm. Effects of quantum confinement (QC) in z ‐direction and grain boundary distortions alter the carrier dynamics of these films considerably. Based on the determination of critical points in the first Brillouin zone of the band structure of materials, we have time‐resolved the relaxation times of surface‐related states and indirect valleys. When decreasing the film thickness down to the QC limit (∼10 nm) new ultrafast relaxation mechanisms start to play a dominant role in carrier dynamics due to the topological disordering of these ultrathin films. These relaxation mechanisms seem to be related with the traping/de‐traping of the excited carriers prior to recombination. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Ultrafast dynamics of carrier-induced absorption changes in highly-excited CdSe nanocrystals

We use femtosecond time-resolved transmission spectroscopy to study the density and time dependence of transient absorption changes of CdSe nanocrystals. Our data show pronounced absorption saturation up to complete bleaching of the lowest optical transition. At high carrier density the nonlinear spectra show several peaks that can be related to the two lowest quantized electron states. Thetime dependence of the carrier-induced absorption changes indicates an ultrafast relaxation process within the strongly broadened absorption lines.     

利用可见-近红外-中红外超快光谱揭示离子交换法制备的少层MoS2中缺陷介导的载流子动力学

本文结合可见-近红外-中红外瞬态吸收光谱技术对离子交换法制备的少层MoS₂中缺陷介导的载流子动力学进行了详细的解析. 在近红外瞬态吸收光谱中观察到的宽带漂白信号表明少层MoS₂纳米片带隙中分布着大量的缺陷态. 实验结果明确揭示了载流子被缺陷态的快速捕获以及进一步的复合过程，证明带隙中的缺陷态对MoS₂光生载流子动力学过程起着至关重要的作用. 在中红外瞬态吸收光谱中观察到的正信号到负信号的转变进一步证实了在导带下小于0.24 eV处存在被载流子占据的缺陷态. 这些在少层MoS₂纳米片中存在的缺陷态可以作为有效的载流子捕获中心来辅助光生载流子在皮秒时间尺度内完成非辐射复合过程.     

Ultrafast softening in InMnAs

We have used two-color time-resolved magneto-optical Kerr effect spectroscopy to manipulate and detect dynamic processes of spin/magnetic order in a ferromagnetic semiconductor InMnAs. We observed ultrafast photo-induced “softening” (i.e., transient decrease of coercivity) due to spin-polarized transient carriers. This transient softening persists only during the carrier lifetime (2 ps) and returns to its original value as soon as the carriers recombine to disappear. Our data clearly demonstrates that magnetic properties, e.g., coercivity, can be strongly and reversibly modified in an ultrafast manner. We attribute the origin of this unusual phenomenon to carrier-mediated ferromagnetic exchange interactions between Mn ions. We discuss the dependence of data on the pump polarization, pump intensity, and sample temperature. Our observation opens up new possibilities for ultrafast optical manipulation of ferromagnetic order as well as providing a new avenue for studying the dynamics of long-range collective order in strongly correlated many-body systems.     

Properties of liquid silicon observed by time-resolved x-ray absorption spectroscopy

Time-resolved x-ray spectroscopy at the Si L edges is used to probe the electronic structure of an amorphous Si foil as it melts following absorption of an ultrafast laser pulse. Picosecond temporal resolution allows observation of the transient liquid phase before vaporization and before the liquid breaks up into droplets. The melting causes changes in the spectrum that match predictions of molecular dynamics and ab initio x-ray absorption codes.