Dynamics of photo-induced terahertz optical activity in metal chiral gratings

We investigated the dynamics of photo-induced optical activity of metal chiral gratings on an Si substrate for terahertz (THz) waves. We employed a new technique that enables optical-pump and THz-probe measurements via broadband THz spectroscopy at the microsecond time scale using a low-repetition-rate pump and a high-repetition-rate probe. We revealed that the THz optical activity decays as a result of the carrier diffusion effect because this optical activity is because of the presence of three-dimensional chiral structures of photo-carriers in the Si substrate.     

Optical-Electrical Characteristics and Carrier Dynamics of Semi-Insulation GaAs by Terahertz Spectroscopic Technique

GaAs has been widely used to fabricate a variety of optoelectronic devices by virtue of its superior performance,and it is very important to accurately measure its electrical and optical properties.In this study,a semiinsulation(SI) GaAs wafer is investigated by the terahertz(THz) non-destructive testing technology.Using an air biased coherent generation and detection THz time domain spectroscopy system,the THz time domain waveform and spectrum of SI-GaAs are obtained by the time domain spectroscopy module,and its opticalelectrical characteristics including complex refractive index,permittivity and dielectric loss angle are calculated.Its carrier lifetime is measured by the optical-pump THz-probe module,and the THz pulse induced intervalley scattering in photo-excited SI-GaAs is discussed.     

Terahertz tomography of a photo-induced carrier based on pump-probe spectroscopy using counterpropagation geometry

A novel technique for the terahertz (THz) tomography of a photo-induced carrier that is based on optical-pump THz-probe time-resolved reflection spectroscopy using counterpropagation geometry of the pump and probe pulses has been proposed. Transient reflection due to the photo-induced carrier provides information about the physical properties and spatial distribution separately. We have experimentally demonstrated this method using a silicon wafer. The obtained complex reflection can be reproduced by the exact solution of Maxwell's equations, assuming an exponential distribution of the photo-induced carrier density.     

Ultrafast control of slow light in THz electromagnetically induced transparency metasurfaces

In this paper,we experimentally demonstrate ultrafast optical control of slow light in the terahertz(THz) range by combining the electromagnetically induced transparency(EIT) metasurfaces with the cut wire made of P~+-implanted silicon with short carrier lifetime.Employing the optical-pump THz-probe spectroscopy,we observed that the device transited from a state with a slow light effect to a state without a slow light effect in an ultrafast time of 5 ps and recovered within 200 ps.A coupled oscillator model is utilized to explain the origin of controllability.The experimental results agree very well with the simulated and theoretical results.These EIT metasurfaces have the potential to be used as an ultrafast THz optical delay device.     

X-ray pump optical probe cross-correlation study of GaAs

Ultrafast dynamics in atomic, molecular and condensed-matter systems are increasingly being studied using optical-pump, X-ray probe techniques where subpicosecond laser pulses excite the system and X-rays detect changes in absorption spectra and local atomic structure(1-3). New opportunities are appearing as a result of improved synchrotron capabilities and the advent of X-ray free-electron lasers(4,5). These source improvements also allow for the reverse measurement: X-ray pump followed by optical probe. We describe here how an X-ray pump beam transforms a thin GaAs specimen from a strong absorber into a nearly transparent window in less than 100 ps, for laser photon energies just above the bandgap. We find the opposite effect-X-ray induced optical opacity-for photon energies just below the bandgap. This raises interesting questions about the ultrafast many-body response of semiconductors to X-ray absorption, and provides a new approach for an X-ray/optical cross-correlator for synchrotron and X-ray free-electron laser applications.     

Enhanced photosusceptibility near Tc for the light-induced insulator-to-metal phase transition in vanadium dioxide

We use optical-pump terahertz-probe spectroscopy to investigate the near-threshold behavior of the photoinduced insulator-to-metal (IM) transition in vanadium dioxide thin films. Upon approaching Tc a reduction in the fluence required to drive the IM transition is observed, consistent with a softening of the insulating state due to an increasing metallic volume fraction (below the percolation limit). This phase coexistence facilitates the growth of a homogeneous metallic conducting phase following superheating via photoexcitation. A simple dynamic model using Bruggeman effective medium theory describes the observed initial condition sensitivity.     

Femtosecond infrared spectroscopy of semiconductors and semiconductor nanostructures

Infrared spectroscopy on ultrafast time scales represents a powerful technique to investigate the nonequilibrium dynamics of elementary excitations in bulk and nanostructured semiconductors. In this article, recent progress in this field is reviewed. After a brief introduction into electronic excitations below the fundamental bandgap and ultrafast processes in semiconductors, infrared pulse generation and the methodology of time-resolved infrared spectroscopy are reviewed. The main part of this paper is devoted to coherent optical polarizations and nonequilibrium excitations of the electronic system in the spectral range below the fundamental band gap. The focus is on the physics of single component plasmas, i.e. electrons or holes. In particular, intraband, inter-valence and intersubband transitions are considered. Processes of phase relaxation, carrier and energy redistribution are analyzed. The potential of ultrafast infrared technology and spectroscopy for future applications is discussed in the final part.     

Time-resolved IR spectroscopy of quantum-optics in semiconductors

Time-resolved IR spectroscopy is a powerful non-destructive technique for probing electron dynamics and plasmonics in semiconductors. We present recent experiments in which intense IR laser pulses are used to induce “quantum-optical” phenomena, including gain without population inversion and slow light, in semiconductor nanostructures. The potential advantages of IR Synchrotron radiation to probe these systems are discussed.     

Microscopic structure of semiconductor surfaces

To study the initial reaction steps of hydrogen, oxygen, and water, on differently prepared single crystal surfaces of silicon, germanium/silicon alloys, indium phosphide and gallium arsenide, we used high-resolution electron energy-loss spectroscopy (HREELS) in combination with low-energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). Very recently, we started a program on the hydrogenation of III–V compound semiconductors, and on the oxidation of Si and III–V compound semiconductors, using alkali metals as a catalyst. This paper summarizes the present stage of our investigations, describing in particular aspects of the microscopic structure of differently prepared semiconductor surfaces.     

Spin noise spectroscopy in GaAs

We observe the noise spectrum of electron spins in bulk GaAs by Faraday-rotation noise spectroscopy. The experimental technique enables the undisturbed measurement of the electron-spin dynamics in semiconductors. We measure exemplarily the electron-spin relaxation time and the electron Landé g factor in -doped GaAs at low temperatures and find good agreement of the measured noise spectrum with a theory based on Poisson distribution probability.     

Few-cycle THz spectroscopy of nanostructures

Optically excited plasma oscillations in n-doped GaAs epilayers emit intense THz pulses. Using a THz-pump and THz-probe technique we observe the response of the intersubband polarization in semiconductor quantum structures. THz Cross-correlation measurements of modulation doped semiconductor quantum structures allow to determine the absorption, the dispersion, and the dephasing times of the quantized electrons.     

Pseudogaps in incommensurate charge density waves and one-dimensional semiconductors

We consider pseudogap effects for electrons interacting with gapless modes. We study generic 1D semiconductors with acoustic phonons and incommensurate charge density waves. We calculate the subgap absorption as it can be observed by means of photoelectron or tunneling spectroscopy. Within the formalism of functional integration and adiabatic approximation, the probabilities are described by nonlinear configurations of an instanton type. Particularities of both cases are determined by the topological nature of stationary excited states (acoustic polarons or amplitude solitons) and by the presence of gapless phonons that change the usual dynamics to the quantum dissipation regime. Below the free-particle edge, the pseudogap starts with an exponential (stretched exponential for gapful phonons) decrease of the transition rates. Deeply within the pseudogap, they are dominated by a power law, in contrast to a nearly exponential law for gapful modes.     

Photoreflectance system based on vacuum ultraviolet laser at 177.3 nm

Photoreflectance (PR) spectroscopy is a powerful and non-destructive experimental technique to explore interband transitions of semiconductors. In most PR systems, the photon energy of the pumping beam is usually chosen to be higher than the bandgap energy of the sample. To the best of our knowledge, the highest energy of pumping laser in reported PR systems is 5.08 eV (244 nm), not yet in the vacuum ultraviolet (VUV) region. In this work, we report the design and construction of a PR system pumped by VUV laser of 7.0 eV (177.3 nm). At the same time, dual-modulated technique is applied and a dual channel lock-in-amplifier is integrated into the system for efficient PR measurement. The system's performance is verified by the PR spectroscopy measurement of well-studied semiconductors, which testifies its ability to probe critical-point energies of the electronic band in semiconductors from ultraviolet to near-infrared spectral region.     

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.     

Submillimeter Reflection Spectroscopy of Narrow-Band Semiconductors

The information possibilities of the submillimeter reflection spectroscopy of semiconductors and semiconductor structures are discussed. Models are presented to describe the reflection spectra of semiconductors and semiconductor structures. The possibility of obtaining information on various parameters and characteristics of semiconductors and semiconductor structures by fitting theoretical spectra to experimental data is shown. An evaluation of the validity and accuracy of the information obtained is given.     

Determination of the optical constants of metals and semiconductors by combining ellipsometry with electron spectroscopy microscopy and X-ray specular reflection analysis

The optical constants n(λ) and k(λ) of metals and semiconductors can be determined by spectroellipsometry, however, their apparent values are considerably affected by the roughness and oxide overlayer thickness dof the sample. Aluminium thin film samples of high perfection and very low roughness (<1 nm) have been studied by cross-disciplinary experimental methods: X-ray specular reflection analysis for determining the structure and thickness of the natural (hydrated) oxide overlayer and roughness of the substrate; plasmon electron energy loss spectroscopy supplied d. For calibration of the d measurements a special thin film multilayer system was developed, suitable for preparing cross-sectional samples for resolution transmission electron microscopy. Knowing the roughness and d-data, the optical constants n(λ) and k(λ) of aluminium were determined by spectroellipsometry in the spectral range λ=365–633 nm. Experimental results and a nomogram are presented for evaluating n(λ) and k(λ). The dependence of the ellipsometric optical constants on roughness and d is discussed. Very good agreement of the optical constants with the corrected ellipsometric results of Blanco and the synchrotron spectroscopy data of Hagemann was found. The cross-disciplinary methods can be applied to metals and semiconductors covered with an overlayer.     

自旋极化扫描隧道显微术

自旋极化扫描隧道显微术是一种新兴的表面自旋分辨技术，文章主要介绍了自旋极化的扫描隧道显微镜和扫描隧道谱实现表面自旋分辨的原理以及在各种磁性表面研究中的应用，采用自旋极化技术的扫描隧道显微镜可以测量表面磁结构，其空间分辨可以达到原子尺度，分辨率超过其他磁显微技术，而自旋极化扫描隧道谱不但可以分辨空间精细磁畴结构，而且能研究表面态的交换劈裂，文章作者还进一步提出了利用自旋极化扫描隧道显微镜实现自旋注入的设想。     

Microscopic defect level characterization of semi-insulating compound semiconductors by TSC and PICTS: Application to the effect of hydrogen in CdTe

Thermally stimulated current (TSC) and photo-induced current transient spectroscopy (PICTS) methods have been developed for the microscopic defect characterization in semi-insulating compound semiconductors. The capabilities of these methods are demonstrated by investigating the effects of hydrogen implantation or diffusion into semi-insulating cadmium telluride.     

Terahertz Time-Domain Spectroscopy of Solids: A Review

Recent development of the terahertz time domain spectroscopy (THz-TDS) and its application to solids have been reviewed. This spectroscopy is unique in that the time-domain wave forms are measured at first and the complex optical constants are deduced directly by the Fourier transformation of them without resort to the Kramers-Kronig analysis. Various types of the THz-TDS systems are briefly described. Applications of the THz-TDS to various solids, i.e., semiconductors, superconductors, polymers, photonic crystals, and so on are also presented to demonstrate how widely this spectroscopy is applicable to characterization of solids.     

Exciton dynamics at interfaces of organic semiconductors

We review recent progress of using time-resolved two-photon photoelectron spectroscopy (2PPE) to study the energetics and dynamics of excitons at surfaces and interfaces of two prototypical organic semiconductors: C₆₀ and pentacene. For C₆₀ thin films epitaxially grown on Au(1 1 1) and Cu(1 1 1) surfaces, we observe both charge transfer and exciton states. For excitons in C₆₀, the proximity of a metal surface leads to rapid, exciton band-mediated quenching. At the surface of pentacene thin films we observe a series of charge-transfer excitons where the electron and the photohole are bound across the interface. The ability of 2PPE to measure and directly relate exciton levels to single-electron levels is illustrated.