Electrical control of dynamic spin splitting induced by exchange interaction as revealed by time-resolved Kerr rotation in a degenerate spin-polarized electron gas

Manipulation of the spin degree of freedom has been demonstrated in a spin-polarized electron plasma in a heterostructure by using exchange-interaction-induced dynamic spin splitting rather than the Rashba and Dresselhaus types, as revealed by time-resolved Kerr rotation. The measured spin splitting increases from 0.256 meV to 0.559 meV as the bias varies from -0.3 V to -0.6 V. Both the sign switch of the Kerr signal and the phase reversal of Larmor precessions have been observed with biases, which all fit into the framework of exchange-interaction-induced spin splitting. The electrical control of it may provide a new effective scheme for manipulating spin-selected transport in spin FET-like devices. Copyright (C) EPLA, 2008.

Spin dynamics in the second subband of a quasi-two-dimensional system studied in a single-barrier heterostructure by time-resolved Kerr rotation

Spin dynamics in the first and second subbands have been examined simultaneously by time resolved Kerr rotation in a single-barrier heterostructure of a 500 nm thick GaAs absorption layer. By scanning the wavelengths of the probe and pump beams towards the short wavelength in the zero magnetic field, the spin coherent time T-2(1)* in the 1st subband E-1 decreases in accordance with the D'yakonov-Perel' (DP) spin decoherence mechanism. Meanwhile, the spin coherence time T-2(2)* in the 2nd subband E-2 remains very low at wavelengths longer than 810 nm, and then is dramatically enhanced afterwards. At 803 nm, T-2(2)* (450 ps) becomes ten times longer than T-2(1)* (50 ps). A new feature has been discovered at the wavelength of 811nm under the bias of -0.3V (807nm under the bias of -0.6V) that the spin coherence times (T-2(1)* and T-2(2)*) and the effective g* factors (vertical bar g*(E-1)vertical bar and vertical bar g*(E-2)vertical bar) all display a sudden change, presumably due to the "resonant" spin exchange coupling between two spin opposite bands. Copyright (C) EPLA, 2008.

Direct observation of coherent spin transfer processes in an InGaAs/GaAs quantum well via two-color time-resolved Kerr rotation measurements

A two-color time-resolved Kerr rotation spectroscopy system was built, with a femtosecond Ti:sapphire laser and a photonic crystal fiber, to study coherent spin transfer processes in an InGaAs/GaAs quantum well sample. The femtosecond Ti:sapphire laser plays two roles: besides providing a pump beam with a tunable wavelength, it also excites the photonic crystal fiber to generate supercontinuum light ranging from 500 nm to 1600 nm, from which a probe beam with a desirable wavelength is selected with a suitable interference filter. With such a system, we studied spin transfer processes between two semiconductors of different gaps in an InGaAs/GaAs quantum well sample. We found that electron spins generated in the GaAs barrier were transferred coherently into the InGaAs quantum well. A model based on rate equations and Bloch-Torrey equations is used to describe the coherent spin transfer processes quantitatively. With this model, we obtain an effective electron spin accumulation time of 21 ps in the InGaAs quantum well.

Doped polycrystalline 3C-SiC films deposited by LPCVD for radio-frequency MEMS applications

Polycrystalline 3C-SiC films are deposited on SiO2 coated Si substrates by low pressure chemical vapour deposition (LPCVD) with C3H8 and SiH4 as precursors. Controlled nitrogen doping is performed by adding NH3 during SiC growth to obtain the low resistivity 3C-SiC films. X-ray diffraction (XRD) patterns indicate that the deposited films are highly textured (111) orientation. The surface morphology and roughness are determined by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The surface features are spherulitic texture with average grain size of 100 nm, and the rms roughness is 20nm (AFM 5 x 5 mu m images). Polycrystalline 3C-SiC films with highly orientational texture and good surface morphology deposited on SiO2 coated Si substrates could be used to fabricate rf microelectromechanical systems (MEMS) devices such as SiC based filters.

High-frequency characterization of packaging network in to-can photodiode modules

A simple method for analyzing the effects of TO packaging network on the high-frequency response of photodiode modules is presented. This method is established based on the relations of the scattering parameters of the packaging network, photodiode chip, and module. It is shown that the results obtained by this method agree well with those obtained by the conventional comparison method. The proposed method is much more convenient since only the electrical domain measurements are required. (C) 2008 Wiley Periodicals, Inc.

High rate deposition of microcrystalline silicon films by high-pressure radio frequency plasma enhanced chemical vapor deposition (PECVD)

Hydrogenated microcrystalline silicon (mu c-Si:H) thin films were prepared by high-pressure radio-frequency (13.56 MHz) plasma enhanced chemical vapor deposition (rf-PECVD) with a screened plasma. The deposition rate and crystallinity varying with the deposition pressure, rf power, hydrogen dilution ratio and electrodes distance were systematically studied. By optimizing the deposition parameters the device quality mu c-Si:H films have been achieved with a high deposition rate of 7.8 angstrom/s at a high pressure. The V-oc of 560 mV and the FF of 0.70 have been achieved for a single-junction mu c-Si:H p-i-n solar cell at a deposition rate of 7.8 angstrom/s.

Effects of disk rotation rate on the growth of ZnO films by low-pressure metal-organic chemical vapor deposition

ZnO films were grown at low pressure in a vertical metal-organic vapor deposition (MOCVD) reactor with a rotating disk. The structural and morphological properties of the ZnO films grown at different disk rotation rate (DRR) were investigated. The growth rate increases with the increase of DRR. The ZnO film grown at the DRR of 450 revolutions per minute (rpm) has the lowest X-ray rocking curve full width at half maximum and shows the best crystalline quality and morphology. In addition, the crystalline quality and morphology are improved as the DRR increased but both are degraded when the DRR is higher than 450 rpm. These results can help improve in understanding the rotation effects on the ZnO films grown by MOCVD. (C) 2007 Elsevier B.V. All rights reserved.

FREQUENCY RESPONSE OF FIBER RING RESONATOR AND ITS APPLICATION IN DFB LASER WAVELENGTH VARIATION MEASUREMENT

Frequency response of a fiber ring resonator (FRR) composed of an ordinary optical coupler and a segment of optical fiber is theoretically and experimentally investigated. The frequency response equation based oil small-signal modulation is derived and studied in detail. It is shown that the shape of the frequency response curve is very sensitive to the wavelength; as a result, the FRR can be applied to measure the wavelength of a lightwave source with high resolution. With this method, we demonstrate the measurement of tiny changes of wavelength of a DFB laser. (C) 2009 Wiley Periodicals. Inc. Microwave Opt Technol Lett 51 2444-2448, 2009 Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.24608

A ridge width varied two-section index-coupled DFB self-pulsation laser with a wide continuously tunable frequency range

A 1.55 mu m InGaAsP-InP index-coupled two-section DFB self-pulsation laser (SPL) with a varied ridge width has been fabricated. A record wide self-pulsation tuning range above 450 GHz has been achieved for this index-coupled DFB SPL. Furthermore, frequency locking to an optically injected modulated signal is successfully demonstrated.

Study on Frequency Coherence Properties of Light Beams

This paper presents a new concept of frequency coherence in the frequency-time domain to describe the field correlations between two lightwaves with different frequencies. The coherence properties of the modulated beams from lightwave sources with different spectral widths and the modes of Fabry-Wrot (FP) laser are investigated. It is shown that the lightwave and its corresponding sidebands produced by the optical intensity modulation are perfectly coherent. The measured linewidth of the beat signal is narrow and almost identical no matter how wide the spectral width of the beam is. The frequency spacing of the adjacent FP modes is beyond the operation frequency range of the measurement instruments. In our experiment, optical heterodyne technique is used to investigate the frequency coherence of the modes of FP laser by means of the frequency shift induced by the optical intensity modulation. Experiments show that the FP modes are partially coherent and the mode spacing is relatively fixed even when the wavelength changes with ambient temperature, bias current and other factors. Therefore, it is possible to generate stable and narrow-linewidth signals at frequencies corresponding to several mode intervals of the laser.

Characteristic analysis of the optical delay in frequency response of resonant cavity enhanced (RCE) photodetectors

With consideration of the modulation frequency of the input lightwave itself, we present a new model to calculate the quantum efficiency of RCE p-i-n photodetectors (PD) by superimposition of multiple reflected lightwaves. For the first time, the optical delay, another important factor limiting the electrical bandwidth of RCE p-i-n PD excluding the transit time of the carriers and RCd response of the photodetector, is analyzed and discussed in detail. The optical delay dominates the bandwidth of RCE p-i-n PD when its active layer is thinner than several 10 nm. These three limiting factors must be considered exactly for design of ultra-high-speed RCE p-i-n PD.

Effect of Ka-band microwave on the spin dynamics of electrons in a GaAs/Al0.35Ga0.65As heterostructure

We report experimental results of the effect of Ka-band microwave on the spin dynamics of electrons in a two-dimensional electron system (2DES) in a GaAs/Al0.35Ga0.65As heterostructure via time-resolved Kerr rotation measurements. While the microwave reduces the transverse spin lifetime of electrons in the bulk GaAs, it significantly increases that in the 2DES, from 745 to 1213 ps, when its frequency is close to the Zeeman splitting of the electrons in the magnetic field. Such a microwave-enhanced spin lifetime is ascribed to the microwave-induced electron scattering which leads to a "motional narrowing" of spins via D'yakonov-Perel' mechanism.

Development of Swept Frequency Method for Measuring Frequency Response of Photodetectors Based on Harmonic Analysis

We develop a swept frequency method for measuring the frequency response of photodetectors; (PDs) based on harmonic analysis. In this technique, a lightwave from a laser source is modulated by a radio-frequency (RF) signal via a Mach-Zehnder LiNbO3 modulator, and detected by a PD under test. The measured second-order harmonic of the RF signal contains information of the frequency responses and nonlinearities of the RF source, modulator, and PD. The frequency response of the PD alone is obtained by deducting the known frequency responses and nonlinearities of the RF source and modulator. Compared with the conventional swept frequency method, the measurement frequency range can be doubled using the proposed method. Experiment results show a good agreement between the measured results and those obtained using other techniques.

Low-frequency noise properties of GaN Schottky barriers deposited on intermediate temperature buffer layers

Metal-semiconductor-metal (MSM) structures were fabricated by RF-plasma-assisted MBE using different buffer layer structures. One type of buffer structure consists of an AlN high-temperature buffer layer (HTBL) and a GaN intermediate temperature buffer layer (ITBL), another buffer structure consists of just a single A IN HTBL. Systematic measurements in the flicker noise and deep level transient Fourier spectroscopy (DLTFS) measurements were used to characterize the defect properties in the films. Both the noise and DLTFS measurements indicate improved properties for devices fabricated with the use of ITBL and is attributed to the relaxation of residue strain in the epitaxial layer during growth process. (C) 2003 Elsevier Ltd. All rights reserved.

Extraction of intrinsic frequency response of p-i-n photodiodes

This letter presents a new method for extracting the intrinsic frequency response of a p-i-n photodiode (PD) from the measured frequency response of the PD at different bias voltages. This method is much simpler than the conventional calibration method, since only the measured scattering parameters are required, and there is no need to calibrate the test fixtures and the lightwave source. Experiment shows that the proposed method is as accurate as the calibration method.