Theory of ultrafast optical manipulation of electron spins in quantum wells

Based on a multiparticle-state stimulated Raman adiabatic passage approach, a comprehensive theoretical study of the ultrafast optical manipulation of electron spins in quantum wells is presented. In addition to corroborating experimental findings [Gupta , Science 292, 2458 (2001)], we improve the expression for the optical-pulse-induced effective magnetic field, in comparison with the one obtained via the conventional single-particle ac Stark shift. Further study of the effect of hole-spin relaxation reveals that, while the coherent optical manipulation of electron spin in undoped quantum wells would deteriorate in the presence of relatively fast hole-spin relaxation, the coherent control in doped systems can be quite robust against decoherence. The implications of the present results on quantum dots will also be discussed. (c) 2005 American Institute of Physics.

Fabrication of thermo-optic switch in silicon-on-insulator

Silicon-on-insulator technology has been used to fabricate 2 x 2 thermo-optic switches. The switch shows crosstalk of -23.4 dB and extinction ratio of 18.1 dB in the bar-state. The switching speed is less than 30 mus and the power consumption is about 420 mW The measured excess loss is 1.8 dB. These merits make the switch more attractive for applications in wavelength division multiplexing.

Atomic hydrogen induced step bunching and fabrication of quantum wire arrays on GaAs (311)A substrate by molecular beam epitaxy

Atomic hydrogen assisted molecular beam epitaxy (MBE) is a novel type of epitaxial growth of nanostructures. The GaAs (311)A surface naturally forms one-dimensional step arrays by step bunching along the direction of (-233) and the space period is around 40nm. The step arrays extend over several mum without displacement. The InGaAs quantum wire arrays are grown on the step arrays as the basis. Our results may prompt further development of more uniform quantum wire and quantum dot arrays.

Temperature and excitation power dependence of the optical properties of InAs self-assembled quantum dots grown between two Al0.5Ga0.5As confining layers

We have investigated the temperature and excitation power dependence of photoluminescence properties of InAs self-assembled quantum dots grown between two Al0.5Ga0.5As quantum wells. The temperature evolutions of the lower-and higher-energy transition in the photoluminescence spectra have been observed. The striking result is that a higher-energy peak appears at 105 K and its relative intensity increases with temperature in the 105-291 K range. We demonstrate that the higher-energy peak corresponds to the excited-state transition involving the bound-electron state of quantum dots and the two-dimensional hole continuum of wetting layer. At higher temperature, the carrier transition associated with the wetting layer dominates the photoluminescence spectra. A thermalization model is given to explain the process of hole thermal transfer between wetting layer and quantum dots. (C) 2000 Published by Elsevier Science B.V. All rights reserved.

Temperature dependence of the optical properties of InAs/GaAs self-organized quantum dots with bimodal size distribution

In this work we report the optical and microscopic properties of self-organized InAs/GaAs quantum dots grown by molecular beam epitaxy on (1 0 0) oriented GaAs substrates. A distinctive double-peak feature of the PL spectra from quantum dots has been observed, and a bimodal distribution of dot sizes has also been confirmed by scanning tunneling microscopy (STM) image for uncapped sample. The power-dependent photoluminescence (PL) study demonstrates that the distinctive PL emission peaks are associated with the ground-state emission of islands in different size branches. The temperature-dependent PL study shows that the PL quenching temperature for different dot families is different. It is shown that the coupling between quantum dots plays a key role in unusual temperature dependence of QD photoluminescence. In addition, we have tuned the emission wavelength of InAs QDs to 1.3 mu m at room temperature. (C) 2000 Elsevier Science B.V. All rights reserved.

Fabrication of InGaAs quantum dots with an underlying InGaAlAs layer on GaAs(100) and high index substrates by molecular beam epitaxy

In this paper, InGaAs quantum dots with an adjusting InGaAlAs layer underneath are grown on (n 1 1)A/B (n = 2-5) and the reference (1 0 0) substrates by molecular beam epitaxy. Small and dense InGaAs quantum dots are formed on (1 0 0) and (n 1 1)B substrates. A comparative study by atomic force microscopy shows that the alignment and uniformity for InGaAs quantum dots are greatly improved on(5 1 1)B but deteriorated on (3 1 1)B surface, demonstrating the great influence of the buried InGaAlAs layer. There is an increase in photoluminescence intensity and a decrease in the full-width at half-maximum when n varies from 2 to 5. Quantum dots formed on (3 1 1)A and (5 1 1)A surfaces are large and random in distribution, and no emission from these dots can be detected. (C) 1999 Elsevier Science B.V. All rights reserved.

Fabrication of luminescent Ge nanocrystals started from unlayered hydrogenated amorphous SiGe films or hydrogenated amorphous Si hydrogenated amorphous Ge multilayers

Nanocrystalline Ge embedded in SiOx matrix is fabricated by oxidizing hydrogenated amorphous Sice alloys or hydrogenated amorphous Si/hydrogenated amorphous Ge multilayers. The structures before and after oxidation are systematically investigated. Visible light emission was observed from both samples. The luminescence peak is located at 2.2 eV which is independent of the starting materials. Compared to the luminescence from unlayered samples, the photoluminescence spectrum from multilayered samples has a narrower band width, which can be attributed to the uniform size distribution. The light emission origin is also discussed briefly and a mechanism different from the quantum size effect is suggested.

Stability improvement of selective oxidation during the fabrication of InGaAs/GaAs vertical cavity surface emitting laser

The effects of the carrier gas flow and water temperature on the oxidation rate for different reaction temperatures were investigated. The optimum conditions for stable oxidation were obtained. Two mechanisms of the oxidation process are revealed. One is the flow-controlling process, which is unstable. The other is the temperature-controlling process, which is stable. The stable region decreases for higher reaction temperatures. The simulation results for the stable oxidation region are also given. With optimum oxidation conditions, the stability and precision of the oxidation can be dramatically improved.

Fabrication of GaN epitaxial films on Al2O3/Si (001) substrates

Single crystal GaN films of hexagonal modification have been fabricated on Al2O3/Si (001) substrates via a low pressure metalorganic chemical deposition (LP-MOCVD) method. The full width at half-maximum of (0002) X-ray diffraction peak for the GaN film 1.1 mu m thick was 72 arcmin. and the mosaic structure of the film was the main cause of broadening to the X-ray diffraction peak. Al room temperature, the photoluminescence (PL) spectrum of GaN exhibited near band edge emission peaking at 365 nm.

Fabrication of Nano-structured VOx Film by Low Temperature Ion Beam Sputtering and Reductive Annealing

VOx thin films have been fabricated by low temperature ion beam sputtering and post reductive annealing process. Semiconductor-metal phase transition is observed for the film annealed at 400 degrees C for 2 hours. The film also shows a polycrystal structure with grain size from 50nm to 150nm. The VOx thin films fabricated by this process have a TCR up to -2.7% at room temperature. Our results indicate a promising fabrication method of the nano-structured VOx film with relatively high TCR and semiconductor-metal phase transition.

Measurement and analysis of microwave frequency response of semiconductor optical amplifiers - art. no. 682406

The measurement and analysis of the microwave frequency response of semiconductor optical amplifiers (SOAs) are proposed in this paper. The response is measured using a vector network analyzer. Then with the direct-subtracting method, which is based on the definition of scattering parameters of optoelectronic devices, the responses of both the optical signal source and the photodetector are eliminated, and the response of only the SOA is extracted. Some characteristics of the responses can be observed: the responses are quasi-highpass; the gain increases with the bias current; and the response becomes more gradient while the bias current is increasing. The multisectional model of an SOA is then used to analyze the response theoretically. By deducing from the carrier rate equation of one section under the steady state and the small-signal state, the expression of the frequency response is obtained. Then by iterating the expression, the response of the whole SOA is simulated. The simulated results are in good agreement with the measured on the three main characteristics, which are also explained by the deduced results. This proves the validity of the theoretical analysis.

Fabrication of improved FD SOIMOSFETs for suppressing edge effect

FD SOI MOSFETs with MESA and Irradiated FD SOI MOSFETs with LOCOS isolation usually show the edge effect, that is, the leakage current called hump is generated in the subthreshold region. According to different reasons for generating the edge effect, rounded corner process and BTS structure are applied to improve device performance. The results indicate that the above two methods are effective to reduce the edge effect and qualified devices are fabricated successfully.

Simulation and fabrication of the SiC-based clamped-clamped filter

In this paper, the SiC-based clamped-clamped filter was designed and fabricated. The filter was composed of two clamped-clamped beam micromechanical resonators coupled by a spring coupling beam. Structural geometries, including the length and width of the resonator beam and coupling beam, were optimized by simulation for high frequency and high Q, under the material properties of SiC. The vibrating modes for the designed filter structure were analyzed by finite element analysis (FEA) method. For the optimized structure, the geometries of resonator beams and coupling beams, as well as the coupling position, the SiC-based clamped-clamped filter was fabricated by surface micromaching technology.

Design and Fabrication of a Photonic Crystal Channel Drop Filter Based on an Asymmetric Silicon-on-Insulator Slab

We report on the design and fabrication of a photonic crystal (PC) channel drop filter based on an asymmetric silicon-on-insulator (SOI) slab. The filter is composed of two symmetric stick-shape micro-cavities between two single-line-defect (W1) waveguides in a triangular lattice, and the phase matching condition for the filter to improve the drop efficiency is satisfied by modifying the positions and radii of the air holes around the micro-cavities. A sample is then fabricated by using electron beam lithography (EBL) and inductively coupled plasma (ICP) etching processes. The measured 0 factor of the filter is about 1140, and the drop efficiency is estimated to be 73% +/- 5% by fitting the transmission spectrum.

The application of parallel optical transmitter and receiver modules in communication subsystems

A new 12 channels parallel optical transmitter module in which a Vertical Cavity Surface Emitting Laser (VCSEL) has been selected as the optical source is capable of transmitting 37.5Gbps date over hundreds meters. A new 12 channels parallel optical receiver module in which a GaAs PIN (p-intrinsic-n-type) array has been selected as the optical receiver unit is capable of responding to 30Gbps date. A transmission system based on a 12 channels parallel optical transmitter module and a 12 channels parallel optical receiver module can be used as a 10Gbps STM-64 or an OC-192 optical transponder. The parallel optical modules and the parallel optical transmission system have passed the test in laboratory.