Spin relaxation in submonolayer and monolayer InAs structures grown in a GaAs matrix

Electron-spin dynamics in InAs/GaAs heterostructures consisting of a single layer of InAs (1/3-1 monolayer) embedded in (001) and (311)A GaAs matrix was studied by means of time-resolved Kerr rotation spectroscopy. The spin-relaxation time of the submonolayer InAs samples is significantly enhanced, compared with that of the monolayer InAs sample. The electron-spin-relaxation time and the effective g factor in submonolayer samples were found to be strongly dependent on the photogenerated carrier density. The contribution from both the D'yakonov-Perel' mechanism and Bir-Aronov-Pikus mechanism are discussed to interpret the temperature dependence of spin decoherence at various carrier densities.

Formation of ferromagnetic clusters in GaAs matrix and GaAs/AlGaAs superlattice through Mn ion implantation at two different temperatures

Different submicron ferromagnets are fabricated into GaAs and GaAs/AlGaAs superlattice through ion implantation at two different temperatures followed by thermal annealing. The structural and magnetic properties of the granular film are studied by an atomic force microscope, X-ray diffraction and alternating gradient magnetometer. By analyzing the saturation magnetization M-s, remanence M-r, coercivity H-c and remanence ratio S-q, it is confirmed that both MnGa and MnAs clusters are formed in the 350degreesC-implanted samples whereas only MnAs clusters are formed in the room-temperature implanted samples. (C) 2004 Elsevier B.V. All rights reserved.

InAs nanostructure grown with different growth rate in InAlAs matrix on InP (001) substrate

InAs self-organized nanostructures in In0.52Al0.48As matrix have been grown on InP (001) substrates by molecular beam epitaxy. The morphologies of the nanostructures are found to be strongly dependent on the growth rate of the InAs layer. By increasing the growth rate from 0.005 to 0.35 ML/s, the morphology of the nanostructure changes from wire to elongated dot and then changes back to wire again. Polarized photoluminescence of the InAs quantum wires and quantum dots are performed at 77 K, which are characterized by strong optical anisotropies. (C) 2003 Elsevier B.V. All rights reserved.

Origin and evolution of photoluminescence from Si nanocrystals embedded in a SiO2 matrix

A detailed analysis of the photoluminescence (PL) from Si nanocrystals (NCs) embedded in a silicon-rich SiO2 matrix is reported. The PL spectra consist of three Gaussian bands (peaks A,B, and C), originated from the quantum confinement effect of Si NCs, the interface state effect between a Si NC and a SiO2 matrix, and the localized state transitions of amorphous Si clusters, respectively. The size and the surface chemistry of Si NCs are two major factors affecting the transition of the dominant PL origin from the quantum confinement effect to the interface state recombination. The larger the size of Si NCs and the higher the interface state density (in particular, Si = O bonds), the more beneficial for the interface state recombination process to surpass the quantum confinement process, in good agreement with Qin's prediction in Qin and Li [Phys. Rev. B 68, 85309 (2003)]. The realistic model of Si NCs embedded in a SiO2 matrix provides a firm theoretical support to explain the transition trend.

Design and fabrication of compact nonblocking 4X4 optical matrix switch on silicon-on-insulator by anisotropic chemical etching

A folding nonblocking 4 X 4 optical matrix switch in simplified-tree architecture was designed and fabricated on a silicon-on-insulator wafer. To compress chip size, switch elements (SEs) were connected by total internal reflection mirrors instead of conventional S-bends. For obtaining smooth interfaces, potassium hydroxide (KOH) anisotropic chemical etching of silicon was employed. The device has a compact size of 20 X 3.2 mm(2) and a fast response of 8 +/- 1 mu s. Power consumption of 2 x 2 SE and excess loss per mirror were 145 mW and -1.1 dB, respectively. (c) 2005 Society of Photo-Optical Instrumentation Engineers.

Rearrangeable nonblocking SOI waveguide thermooptic 4x4 switch matrix with low insertion loss and fast response

A rearrangeable nonblocking 4 x 4 thermooptic silicon-on-insulator waveguide switch matrix at 1.55-mu m integrated spot size converters is designed and fabricated for the first time. The insertion losses and polarization-dependent losses of the four channels are less than 10 and 0.8 dB, respectively. The extinction ratios are larger than 20 dB. The response times are 4.6 mu s for rising edge and 1.9 mu s for failing edge.

Rearrangeable nonblocking thermo-optic 4 x 4 switching matrix in silicon-on-insulator

A rearrangeable nonblocking thermo-optic 4 x 4 switching matrix is demonstrated. The matrix, which consists of five 2 x 2 multimode interference-based Mach-Zehnder interferometer (MMI-MZI) switch elements, is fabricated in silicon-on-insulator waveguide system. The average excess loss for the optical path experiencing 2 and 3 switch elements is 6.6 and 10.1 dB respectively. The crosstalk in the matrix is measured to be between -12 and -19 dB. The switching time of the device is less than 30 mu s.

Integrated folding 4x4 optical matrix switch with total internal reflection mirrors on SOI by anisotropic chemical etching

A folding rearrangeable nonblocking 4 x 4 optical matrix switch was designed and fabricated on silicon-on-insulator wafer. To compress chip size, switch elements (SEs) were interconnected by total internal reflection (TIR) mirrors instead of conventional S-bends. For obtaining smooth interfaces, potassium hydroxide anisotropic chemical etching of silicon was utilized to make the matrix switch for the first time. The device has a compact size of 20 x 1.6 mm(2) and a fast response of 7.5 mu s. The power consumption of each 2 x 2 SE and the average excess loss per mirror were 145 mW and -1.1 dB, respectively. Low path dependence of +/- 0.7 dB in total excess loss was obtained because of the symmetry of propagation paths in this novel matrix switch.

A 4x4 strictly nonblocking silicon-on-insulator thermo-optic switch matrix

A 4 x 4 strictly nonblocking thermo-optic switch matrix implemented with a 2 x 2 Mach-Zehnder switch unit was fabricated in silicon-on-insulator wafer. Insertion losses of the shortest and the longest path in the device are about 14.8 dB and 19.2 dB, respectively. The device presents a very low loss dependent on wavelength. For one switch unit, the power consumption needed for operation is measured to be 0.270 W-0.288 W and the switching time is about 13 +/- 1 mu s.

Fabrication of a 4x4 strictly nonblocking SOI switch matrix

A 4 x 4 strictly nonblocking thermo-optical switch matrix based on Mach-Zehnder (MZ) switching unit was designed and fabricated in silicon-on-insulator (SOI) wafer. The paired multi-mode interferometers (MMI) were used as power splitters and combiners in MZ structures. The device presents an average insertion loss of 17 dB and an average crosstalk of 16.5 dB. The power consumption needed for operation is reduced to 0.288 W by adding isolating trenches. The switching time of the device is about 15 mu s, which is much faster than that of silica-based switches. (C) 2005 Elsevier B.V. All rights reserved.

Research progresses of SOI optical waveguide devices and integrated optical switch matrix

SOI (silicon-on-insulator) is a new material with a lot of important performances such as large index difference, low transmission loss. Fabrication processes for SOI based optoelectronic devices are compatible with conventional IC processes. Having the potential of OEIC monolithic integration, SOI based optoelectronic devices have shown many good characteristics and become more and more attractive recently. In this paper, the recent progresses of SOI waveguide devices in our research group are presented. By highly effective numerical simulation, the single mode conditions for SOI rib waveguides with rectangular and trapezoidal cross-section were accurately investigated. Using both chemical anisotropic wet etching and plasma dry etching techniques, SOI single mode rib waveguide, MMI coupler, VOA (variable optical attenuator), 2X2 thermal-optical switch were successfully designed and fabricated. Based on these, 4X4 and 8X8 SOI optical waveguide integrated switch matrixes are demonstrated for the first time.

Thermo-optical switch matrix based on silicon-on-insulator waveguides

A thermo-optical waveguide switch matrix is designed and fabricated on silicon-on-insulator wafer. Multi-mode interferometers are used as power splitters and combiners in a Mach-Zehnder structure. Inductively coupled plasma reactive ion etching is used to fabricate the waveguides. The rise and fall times of the switch matrix are 13 mu s and 7 mu s, respectively. Switch cells have an average switching power consumption of 340 mW.

Novel folding large-scale optical switch matrix with total internal reflection mirrors on silicon-on-insulator by anisotropy chemical etching

A compact optical switch matrix was designed, in which light circuits were folded by total internal reflective (TIR) mirrors. Two key elements, 2 x 2 switch and TIR mirror, have been fabricated on silicon-on-insulator wafer by anisotropy chemical etching. The 2 x 2 switch showed very low power consumption of 140 mW and a very high speed of 8 +/- 1 mus. An improved design for the TIR mirror was developed, and the fabricated mirror with smooth and vertical reflective facet showed low excess loss of 0.7 +/- 0.3 dB at 1.55 mum.

Density matrix Loschmidt echo and quantum phase transitions

We introduce the concept of the Loschmidt echo (LE) to the space of the reduced density matrix of spin and fermionic systems to study the density matrix LEs (DMLEs) of the one-dimensional extended Hubbard model and the transverse field Ising model. Our results show that the DMLEs are remarkably influenced by the criticality of the system, and the method is a convenient way to study quantum phase transitions.

Effect of substrate misorientation on the InAs/InAlAs/InP nanostructure morphology and lateral composition modulation in the InAlAs matrix

The authors report the self-organized growth of InAs/InAlAs quantum wires on nominal (001) InP substrate and (001) InP substrates misoriented by 2 degrees, 4 degrees, and 8 degrees towards both [-110] and [110]. The influence of substrate misorientation on the structural and optical properties of these InAs/InAlAs quantum wires is studied by transmission electron microscopy and photoluminescence measurements. Compared with that grown on nominal (001) InP substrate, the density of InAs/InAlAs quantum wires grown on misoriented InP(001) substrates is enhanced. A strong lateral composition modulation effect take place in the InAlAs buffer layers grown on misoriented InP substrates with large off-cut angles (4 degrees and 8 degrees), which induces a nucleation template for the first-period InAs quantum wires and greatly improve the size distribution of InAs quantum wires. InAs/InAlAs quantum wires grown on InP (001) substrate 8 degrees off cut towards [-110] show the best size homogeneity and photoluminescence intensity. (c) 2007 American Institute of Physics.