New type of multimode-interference-type thermo-optic variable optical attenuator

A thermo-optic variable optical attenuator (VOA) based on a Mach-Zehnder interferometer and multimode-interference coupler is fabricated. Not a single-mode but a multimode waveguide is used as the input and output structures of the optical field, which greatly reduces the coupling loss of the VOA with a normal single-mode fiber. The insertion loss of the fabricated VOA is 2.52 to 2.82 dB at the wavelength of 1520 to 1570 nm. The polarization dependent loss is 0.28 to 0.45 dB at the same wavelength range. Its maximum attenuation range is up to 26.3 dB when its power consumption is 369 mW. The response frequency of the fabricated VOA is about 10 kHz. (C) 2004 Society of Photo-Optical Instrumentation Engineers.

Thermal annealing effect on InAs/InGaAs quantum dots grown by atomic layer molecular beam epitaxy

The effect of rapid thermal annealing on the InAs quantum dots (QDs) grown by atomic layer molecular beam epitaxy and capped with InGaAs layer has been investigated using transmission electron microscopy and photoluminescence (PL). Different from the previously reported results, no obvious blueshift of the PL emission of QDs is observed until the annealing temperature increases up to 800 degreesC. The size and shape of the QDs annealed at 750 degreesC have hardly changed indicating the relatively weak Ga/In interdiffusion, which is characterized by little blueshift of the PL peak of QDs. The QD size increases largely and a few large clusters can be observed after 800 degreesC RTA, implying the fast interdiffusion and the formation of InGaAs QDs. These results indicate that the delay of the blueshift of the PL peak of QDs is correlated with the abnormal interdiffusion process, which can be explained by two possible reasons: the reduction of excess-As-induced defects and the redistribution of In, Ga atoms around the InAs QDs resulted from the sub-monolayer deposition of InGaAs capping layer. (C) 2004 Elsevier B.V. All rights reserved.

Preferred growth of nanocrystalline silicon in boron-doped nc-Si : H Films

Preferred growth of nanocrystalline silicon (nc-Si) was first found in boron-doped hydrogenated nanocrystalline (nc-Si:H) films prepared using plasma-enhanced chemical vapor deposition system. The films were characterized by high-resolution transmission electron microscope, X-ray diffraction (XRD) spectrum and Raman Scattering spectrum. The results showed that the diffraction peaks in XRD spectrum were at 2theta approximate to 47degrees and the exponent of crystalline plane of nc-Si in the film was (220). A considerable reason was electric field derived from dc bias made the bonds of Si-Si array according to a certain orient. The size and crystalline volume fraction of nc-Si in boron-doped films were intensively depended on the deposited parameters: diborane (B2H6) doping ratio in silane (SiH4), silane dilution ratio in hydrogen (H-2), rf power density, substrate's temperature and reactive pressure, respectively. But preferred growth of nc-Si in the boron-doped nc-Si:H films cannot be obtained by changing these parameters. (C) 2004 Elsevier Ltd. All rights reserved.

Structural phase transformations of ZnS nanocrystalline under high pressure

In-situ energy dispersive x-ray diffraction on ZnS nanocrystalline was carried out under high pressure by using a diamond anvil cell. Phase transition of wurtzite of 10 nm ZnS to rocksalt occurred at 16.0 GPa, which was higher than that of the bulk materials. The structures of ZnS nanocrystalline at different pressures were built by using materials studio and the bulk modulus, and the pressure derivative of ZnS nanocrystalline were derived by fitting the equation of Birch-Murnaghan. The resulting modulus was higher than that of the corresponding bulk material, which indicates that the nanomaterial has higher hardness than its bulk materials.

Cleaved-edge overgrowth of aligned InAs islands on GaAs(110)

A novel approach for positioning InAs islands on GaAs(110) by cleaved-edge overgrowth is reported. The first growth sample contains a strained InxGa1-xAs/GaAs superlattice of varying indium fraction and thickness, which acts as a strain nanopattern for the cleaved edge overgrowth. The formation of aligned islands is observed by means of atomic force microscopy. The ordering of the aligned islands and the structure of a single InAs island are found to depend on the properties of the underlying InxGa1-xAs/GaAs superlattice and molecular beam epitaxy growth conditions.

A novel method for positioning of InAs islands on GaAs(110)

A novel method for positioning of InAs islands on GaAs (110) by cleaved edge overgrowth is reported. The first growth sample contains strained InxGa1-xAs/GaAs superlattice (SL) of varying indium fraction, which acts as a strain nanopattern for the cleaved-edge overgrowth. Atoms incident on the cleaved edge will preferentially migrate to InGaAs regions where favorable bonding sites are available. By this method InAs island chains with lateral periodicity defined by the thickness of InGaAs and GaAs of SL have been realized by molecular beam epitaxy (MBE). They are observed by means of atomic force microscopy (AFM). The strain nanopattern's effect is studied by the different indium fraction of SL and MBE growth conditions. (c) 2005 Elsevier B.V. All rights reserved.

Quantum-dot growth simulation on periodic stress of substrate

InAs quantum dots (QDs) are grown on the cleaved edge of an InxGa1-xAs/GaAs supperlattice experimentally and a good linear alignment of these QDs on the surface of an InxGa1-xAs layer has been realized. The modulation effects of periodic strain on the substrate are investigated theoretically using a kinetic Monte Carlo method. Our results show that a good alignment of QDs can be achieved when the strain energy reaches 2% of the atomic binding energy. The simulation results are in excellent qualitative agreement with our experiments. (C) 2005 American Institute of Physics.

Site controlling of InAs quantum wires on cleaved edges of AlGaAs/GaAs superlattices

Site-controlled InAs quantum wires were fabricated on cleaved edges of AlGaAs/GaAs superlattices (SLs) by solid source molecular beam epitaxy. The cleaved edge of AlGaAs/GaAs SLs acted as a nanopattern for selective overgrowth after selective etching. By just growing 2.0 ML InAs without high temperature degassing, site-controlled InAs quantum wires were fabricated on the cleaved edge. Furthermore, atomic force microscopy demonstrates the diffusion of In atoms is strong toward the [00 (1) over bar] direction on the (110) surface.

Low power-consumption and high response frequency thermo-optic variable optical Attenuators based on silicon-on-insulator materials

A novel silicon-on-insulator thermo-optic variable optical attenuator with isolated grooves based on a multimode interference coupler principle is fabricated by the inductive coupled plasma etching technology. The maximum fibre-to-fibre insertion loss is lower than 2.2 dB, the dynamic attenuation range is from 0 to 30 dB in the wavelength range 1500-1600 nm, and the maximum power consumption is only 140 mW. The response frequency of the fabricated variable optical attenuator is about 30 kHz. Compared to the variable optical attenuator without isolated grooves, the maximum power consumption decreases more than 220 mW, and the response frequency rises are more than 20 kHz.

The structural and photoluminescence character of InAs quantum dots grown on a combined InAlAs and GaAs strained buffer

The structural and photoluminescence (PL) properties of the InAs quantum dots (QDs) grown on a combined InAlAs and GaAs strained buffer layer have been investigated by AFM and PL measurements. The dependence of the critical thickness for the transition from 2D to 3D on the thickness of GaAs layer is demonstrated directly by RHEED. The effects of the introduced-InAlAs layer on the density and the aspect ratio of QDs have been discussed.

In-situ resistivity measurement of ZnS in diamond anvil cell under high pressure

An effective method is developed to fabricate metallic microcircuits in diamond anvil cell (DAC) for resistivity measurement under high pressure. The resistivity of nanocrystal ZnS is measured under high pressure up to 36.4 GPa by using designed DAC. The reversibility and hysteresis of the phase transition are observed. The experimental data is confirmed by an electric current field analysis accurately. The method used here can also be used under both ultrahigh pressure and high temperature conditions.

Photonic crystal vertical-cavity surface-emitting laser based on GaAs material

A photonic crystal vertical-cavity-surface-emitting laser ( PC-VCSEL) with a wavelength of about 850 nm was realized. The direct-current electrically-driven PC-VCSELs with a minimum threshold current of 2 mA and a maximum threshold current of 13.5 mA were obtained. We fabricated a series of PC-VCSEL chips whose lattice constants are in the range from 0.5 to 3 mu m with different filling factors, and found that the laser characterization depends on the lattice constant, the filling factor, the size of cavity, etc.

Structure tuning of line-defect waveguides based on silicon-on-insulator photonic crystal slabs

We present fabrication and experimental measurement of a series of photonic crystal waveguides. The complete devices consist of an injector taper down from 3 mu m into a triangular-lattice air-hole single-line-defect waveguide with lattice constant from 410nm to 470nm and normalized radius 0.31. We fabricate these devices on a siliconon-insulator substrate and characterize them using a tunable laser source over a wavelength range from 1510nm to 1640nm. A sharp attenuation at photonic crystal waveguide mode edge is observed for most structures. The edge of guided band is shifted about 30nm with the 10nm increase of the lattice constant. We obtain high-efficiency light propagation and broad flat spectrum response of the photonic crystal waveguides.

Experimental investigation of intracavity absorber low temperature GaAs in diode-pumped Nd : GdVO4 laser

A passively Q-switched and mode-locked diode-pumped Nd:GdVO4 laser was demonstrated using a low-temperature-grown GaAs wafer (LT-GaAs) as an intracavity saturable absorber. The maximal Q-switched mode-locked average output power was 750 mW with the Q-switched envelop having a repetition rate of 167 kHz. The mode-locked pulse trains inside the Q-switched pulse envelope had a repetition rate of similar to 790 MHz.

Light-propagation characteristics of photonic crystal waveguide based on SOI materials at different polarized states

Straight single-line defect optical waveguides in photonic crystal slabs are designed by the plane wave expansion method and fabricated into silicon-on-insulator (SOI) wafer by 248-nm deep UV lithography. We present an efficient way to measure the light transmission spectrum of the photonic crystal waveguide (PhC WG) at given polarization states. By employing the Mueller/Stokes method, we measure and analyse the light propagation properties of the PhC WG at different polarized states. It is shown that experimental results are in agreement with the simulation results of the three-dimensional finite-difference-time-domain method.