Super-resolution with a nonlinear thin film: Beam reshaping via internal multi-interference

In laser applications, the size of the focus spot can be reduced beyond the diffraction limit with a thin film of strong nonlinear optical Kerr effect. We present a concise theoretical simulation of the device. The origin of the super-resolution is found to be mainly from the reshaping effect due to the strongly nonlinear refraction mediated multi-interference inside the thin film. In addition, both diffraction and self-focusing effects have been explored and found negligible for highly refractive and ultrathin films in comparison with the reshaping effect. Finally, the theoretic model has been verified in experiments with single Ge2Sb2Te5 film and SiN/Si/SiN/Ge2Sb2Te2 multilayer structures. (c) 2006 American Institute of Physics.

Ferromagnetic modification of GaN film by Cu+ ions implantation

The structural and magnetic properties of Cu+ ions-implanted GaN films have been reported. Eighty kilo-electron-volt Cu+ ions were implanted into n-type GaN film at room temperature with fluences ranging from 1 x 10(16) to 8 x 10(16) cm(-2) and subsequently annealed at 800 degrees C for 1 h in N-2 ambient. PIXE was employed to determine the Cu-implanted content. The magnetic property was measured by the Quantum Design MPMS SQUID magnetometer. No secondary phases or clusters were detected within the sensitivity of XRD. Raman spectrum measurement showed that the Cu ions incorporated into the crystal lattice positions of GaN through substitution of Ga atoms. Apparent ferromagnetic hysteresis loops measured at 10 K were presented. The experimental result showed that the ferromagnetic signal strongly increased with Cu-implanted fluence from 1 x 10(16) to 8 x 10(16) cm(-2).

Influence of AlN Buffer Thickness on GaN Grown on Si(111) by Gas Source Molecular Beam Epitaxy with Ammonia

Hexagonal GaN is grown on a Si(111) substrate with AlN as a buffer layer by gas source molecular beam epitaxy (GSMBE) with ammonia. The thickness of AlN buffer is changed from 9 to 72 nm. When the thickness of AlN buffer is 36 nm, the surface morphology and crystal quality of GaN is optimal. The in-situ reflection high energy electron diffraction (RHEED) reveals that the transition to a two-dimensional growth mode of AlN is the key to the quality of GaN. However, the thickness of AlN buffer is not so critical to the residual in-plane tensile stress in GaN grown on Si(111) by GSMBE for AlN thickness between 9 to 72 nm.

Design of a compact polarization beam splitter based on a deformed photonic crystal directional coupler

In this paper a compact polarization beam splitter based on a deformed photonic crystal directional coupler is designed and simulated. The transverse-electric (TE) guided mode and transverse-magnetic (TM) guided mode are split due to different guiding mechanisms. The effect of the shape deformation of the air holes on the coupler is studied. It discovered that the coupling strength of the coupled waveguides is strongly enhanced by introducing elliptical airholes, which reduce the device length to less than 18.5 mu m. A finite-difference time-domain simulation is performed to evaluate the performance of the device, and the extinction ratios for both TE and TM polarized light are higher than 20 dB.

Influence of V/III ratio on the structural and photoluminescence properties of In0.52AlAs/In0.53GaAs metamorphic high electron mobility transistor grown by molecular beam epitaxy

A series of metamorphic high electron mobility transistors (MMHEMTs) with different V/III flux ratios are grown on GaAs (001) substrates by molecular beam epitaxy (XIBE). The samples are analysed by using atomic force microscopy (AFM), Hall measurement, and low temperature photoluminescence (PL). The optimum V/III ratio in a range from 15 to 60 for the growth of MMHEMTs is found to be around 40. At this ratio, the root mean square (RMS) roughness of the material is only 2.02 nm; a room-temperature mobility and a sheet electron density are obtained to be 10610.0cm(2)/(V.s) and 3.26 x 10(12)cm(-2) respectively. These results are equivalent to those obtained for the same structure grown on InP substrate. There are two peaks in the PL spectrum of the structure, corresponding to two sub-energy levels of the In0.53Ga0.47 As quantum well. It is found that the photoluminescence intensities of the two peaks vary with the V/III ratio, for which the reasons are discussed.

Optimization of metamorphic InGaAs quantum wells on GaAs grown by molecular beam epitaxy

We investigate the molecular beam epitaxy growth of metamorphic InxGa(1-x)As materials (x up to 0.5) on GaAs substrates systematically. Optimization of structure design and growth parameters is aimed at obtaining smooth surface and high optical quality. The optimized structures have an average surface roughness of 0.9-1.8 nm. It is also proven by PL measurements that the optical properties of high indium content (55%) InGaAs quantum wells are improved apparently by defect reduction technique and by introducing Sb as a surfactant. These provide us new ways for growing device quality metamorphic structures on GaAs substrates with long-wavelength emissions.

GaAs-Based Metamorphic Long-Wavelength InAs Quantum Dots Grown by Molecular Beam Epitaxy

A bilayer stacked InAs/GaAs quantum dot structure grown by molecular beam epitaxy on an In0.05Ga0.95As metamorphic buffer is investigated. By introducing a InGaAs Sb cover layer on the upper InAs quantum dots (QDs) layers, the emission wavelength of the QDs is extended successfully to 1.533 mu m at room temperature, and the density of the QDs is in the range of 4 x 10(9) -8 x 10(9) cm(-2). Strong photoluminescence (PL) intensity with a full width at half maximum of 28.6 meV of the PL spectrum shows good optical quality of the bilayer QDs. The growth of bilayer QDs on metamorphic buffers offers a useful way to extend the wavelengths of GaAs-based materials for potential applications in optoelectronic and quantum functional devices.

Enhancement of Exciton-Phonon Interaction in InGaN Quantum Wells Induced by Electron-Beam Irradiation

InGaN/GAN multiple quantum wells grown by metal-organic chemical vapor deposition were irradiated with the electron beam from a low energy accelerator. The electron irradiation induced a redshift by 50 meV in the photoluminescence spectra of the electron-irradiated InGaN/GaN quantum wells, irrespective of the exposure time to the electron beam which ranges from 10 to 1000s. The localization parameter extracted from the temperature-dependent photoluminescence spectra was found to increase in the Irradiated samples. Analysis of the intensity of the longitudinal optical phonon sidebands showed the enhancement of the exciton-phonon coupling, indicating that the excitons are more strongly localized in the irradiated InGaN wells. The change in the pholotuminescence spectra. In the irradiated InGa/GAN quantum wells were explained in terms of the increase of indium concentration in indium rich clusters induced by the electron irradiation (C) 2009 The Japan Society of Applied Physics

Interfaces in InAs/GaSb Superlattices Grown by Molecular Beam Epitaxy

Short period InAs(4 ML)/GaSb(8 ML) superlattices (SLs) with InSb- and mixed-like (or Ga(1-x)In(x)As(1-)ySb(y)-like) interfaces (IFs) are grown by molecular-beam epitaxy (MBE) on (001) GaSb substrates at optimized growth temperature. Raman scattering reveals that two kinds of IFs can be formed by controlling shutter sequences. X-ray diffraction (XRD) and atomic force microscopy (AFM) demonstrate that SLs with mixed-like IFs are more sensitive to growth temperature than that with InSb-like IFs. The photoluminescence (PL) spectra of SLs with mixed-like IFs show a stronger intensity and narrower line width than with InSb-like IFs. It is concluded that InAs/GaSb SLs with mixed-like IFs have better crystalline and optical properties.

GaAs Based InAs/GaSb Superlattice Short Wavelength Infrared Detectors Grown by Molecular Beam Epitaxy

InAs/GaSb superlattice (SL) short wavelength infrared photoconduction detectors are grown by molecular beam epitaxy on GaAs(001) semi-insulating substrates. An interfacial misfit mode AlSb quantum dot layer and a thick GaSb layer are grown as buffer layers. The detectors containing a 200-period 2ML/8ML InAs/GaSb SL active layer are fabricated with a pixel area of 800 x 800 mu m(2) without using passivation or antireflection coatings. Corresponding to the 50% cutoff wavelengths of 2.05 mu m at 77K and 2.25 mu m at 300 K, the peak detectivities of the detectors are 4 x 10(9) cm.Hz(1/2)/W at 77K and 2 x 10(8) cm.Hz(1/2)/W at 300 K, respectively.

Metamorphic InGaAs Quantum Well Laser Diodes at 1.5 mu on GaAs Grown by Molecular Beam Epitaxy

We report a 1.5-mu m InGaAs/GaAs quantum well laser diode grown by molecular beam epitaxy on InGaAs metamorphic buffers. At 150 K, for a 1500 x 10 mu m(2) ridge waveguide laser, the lasing wavelength is centred at 1.508 mu m and the threshold current density is 667 A/cm(2) under pulsed operation. The pulsed lasers can operate up to 286 K.

Molecular Beam Epitaxy of GaSb on GaAs Substrates with AlSb Buffer Layers

We investigate the molecular beam epitaxy growth of GaSb films on GaAs substrates using AlSb buffer layers. Optimization of AlSb growth parameter is aimed at obtaining high GaSb crystal quality and smooth GaSb surface. The optimized growth temperature and thickness of AlSb layers are found to be 450 degrees C and 2.1 nm, respectively. A rms surface roughness of 0.67 nm over 10 x 10 mu m(2) is achieved as a 0.5 mu m GaSb film is grown under optimized conditions.

Optical matching layer structures in evanescent coupling photodiodes at a wavelength of 1.55 mu m: physics, design and simulation

We have studied the optical matching layers (OMLs) and external quantum efficiency in the evanescent coupling photodiodes (ECPDs) integrating a diluted waveguide as a fibre-to-waveguide coupler, by using the semi-vectorial beam propagation method (BPM). The physical basis of OML has been identified, thereby a general designing rule of OML is developed in such a kind of photodiode. In addition, the external quantum efficiency and the polarization sensitivity versus the absorption and coupling length are analysed. With an optical matching layer, the absorption medium with a length of 30 mu m could absorb 90% of the incident light at 1.55 mu m wavelength, thus the total absorption increases more than 7 times over that of the photodiode without any optical matching layer.

Molecular beam epitaxy growth and photoluminescence of type-II (GaAs1-xSbx/InyGa1-yAs)/GaAs bilayer quantum well

We report a systematical study on the molecular beam epitaxy growth and optical property of (GaAs1-xSbx/In-y Ga1-yAs)/GaAs bilayer quantum well (BQW) structures. It is shown that the growth temperature of the wells and the sequence of layer growth have significant influence on the interface quality and the subsequent photoluminescence (PL) spectra. Under optimized growth conditions, three high-quality (GaAsSb0.29/In0.4GaAs)/GaAs BQWs are successfully fabricated and a room temperature PL at 1314 nm is observed. The transition mechanism in the BQW is also discussed by photoluminescence and photoreflectance measurements. The results confirm experimentally a type-II band alignment of the interface between the GaAsSb and InGaAs layers.