Finite-difference time-domain analysis of deformed square cavity filters with a traveling-wave-like filtering response by mode coupling

An add-drop filter based on a perfect square resonator can realize a maximum of only 25% power dropping because the confined modes are standing-wave modes. By means of mode coupling between two modes with inverse symmetry properties, a traveling-wave-like filtering response is obtained in a two-dimensional single square cavity filter with cut or circular corners by finite-difference time-domain simulation. The optimized deformation parameters for an add-drop filter can be accurately predicted as the overlapping point of the two coupling modes in an isolated deformed square cavity. More than 80% power dropping can be obtained in a deformed square cavity filter with a side length of 3.01 mu m. The free spectral region is decided by the mode spacing between modes, with the sum of the mode indices differing by 1. (c) 2007 Optical Society of America.

Net-like ferromagnetic MnSb film deposited on porous silicon substrates

MnSb films were deposited on porous silicon substrates by physical vapor deposition (PVD) technique. Modulation effects due to the substrate on microstructure and magnetic properties of the MnSb film's were studied by scanning electron microscope (SEM), X-ray diffraction (XRD) and measurements of hysteresis loops. SEM images of the MnSb films indicate that net-like structures were obtained because of the special morphology of the substrates. The net-like MnSb films exhibit some novel magnetic properties different from the unpatterned referenced samples. For example, in the case of net-like morphology, the coercive field is as low as 60 Oe. (c) 2006 Elsevier B.V. All rights reserved.

WG-like modes selectivity in a square cavity with posts

We investigate the characteristics of Whispering-Gallery(WG)-like modes in a square cavity with posts by employing the two-dimentional (2D) finite-difference time-domain (FDTD) technique combined with the effective index method. The results indicate that the posts can result in mode selection in the WG-like modes. The WG-like modes with odd mode numbers are not much sensitive to the sizes of the posts. However, the quality factor (i.e. Q-factor) of the WG-like modes with even mode numbers decreases sharply with the increasing size of the posts. The decreasing Q-factor is attributed to mode leakage and scattering loss due to the presence of the post. The mode selection increases with the mode spacing of square cavity twice in an optimized strucure.

Synthesis and temperature-dependent near-band-edge emission of chain-like Mg-doped ZnO nanoparticles

Chain-like Mg-doped ZnO nanoparticles were prepared using a wet chemical method combined with subsequent heat treatment. The blueshifted near-band-edge emission of the doped ZnO sample with respect to the undoped one was investigated by temperature-dependent photoluminescence. Based on the energy shift of the free-exciton transition, a band gap enlargement of similar to 83 meV was estimated, which seems to result in the equivalent shift of the bound-exciton transition. At 50 K, the transformation from the donor-acceptor-pair to free-to-acceptor emissions was observed for both the undoped and doped samples. The results show that Mg doping leads to the decrease of the acceptor binding energy. (c) 2006 American Institute of Physics.

Energy spectra of two-electron two-dimensional quantum dots confined by elliptical and bowl-like potentials

The laterally confining potential of quantum dots (QDs) fabricated in semiconductor heterostructures is approximated by an elliptical two-dimensional harmonic-oscillator well or a bowl-like circular well. The energy spectrum of two interacting electrons in these potentials is calculated in the effective-mass approximation as a function of dot size and characteristic frequency of the confining potential by the exact diagonalization method. Energy level crossover is displayed according to the ratio of the characteristic frequencies of the elliptical confinement potential along the y axis and that along the x axis. Investigating the rovibrational spectrum with pair-correlation function and conditional probability distribution, we could see the violation of circular symmetry. However, there are still some symmetries left in the elliptical QDs. When the QDs are confined by a "bowl-like" potential, the removal of the degeneracy in the energy levels of QDs is found. The distribution of energy levels is different for the different heights of the barriers. (C) 2003 American Institute of Physics.

Semiconductor optical amplifier optical gate with graded strained bulk-like active structure

A novel semiconductor optical amplifier (SOA) optical gate with a graded strained bulk-like active structure is proposed. A fiber-to-fiber gain of 10 dB when the coupling loss reaches 7 dB/factet and a polarization insensitivity of less than 0.9 dB for multiwavelength and different power input signals over the whole operation current are obtained. Moreover, for our SOA optical gate, a no-loss current of 50 to 70 mA and an extinction ratio of more than 50 dB are realized when the injection current is more than no-loss current, and the maximum extinction ratio reaches 71 dB, which is critical for crosstalk suppression. (C) 2003 society of Photo-Optical Instrumentation Engineers.

Coherent transport through a quantum dot embedded in a double-slit-like Aharonov-Bohm ring

Coherent transport through a quantum dot embedded in one arm of a double-slit-like Aharonov-Bohm (AB) ring is studied using the Green's function approach. We obtain experimental observations such as continuous phase shift along a single resonance peak and sharp inter-resonance phase drop. The AB oscillations of the differential conductance of the whole device are calculated by using the nonequilibrium Keldysh formalism. It is shown that the oscillating conductance has a continuous bias-voltage-dependent phase shift and is asymmetric in both linear and nonlinear response regimes.

Anisotropic radiation pattern from InGaAlP quantum well mesa-like microdisks

To overcome the isotropic directional emission of an ideal circular microdisk, two kinds of cylindrical mesa-like InGaAlP single quantum well (SQW) microdisks emitting at a visible red wavelength of 0.66 mu m have been fabricated. An anisotropic luminescence pattern was revealed by the microscopic fluorescence (FL) image. FL intensity, preferentially enhanced with twofold symmetry, appeared at the circumference of the InGaAlP SQW microdisks. Our results demonstrated that anisotropic radiation can be achieved by geometry shaping of the disks on the top view two-dimensional boundary slightly deformed from circular shape and/or on the side-view cross-section of the circular mesa by wet etching anisotropic undercut. (C) 2000 Elsevier Science Ltd. All rights reserved.

DX-like centers in n-type Al-doped ZnS1-xTex grown by molecular-beam epitaxy

Al-related DX-like centers were observed in n-type Al-doped ZnS1-xTex epilayers grown by molecular-beam epitaxy on GaAs substrates. The capacitance-voltage measurement, deep-level transient spectroscopy, and photoconductivity spectroscopy revealed that the behaviors of Al donors in ZnS1-xTex were similar to the so-called DX centers in AlxGa1-xAs. The optical ionization energies (E-i) and emission barriers (E-e) for the observed two Al-related DX-like centers were determined as E-i similar to 1.0 and 2.0cV and E-e similar to 0.21 and 0.39 eV, respectively. It was also shown that the formation of Al-related DX-like centers resulted in a significantly large lattice relaxation in ZnS1-xTex. (C) 2000 Elsevier Science B.V. All rights reserved.

Self-organization of wire-like InAs nanostructures on InP

The initial InAs growth on InP(1 0 0) during molecular beam epitaxy has been investigated. The as-grown islands were shaped like nanowires and formed dense arrays over the entire surface in the 3-6 monolayer InAs deposition range. The wires were oriented along the [(1) over bar 1 0] direction. Transmission electron microscopy images confirm that the wires are coherently grown on the substrates. Our results suggest that the coherent wire-shaped island formation may be a possible method to fabricate self-organized InAs nanowires. (C) 1999 Elsevier Science B.V. All rights reserved.

Void-like defects in annealed Czochralski silicon

Void-like defects of octahedron structure having {111} facets were observed in annealed Czochralski silicon. The amorphous coverage of SiOx and SiCx on the inner surface of the defects was identified using transmission electron microscopy and electron energy-loss spectroscopy. It is suggested that these defects are a kind of amorphous precipitate origin. A mechanism for the generation of these defects and the previously reported solid amorphous precipitates is proposed. (C) 1998 American Institute of Physics. [S0003-6951(98)02842-3].

Synthesis of GaN nanorods with vertebra-like morphology

GaN nanorods with vertebra-like morphology were synthesized by nitriding Ga2O3/ZnO films at 1000 degrees C for 20min. Ga2O3 thin films and ZnO middle layers were pre-deposited in turn on Si(111) substrates by r.f. magnetron sputtering system. In the flowing ammonia ambient, ZnO was reducted to Zn and Zu sublimated at 1000 degrees C. Ga2O3 was reducted to Ga2O and Ga2O reacted with NH3 to synthesize GaN nanorods in the help of the sublimation of Zn. The structure and morphology of the nanorods were studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM), The composition of GaN nanorods was studied by energy dispersive spectroscopy (EDS) and fourier transform infrared (FTIR) system.