High refractive index TiO2 film deposited by electron beam evaporation

The well known 'crystal seed' theory is first applied in this work to prepare TiO2 film: a high refractive index rutile TiO2 film is grown by electron beam evaporation on the rutile seed formed by 1100 degrees C annealing. The average n is larger than 2.4, by far the highest in all the authors' TiO2 films. The films are characterised by optical properties, microstructure and surface morphologies. It is found that the refractive index shows positive relation with the crystal structure, grain size, and packing density and roughness of the film. The film has lower density of granularity and nodule defects on the surface than those of the film deposited by magnetron sputtering. The result shows attractive application in complex filter and laser coatings.

Off-axis sonar beam pattern of free-ranging finless porpoises measured by a stereo pulse event data logger

The off-axis sonar beam patterns of eight free-ranging finless porpoises were measured using attached data logger systems. The transmitted sound pressure level at each beam angle was calculated from the animal's body angle, the water surface echo level, and the swimming depth. The beam pattern of the off-axis signals between 45 and 115 (where 0 corresponds to the on-axis direction) was nearly constant. The sound pressure level of the off-axis signals reached 162 dB re 1 mPa peak-to-peak. The surface echo level received at the animal was over 140 dB, much higher than the auditory threshold level of small odontocetes. Finless porpoises are estimated to be able to receive the surface echoes of off-axis signals even at 50-m depth. Shallow water systems (less than 50-m depth) are the dominant habitat of both oceanic and freshwater populations of this species. Surface echoes may provide porpoises not only with diving depth information but also with information about surface direction and location of obstacles (including prey items) outside the on-axis sector of the sonar beam. 2005 Acoustical Society of America.

Fabrication of a Low-Loss SSC Using High-Dose Electron Beam Lithography Exposure With Negative PMMA Resist

A silicon-on-insulator optical fiber-to-waveguide spot-size converter (SSC) using Poly-MethylMethAcrylate (PMMA) is presented for integrated optical circuits. Unlike the conventional use of PMMA as a positive resist, it has been successfully used as a negative resist with high-dose electron exposure for the fabrication of ultrafine silicon wire waveguides. Additionally, this process is able to reduce the side-wall roughness, and substantially depresses the unwanted propagation loss. Exploiting this technology, the authors demonstrated that the SSC can improve coupling efficiency by as much as over 2.5 dB per coupling facet, compared with that of SSC fabricated with PMMA as a positive resist with the same dimension.

Design of a compact polarizing beam splitter based on a photonic crystal ring resonator with a triangular lattice

We propose and simulate a new kind of compact polarizing beam splitter (PBS) based on a photonic crystal ring resonator (PCRR) with complete photonic bandgaps. The two polarized states are separated far enough by resonant and nonresonant coupling between the waveguide modes and the microring modes. Some defect holes are utilized to control the beam propagation. The simulated results obtained by the finite-difference time-domain method show that high transmission (over 95%) is obtained and the polarization separation is realized with a length as short as 3.1 mu m. The design of the proposed PBS can be flexible, thanks to the advantages of PCRRs.

Band-tail shape and transport near the metal-insulator transition in Si-doped

In the present work, an infrared light-emitting diode is used to photodope molecular-beam-epitaxy-grown Si: Al0.3Ga0.7As, a well-known persistent photoconductor, to vary the effective electron concentration of samples in situ. Using this technique, we examine the transport properties of two samples containing different nominal doping concentrations of Si [1 x 10(19) cm(-3) for sample 1 (S1) and 9 x 10(17) cm(-3) for sample 2 (S2)] and vary the effective electron density between 10(14) and 10(18) cm(-3). The metal-insulator transition for S1 is found to occur at a critical carrier concentration of 5.7 x 10(16) cm(-3) at 350 mK. The mobilities in both samples are found to be limited by ionized impurity scattering in the temperature range probed, and are adequately described by the Brooks-Herring screening theory for higher carrier densities. The shape of the band tail of the density of states in Al0.3Ga0.7As is found electrically through transport measurements. It is determined to have a power-law dependence, with an exponent of -1.25 for S1 and -1.38 for S2.

Design of novel polarization beam splitter in two-dimensional photonic crystal

We present the design and the simulation of an ultracompact high efficiency polarization beam splitter (PBS) based on the properties of the light waves propagating in straight waveguide and composite structure photonic crystal. The splitting properties of the PBS are numerically simulated and analyzed by using the plane wave expansion (PWE) method and finite difference time domain (FDTD) method. The PBS consists of three parts, namely, input waveguide, beam structure and output waveguide. It is shown that a high efficiency and a large separating angle for TE mode and TM mode can be achieved. Owing to these excellent features, including small size and high rate, the PBS makes a promising candidate in the future photonic integrated circuits.

Dislocation scattering in AlxGa1-xN/GaN heterostructures

The theoretical electron mobility limited by dislocation scattering of a two-dimensional electron gas confined near the interface of AlxGa1-xN/GaN heterostructures was calculated. Based on the model of treating dislocation as a charged line, an exponentially varied potential was adopted to calculate the mobility. The estimated mobility suggests that such a choice can simplify the calculation without introducing significant deviation from experimental data, and we obtained a good fitting between the calculated and experimental results. It was found that the measured mobility is dominated by interface roughness and dislocation scattering at low temperatures if dislocation density is relatively high (>10(9) cm(-2)), and accounts for the nearly flattening-out behavior with increasing temperature.

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.

Structural, electrical, and optical properties of InAsxSb1-x epitaxial films grown by liquid-phase epitaxy

The InAsxSb1-x films were grown on (100) GaSb substrates by liquid-phase epitaxy, and their structural, electrical, and optical properties were investigated. The high-resolution x-ray diffraction results reveal that the single crystalline InAsxSb1-x films with a midrange composition are epitaxially grown on the GaSb substrates. Temperature dependence of the Hall mobility was theoretically modeled by considering several predominant scattering mechanisms. The results indicate that ionized impurity and dislocation scatterings dominate at low temperatures, while polar optical phonon scattering is important at room temperature (RT). Furthermore, the InAsxSb1-x films with the higher As composition exhibit the better crystalline quality and the higher mobility. The InAs0.35Sb0.65 film exhibits a Hall mobility of 4.62x10(4) cm(2) V-1 s(-1). The cutoff wavelength of photoresponse is extended to about 12 mu m with a maximum responsivity of 0.21 V/W at RT, showing great potential for RT long-wavelength infrared detection. (C) 2008 American Institute of Physics. [DOI: 10.1063/1.2989116]

Raman scattering characterization of Mn composition and strain in Ga1-xMnxSb/GaSb epitaxial layers

Raman scattering (RS) experiments have been performed for simultaneous determination of Mn composition and strain in Ga1-xMnxSb thin films grown on GaSb substrate by liquid phase epitaxy technique. The Raman spectra obtained from various Ga1-xMnxSb samples show only GaSb-like phonon modes whose frequency positions are found to have Mn compositional dependence. With the combination of epilayer strain model, RS and energy dispersive x-ray (EDX) experiments, the compositional dependence of GaSb-like LO phonon frequency is proposed both in strained and unstrained conditions. The proposed relationships are used to evaluate Mn composition and strain from the Ga1-xMnxSb samples. The results obtained from the RS data are found to be in good agreement with those determined independently by the EDX analysis. Furthermore, the frequency positions of MnSb-like phonon modes are suggested by reduced-mass model. (C) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Finite element beam propagation method for analysis of plasmonic waveguide

The basic idea of the finite element beam propagation method (FE-BPM) is described. It is applied to calculate the fundamental mode of a channel plasmonic polariton (CPP) waveguide to confirm its validity. Both the field distribution and the effective index of the, fundamental mode are given by the method. The convergence speed shows the advantage and stability of this method. Then a plasmonic waveguide with a dielectric strip deposited on a metal substrate is investigated, and the group velocity is negative for the fundamental mode of this kind of waveguide. The numerical result shows that the power flow direction is reverse to that of phase velocity.

Enhancement of the far-field output power and the beam properties of plasmonic very small aperture lasers

The properties of plasmonic very small aperture lasers are shown: these integrate surface plasmon structures with very small aperture lasers. The transmission field can be confined to a spot of subwavelength width in the far field, and according to the finite difference time domain simulation results the focal length of the spot can be modulated using different ring periods. Scanning of the subwavelength gating in the far field has been realized numerically. Such a device can be used with a high-resolution far-field scanning optical microscope.

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.