Investigation of near-collinear degenerated quasi-phase matching optical parametric amplification using PPKTP crystal

The gain properties of near-collinear degenerated phase-matched optical parametric amplification (OPA) using PPKTP crystal are investigated theoretically. The results indicate that the type-0 phase matching of PPKTP has larger accepted angle and better gain spectrum by tuning crystal temperature or rotating crystal angle.

Group velocity of the light pulse in an open V-type system

We investigate the group velocity of the probe light pulse in an open V-type system with spontaneously generated coherence. We find that, not only varying the relative phase between the probe and driving pulses can but varying the atomic exit rate or incoherent pumping rate also can manipulate dramatically the group velocity, even make the pulse propagation switching from subluminal to superluminal; the subliminal propagation can be companied with gain or absorption, but the superluminal propagation is always companied with absorption. (c) 2006 Elsevier GmbH. All rights reserved.

Phase-space analysis of wavefront coding imaging systems

We explore the use of the Radon-Wigner transform, which is associated with the fractional Fourier transform of the pupil function, for determining the point spread function (PSF) of an incoherant defocused optical system. Then we introduce these phase-space tools to analyse the wavefront coding imaging system. It is shown that the shape of the PSF for such a system is highly invarient to the defocous-related aberrations except for a lateral shift. The optical transfer function of this system is also investigated briefly from a new understanding of ambiguity function.

Phase interpretation for polarization-dependent near-field images of high-density gratings

It has been described that the near-field images of a high-density grating at the half self-imaging distance could be different for TE and TM polarization states. We propose that the phases of the diffraction orders play an important role in such polarization dependence. The view is verified through the coincidence of the numerical result of finite-difference time-domain method and the reconstructed results from the rigorous coupled-wave analysis. Field distributions of TE and TM polarizations are given numerically for a grating with period d = 2.3 lambda, which are verified through experiments with the scanning near-field optical microscopy technique. The concept of phase interpretation not only explains the polarization dependence at the half self-imaging distance of gratings with a physical view, but also, it could be widely used to describe the near-field diffraction of a variety of periodic diffractive optical elements whose feature size comparable to the wavelength. (C) 2008 Elsevier B.V. All rights reserved.

Phase mapping of domain kinetics in lithium niobate by digital holographic interferometry

The phase mapping of domain kinetics under the uniform steady-state electric field is achieved and investigated in the LiNbO3 crystals by digital holographic interferometry. We obtained the sequences of reconstructed three-dimensional and two-dimensional wave-field phase distributions during the electric poling in the congruent and near stoichiometric LiNbO3 crystals. The phase mapping of individual domain nucleation and growth in the two crystals are obtained. It is found that both longitudinal and lateral domain growths are not linear during the electric poling. The phase mapping of domain wall motions in the two crystals is also obtained. Both the phase relaxation and the pinning-depinning mechanism are observed during the domain wall motion. The residual phase distribution is observed after the high-speed domain wall motion. The corresponding analyses and discussions are proposed to explain the phenomena.

Determination of odorous compounds in water by headspace solid-phase microextraction-gas chromatography-mass spectrometry

Three major odorous compounds are 2-methylisoborneol ( 2-MIB), geosmin and beta-cyclocitral, which in water were determined by coupling headspace solid-phase microextraction ( HS-SPME) with gas chromatography-mass spectrometry (GC-MS). The operating conditions of HS-SPME, such as fibre type, salt concentration, water temperature, stirring, absorption time and desorption time were studied and discussed.The highest absorption of the odorous compounds were obtained under the following operating conditions as the addition of 30% ( m/V) NaCl, stirring at 60 degrees C for 40 min, using 65 mu m polydimethyl siloxane/divinylbenzene coated fibre. After the odorous compounds had been absorbed in the fibre under the optimal conditions of HSSPME, they were desorbed at 250 degrees C and determined by GC-MS. The limits of detection for geosmin, beta-cyclocitral and 2-MIB in water were 1. 0, 1. 3, 1. 7 ng/L, and the relative standard deviations for them were 4. 9%, 8. 4%, 6. 2%,respectively. There were good linear correlation (the calibration coefficients were all above 0. 997) for the three odorous compounds in the range of 5 similar to 1000 ng/ L. Therefore, trace levels of the odorous compounds at ng/L in water could be quantified by the simple method with satisfactory result.

Ga1-xMnxSb grown on GaSb substrate by liquid phase epitaxy

The Gal(1-x)Mn(x)Sb epilayer was prepared on the n-type GaSb substrate by liquid phase epitaxy. The structure of the Gal(1-x)Mn(x)Sb epilayer was analyzed by double-crystal X-ray diffraction. From the difference of the lattice constant between the GaSb substrate and the Ga1-xMnxSb epilayer, the Mn content in the Ga1-xMnxSb epilayer were calculated as x = 0.016. The elemental composition of Ga1-xMnxSb epilayer was analyzed by energy dispersive spectrometer. The carrier concentration was obtained by Hall measurement. The hole concentration in the Ga1-xMnxSb epilayer is 4.06 x 10(19)cm(-3). It indicates that most of the Mn atoms in Ga1-xMnxSb take the site of Ga, and play a role of acceptors. The current-voltage curve of the Ga1-xMnxSb/GaSb heterostructure was measured, and the rectifying effect is obvious. (C) 2003 Elsevier B.V. All rights reserved.

Frequency mixing and phase detection functionalities of three-terminal ballistic junctions

Three-terminal ballistic junctions (TBJs) are fabricated from a high-mobility InP/In0.75Ga0.25As heterostructure by electron-beam lithography. The voltage output from the central branch is measured as a function of the voltages applied to the left and right branches of the TBJs. The measurements show that the TBJs possess an intrinsic nonlinearity. Based on this nonlinearity, a novel room-temperature functional frequency mixer and phase detector are realized. The TBJ frequency mixer and phase detector are expected to have advantages over traditional circuits in terms of simple structure, small size and high speed, and can be used as a new type of building block in nanoelectronics.

Ultrafast geometric manipulation of electron spin and detection of the geometric phase via Faraday rotation spectroscopy

Time-resolved Faraday rotation spectroscopy is currently exploited as a powerful technique to probe spin dynamics in semiconductors. We propose here an all-optical approach to geometrically manipulate electron spin and to detect the geometric phase by this type of extremely sensitive experiment. The global nature of the geometric phase can make the quantum manipulation more stable, which may find interesting applications in quantum devices.

Nitrogen vacancy scattering in GaN grown by metal-organic vapor phase epitaxy

Electron mobility limited by nitrogen vacancy scattering was taken into account to evaluate the quality of n-type GaN grown by metal-organic vapor phase epitaxy. Two assumptions were made for this potential for the nitrogen vacancy (1) it acts in a short range, and (2) does not diverge at the vacancy core. According to the above assumptions, a general expression to describe the scattering potential U(r) = - U-0 exp[- (r/beta)(n)], (n = 1, 2,...,infinity) was constructed, where beta is the potential well width. The mobilities for n = 1, 2, and infinity were calculated based on this equation, corresponding to the simple exponential, Gaussian and square well scattering potentials, respectively. In the limiting case of kbeta << 1 (where k is the wave vector), all of the mobilities calculated for n = 1, 2, and infinity showed a same result but different prefactor. Such difference was discussed in terms of the potential tail and was found that all of the calculated mobilities have T-1/2 temperature and beta(-6) well width dependences. A mobility taking account of a spatially complicate scattering potential was studied and the same temperature dependence was also found. A best fit between the calculated results and experimental data was obtained by taking account of the nitrogen vacancy scattering. (C) 2002 Elsevier Science Ltd. All rights reserved.

Theory and experiment of two-photon 'direct' interference for type-II spontaneous parametric down-conversion with an ultrafast pulse laser pump

Fourth-order spatial interference of entangled photon pairs generated in the process of spontaneous parametric down-conversion pumped by a femtosecond pulse laser has been performed for the first time. In theory, it takes into account the transverse correlation between the two photons and is used to calculate the dependence of the visibility of the interference pattern obtained in Young's double-slit experiment. In this experiment, a short focal length tens and two narrow band interference filters were adopted to eliminate the effects of the broadband pump laser and improve the visibility of the interference pattern under the condition of nearly collinear light and degenerate phase matching.

Nitrogen vacancy scattering in n-GaN grown by metal-organic vapor phase epitaxy

Electron mobility limited by nitrogen vacancy scattering was taken into account to evaluate the quality of n-type GaN grown by metal-organic vapor phase epitaxy. The nitrogen vacancy scattering potential used for our mobility calculation has to satisfy two requirements: such potential is (1) spatially short range, and (2) finite and not divergent at the vacancy core. A square-well potential was adopted to calculate the mobility, because it satisfies not only these two requirements, but also simplifies the calculation. As a result, the estimated mobility shows a T-1/2 temperature dependence, and is very sensitive to the potential well width. After introducing the nitrogen vacancy scattering, we obtained the best fitting between the calculated and experimental results for our high quality sample, and it was found that the measured mobility is dominated by ion impurity and dislocation scatterings at the low temperatures, but dominated by optical phonon and nitrogen vacancy scatterings at the high temperatures. (C) 2000 American Institute of Physics. [S0003-6951(00)04112-7].

Quantum-confinement-effect-driven type-I-type-II transition in inhomogeneous quantum dot structures

We investigate the electronic structures of the inhomogeneous quantum dots within the framework of the effective mass theory. The results show that the energies of electron and hole states depend sensitively on the relative magnitude 77 of the core radius to the capped quantum dot radius. The spatial distribution of the electrons and holes vary significantly when the ratio eta changes. A quantum-confinement-driven type-II-type-I transition is found in GaAs/AlxGa1-xAs-capped quantum dot structures. The phase diagram is obtained for different capped quantum dot radii. The ground-state exciton binding energy shows a highly nonlinear dependence on the innner structures of inhomogeneous quantum dots, which originates from the redistribution of the electron and hole wave functions.

Effect of thermal annealing on hole trap levels in Mg-doped GaN grown by metalorganic vapor phase epitaxy

Deep trap levels in a Mg-doped GaN grown by metalorganic vapor phase epitaxy are studied with deep level transient spectroscopy (DLTS). The Mg concentration of the sample was 4.8 x 10(19) cm(-3), but the hole concentration was as low as 1.3x10(17) cm-3 at room temperature. The DLTS spectrum has a dominant peak D-1 with an activation energy of 0.41+/-0.05 eV, accompanied by two additional peaks with activation energies of 0.49+/-0.09 eV (D-2) and 0.59+/-0.05 eV (D-3). It was found that the dominant peak D-1 consists of five peaks, each of which has different activation energy and capture cross section. In order to investigate these deep levels further, we performed heat treatment on the same samples to observe the variations of activation energy, capture cross section, and amplitude of DLTS signals. It was found that the longer the heat treatment duration is, the lower the amplitude of DLTS peaks become. This suggests that the decrease of the DLTS signal originates from hydrogen atom outgoing from the film during the annealing process. The possible originality of multiple trap levels was discussed in terms of the Mg-N-H complex. (C) 2000 American Vacuum Society. [S0734-2101(00)01701-2].

The growth and characterization of GaN grown on an Al2O3 coated (001)Si substrate by metalorganic vapor phase epitaxy

Single crystal GaN films have been grown on to an Al2O3 coated (001)Si substrate in a horizontal-type low-pressure MOVPE system. A thin Al2O3 layer is an intermediate layer for the growth of single crystal GaN on to Si although it is only an oriented polycrystal him as shown by reflection high electron diffraction. Moreover, the oxide was not yet converted to a fully single crystal film, even at the stage of high temperature for the GaN overlayer as studied by transmission electron microscopy. Double crystal X-ray diffraction showed that the linewidth of (0002) peak of the X-ray rocking curve of the 1.3 mu m sample was 54 arcmin and the films had heavy mosaic structures. A near band edge peaking at 3.4 eV at room temperature was observed by photoluminescence spectroscopy. (C) 1998 Elsevier Science B.V. All rights reserved.