Single LP0,n mode excitation in multimode fibers

We analyze the transmission of a Single mode - Multimode - Multimode (SMm) fiber structure with the aim of exciting a single radial mode in the second multimode fiber. We show that by appropriate choice of the length of the central multimode fiber one can obtain > 90% of the total core power in a chosen mode. We also discuss methods of removing undesirable cladding and radiation modes and estimate tolerances for practical applications. (C) 2014 Optical Society of Americ

Temperature dependence of the lowest excitonic transition for an InAs ultrathin quantum well

Temperature dependent photoluminescence and photoreflectance techniques are used to investigate the lowest excitonic transition of InAs ultrathin quantum well. It is shown that the temperature dependence of the lowest energy transition follows the band gap variation of GaAs barrier, which is well reproduced by calculated results based on the envelope function approximation with significant corrections due to strain and temperature dependences of the confinement potential. A redshift in photoluminescence peak energy compared to photoreflectance is observed at low temperatures. This is interpreted to show that the photoluminescence signal originates from the recombination of carriers occupying the band-tail states below the lowest critical point. (c) 200

Linear and nonlinear second-order polarizabilities of hemispherical and sector-shaped metal nanoparticles

In this paper, we present results of calculations of linear and second-order nonlinear polarizabilities of sector-shaped metallic nanoparticles (hemisphere is a special case) using free electron theory. The dependences of the ground state electron density distribution and polarizabilities on various shape parameters of sector are analyzed. The ground state electron densities near the corners and edges of sector-shaped nanoparticle are very low and do not contribute to the linear and second-order polarizabilities. The second-order polarizability is found to depend strongly on the angle of the sector and is shown to be proportional to the product of an appropriately defined asymmetric volume of the particle and the third power of the electron cloud length

Re-examination of the SiGe Raman spectra: Percolation/one-dimensional-cluster scheme and ab initio calculations

We report on the detailed assignment of various features observed in the Raman spectra of SiGe alloys along the linear chain approximation (LCA), as achieved based on remarkable intensity interplays with composition between such neighboring features known from the literature but which so far have not been fully exploited. Such an assignment is independently supported by ab initio calculation of the frequencies of bond-stretching modes taking place in different local environments, which we define at one dimension (1D) for consistency with the LCA. Fair contour modeling of the SiGe Raman spectra is eventually obtained via a so-called 1D-cluster version of the phenomenological (LCA-based) percolation scheme, as originally developed for zincblende alloys, after ab initio calibration of the intrinsic Si-Si, Si-Ge, and Ge-Ge Raman efficiencies. The 1D-cluster scheme introduces a seven-oscillator [1 x (Ge-Ge), 4 x (Si-Ge), 2 x (Si-Si)] Raman behavior for SiGe, which considerably deviates from the currently accepted six-oscillator [1 x (Ge-Ge), 1 x (Si-Ge), 4 x (Si- Si)] behavior. Different numbers of Raman modes per bond are interpreted as different sensitivities to the local environment of Ge-Ge (insensitive), Si-Si (sensitive to first neighbors), and Si- Ge (sensitive to second neighbors) bond stretching. The as-obtained SiGe 1D-cluster/percolation scheme is also compared with the current version for zincblende alloys, using GaAsP as a natural reference. A marked deviation is concerned with an inversion of the like phonon branches in each multiplet. This is attributed either to the considerable Si and Ge phonon dispersions (Si-Si doublet) or to a basic difference in the lattice relaxations of diamond and zincblende alloys (Si-Ge multiplet). The SiGe vs GaAsP comparison is supported by ab initio calculation of the local lattice relaxation/dynamics related to prototype impurity motifs that are directly transposable to the two crystal structures

Growth of strained ZnSe layers on GaAs substrates by pulsed laser deposition carried out in an off-axis deposition geometry

We have deposited thin layers of ZnSe on (001) oriented GaAs substrates by pulsed laser deposition at different incident laser fluence (referred to as normal geometry) and in an off-axis geometry where the plasma plume direction is at an angle of similar to 25 degrees away from the direction of the substrate. The crystalline quality of these layers has been studied by high-resolution X-ray diffraction measurements and Raman scattering. We find that we are in a position to deposit pseudomorphic strained layers of ZnSe on GaAs in the off-axis deposition geometry when the ZnSe layer thickness is less than the critical thickness of ZnSe on GaAs i.e. 150 nm. Secondary ion mass spectroscopy, scanning electron microscopy, photoluminescence and electrical transport measurements have also been carried out in all the ZnSe layers and the results of all the above characterizations have been compared for the normal geometry and the off-axis geometry of deposition. All the results indicate that the ZnSe layers deposited in the off-axis geometry have better crystalline quality and an improved interface as compared to the ones deposited in the normal geometry. We attribute this improvement in the overall quality of the ZnSe layers in the off-axis geometry to the reduction in the average energy of the plume particles that reach the GaAs substrate in the off-axis geometry. (C) 200