Bulletin of the American Physical Society
2013 Annual Fall Meeting of the APS Ohio-Region Section
Volume 58, Number 9
Friday–Saturday, October 4–5, 2013; Cincinnati, Ohio
Session E2: Condensed Matter B |
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Room: Braunstein Hall 301 |
Saturday, October 5, 2013 8:00AM - 8:12AM |
E2.00001: Band Structure in ZnO Based 3D Photonic Crystals Donald Priour We calculate the band gap and dispersion curves for photonic crystals comprised of hexagonal arrays of ZnO nano-pillars with a periodic modulation of the refraction index along the axis of the pillars. Length scales of the intra-pillar refraction index variations and the unit cell of the underlying hexagonal lattice are in the optical range, on the order of 500 nm. We calculate photonic dispersion curves with a perturbative analysis where multidimensional integrals arising in the Rayleigh-Schr\"{o}dinger series are evaluated using adaptive Monte Carlo sampling in conjunction with a Fourier decomposition in the Brillouin Zone to mitigate singularities at the Brillouin zone boundaries. The Fourier decomposition also yields band frequencies throughout the Brillouin Zone with the same series. Results are validated by comparison with band structures obtained from an alternative technique for special lattice geometries. Parameters such at the pillar radius, the hexagonal lattice unit cell size, and the wavelength and amplitude of the refraction index modulations are varied to investigate the effect on salient features of the band structure such as the mean width and uniformity of the band gap. [Preview Abstract] |
Saturday, October 5, 2013 8:12AM - 8:24AM |
E2.00002: Fabrication of Microlens Arrays on Layered Polymers Using Gray Scale mask Tom Oder, Cory Merlo, Michael McMaster, Joshua Petrus, Camron Bagheri, Anthony Mazzocco, Michael Crescimanno Microlens arrays of different diameters ranging from 20 -- 80 micrometers were fabricated on layered polymers using dry etching. The patterns of the arrays were first transferred onto a thick photoresist that was spin-coated on the polymers by a photolithographic process employing a gray scale glass mask. The final transfer of the array patterns to the polymers was carried using dry etching in a home-made reactive ion etching system. The optimized etch condition included a mixture of sulfur hexafluoride and oxygen. The etching process successfully exposed the individual sub-micron thick layers in the polymers. Physical characterization of the microlens arrays was done using atomic force microscope and scanning electron microscope. We will also present preliminary data of the optical characterization of the arrays that has so far been carried. [Preview Abstract] |
Saturday, October 5, 2013 8:24AM - 8:36AM |
E2.00003: Nanoscale Pattern Generation Using Laser Interference Lithography Tom Oder, Andrew Smith, Joshua Petrus Nanotechnology, which encompasses research and development of devices at the atomic, molecular or macromolecular level in the length scale of 1-100 nanometers, has recently sparked a huge interest due to the exciting and novel potentials envisaged. At the nanometer regime, fascinating changes in the properties and functionalities of materials occur. Effective methods to fabricate these structures and to carefully and systematically study their properties in order to harness them into practical devices are required. We present here our investigation of laser interference lithography to fabricate nanometer size device patterns. The technique is a relatively simple way of fabricating nanometer structures. A 325 nm He-Cd laser source was used to expose thin photoresist coated on polymers and silicon substrates. Scanning electron microscopy and atomic force microscopy were used to characterize the resulting patterns. Waveguide patterns with widths as small as 120 nm, and 300 nm square array patterns were fabricated. We will discuss the possibilities this work opens in fabricating three dimensional photonic crystals on layered polymeric systems. [Preview Abstract] |
Saturday, October 5, 2013 8:36AM - 8:48AM |
E2.00004: Transient Rayleigh Spectroscopy measure of Carrier Dynamics in Wurtzite and Zincblende InP nanowires Yuda Wang, Mohammad Montazeri, Howard Jackson, Leigh Smith, Jan Yarrison-Rice, Tim Burgess, Chennupati Jagadish Pump-probe two color transient Rayleigh spectroscopy is used to study the carrier dynamics of the WZ and ZB InP nanowires. Both nanowires were grown by MOCVD with both diameters $\sim$100nm. Utilizing wavelength-tunable pulse laser generated by super-continuum fiber excited with Ti-Sapphire Laser and a time delay line adjusting the time difference between the pump and probe pulse, the change of the reflectivity as a function of excitation energy and time delay after pump can be measured from a single nanowire. The results are fitted based on the absorption coefficient calculated by a band to band transition model and index of refraction correlated by the Kramer-Kronig relation. The temperature of the electron-hole plasma(EHP) is cooled by the interaction with LO phonons and Acoustic phonons. The EHP-LO phonon cooling mechanism is dominating at early times from 0 to 200ps(ZB) and 500ps(WZ), after which the EHP-acoustic phonon interactions begin to contribute more. The carrier relaxation time constants were also fitted for different bands. The conduction bands of ZB and WZ InP show lifetimes of 1500ps and 2000ps, respectively. Both the Split-Off band in ZB InP and C band in WZ InP show slow relaxation time constants compared to the rest of the valence bands by a factor of 2-3. [Preview Abstract] |
Saturday, October 5, 2013 8:48AM - 9:00AM |
E2.00005: Photocurrent spectroscopy studies of single ZB GaAs, GaAs/AlGaAs core-shell and quantum well tube nanowires Bekele Badada, Leigh Smith, Howard Jackson, Jan Yarrison-Rice, Tim Burgess, Qiang Gao, Chennupati Jagadish We employ photocurrent spectroscopy technique to investigate the energy band structure of single bare GaAs, GaAs/AlGaAs core-shell nanowire and quantum well tube. These nanowires were gown by MOCVD techniques. A single nanowire device is fabricated using photolithography followed by deposition of Ti (20nm)/Al (300-500nm) as contacts on the ends of the nanowires. Photocurrent measurements were performed using a tunable CW Ti-Sapphire laser (775nm-890nm) and a broadly tunable (550-960 nm) pulsed excitation from a coherent super continuum photonic crystal fiber at a fixed bias across the nanowire. The photocurrent spectra obtained from single bare GaAs nanowires show the band gap energy of 1.42eV and 1.51eV at room (293K) and low (10K) temperatures respectively. At 10K a peak was observed near the band edge suggesting excitonic resonance. In GaAs/AlGaAs core-shell nanowire, we observe both core and shell absorptions which allow us to estimate the concentration of Al in such structures. The photocurrent spectra of quantum well tube show evidences of confined energy states in the quantum well in addition to the absorption of the GaAs core and the AlGaAs shells. [Preview Abstract] |
Saturday, October 5, 2013 9:00AM - 9:12AM |
E2.00006: Optical investigation of GaAs/AlGaAs heterostructure nanowires Teng Shi, Howard Jackson, Leigh Smith, Jan Yarrison-Rice, Bryan Wong, Nian Jiang, Qiang Gao, Hoe Tan, Chennupati Jagadish, Joanne Etheridge A set of GaAs/AlGaAs heterosturcture nanowires which contain quantum well tubes (QWTs) with AlGaAs layers on both sides wrapped around a 50nm GaAs core are grown by Au-seeded MOCVD. Individual nanowires are studied by low temperature photoluminescence (PL) experiments, which give evidence of increasing quantum confinement with decreased QW growth time. Using structural and alloy concentration information obtained from high resolution transmission electron microscope measurements, we are able to carry out an eigenfunction expansion calculation using a cylindrical QW. By employing this theoretical modeling, we can calculate the QW widths from the ground state emission of these QWTs. A linear relationship between the QW growth time and QW width is found. Localized quantum dots in very narrow GaAs QWTs are also observed. In addition, spatially-resolved PL measurements show that these localized states are randomly distributed along the long axis of the nanowire. [Preview Abstract] |
Saturday, October 5, 2013 9:12AM - 9:24AM |
E2.00007: Andreev Reflection Studies in GaMnAs/Nb Micro-Structures Hussein AbuJeib, Diana Dahliah, Justin Guenther, Khalid Eid, Xinyu Liu, Jacek Furdyna The ability to measure the electron spin polarization of ferromagnetic materials at the interface with non-ferromagnetic materials is essential for spintronics applications. The spin polarization at ferromagnetic/non-ferromagnetic interfaces can be very different from the bulk spin polarization due to the surface states. Andreev spectroscopy has the ability of estimating the spin polarization for ferromagnetic materials. We use the Circular Transfer Line Method (CTLM) [1-2] to measure the Andreev reflection effect at GaMnAs/superconductor interface and to extract GaMnAs spin polarization. This technique works well for high-resistivity films and has other advantages over point contact and planar Andreev effect measurement geometries. We will present our recent results and contrast them with previous work, where we easily eliminate the bulk resistance contribution and the broadening of the superconducting gap. Furthermore, we found that a Schottky barrier is likely to form at the interface unless the interface is cleaned carefully and a high quality sample is used. This Schottky barrier can be confused as an actual Andreev effect. [Preview Abstract] |
Saturday, October 5, 2013 9:24AM - 9:36AM |
E2.00008: Giant Magnetoresistance in GaMnAs/Py Bilayers Justin Guenther, Robert Tolley, Hussein Abujeib, Taylor Reid, A. Sokolov, X. Liu, J.K. Furdyna, K.F. Eid Giant magnetoresistance (GMR) requires a trilayer, typically consisting of a diamagnetic layer sandwiched between two ferromagnetic layers. The middle layer eliminates coupling between the ferromagnetic ones. Using a ferromagnet bilayer might be an advantageous alternative, so it is important to study GMR effect in such bilayers. We fabricated and measured the magnetoresistance of GaMnAs/Py bilayer micro-structures, two ferromagnets which do not couple magnetically. Our device geometry (the circular transfer line method) eliminates the bulk contribution to resistance, which allows us to study the interface magnetoresistance. Contrary to reports in literature (S. Mark et al., PRL 103, 017204, 2009), our results show a measurable coupling between Py and GaMnAs. This is manifested when we replace the Py layer with a superconducting Nb layer. Furthermore, we conclude that the observed magnetoresistance effect is due to anisotropic magnetoresistance in bulk GaMnAs rather than the interface. [Preview Abstract] |
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