Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session W11: Electronic Structure: Thermodynamics and Optical Properties |
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Sponsoring Units: FIAP Room: D222 |
Thursday, March 24, 2011 11:15AM - 11:27AM |
W11.00001: Comparison of Experimental and Theoretical Vibrational Spectra for Pentacene Derivatives Gregory Maslak, Mark Stewart, Lillie Ghobrial, Wes Laurion, Jinyue Jiang, Li Tan, Carolina C. Ilie The practical use of pentacene in the area of organic field-effect transistors is limited by its sensitivity to oxygen and poor solubility in organic solvents. To overcome these disadvantages, new organic semiconductors as 2,3,9,10-tetrakis(3,5-di-t-butylphenylethynyl)-6,13- bis (trimethylsilylethynyl) pentacene are synthesized. The new pentacene derivatives may be useful for electronic devices such as organic field-effect transistors or organic light-emitting diodes. Here we compare the calculated vibrational spectra to the experimental data in order to characterize the new derivative. The methods and similarities between the theoretical calculations and the experimental data are discussed. [Preview Abstract] |
Thursday, March 24, 2011 11:27AM - 11:39AM |
W11.00002: Optical characterization of p-doped InP epitaxial layers in mid and far infrared region R.C. Jayasinghe, Y.F. Lao, A.G.U. Perera, M. Hammar, C.F. Cao, H. Wu The optical properties of p-doped Indium Phosphide (InP) epitaxial thin films with 1, 3, and 24 $\times $ 10$^{18}$ cm$^{-3}$ carrier concentrations were investigated by infrared reflection, transmission, and absorption measurements in 5 - 40 $\mu $m wavelength range. The absorption spectra were modeled by complex dielectric function using the classical Lorentz--Drude model. The phonon absorption in InP was modeled using eight Lorentzian oscillators. This method gives a straightforward approach for modeling the experimental absorption spectra when compared to the two-phonon absorption spectroscopy technique. The calculated spectra are in a good agreement with experimental spectra. The effects of doping on fitting parameters are also investigated. [Preview Abstract] |
Thursday, March 24, 2011 11:39AM - 11:51AM |
W11.00003: Photoluminescence intensity oscillations with magnetic field in InGaAs quantum wells Lars Schweidenback, Andreas Russ, Tariq Ali, Joseph Murphy, Alexander Cartwright, Athos Petrou, Alexander Govorov, Connie Li, Aubrey Hanbicki, Berend Jonker, George Kioseoglou We have observed magnetic field oscillations in the photoluminescence (PL) intensity from InGaAs quantum wells (QWs) with indium compositions of 5{\%} and 15{\%} with laser excitation close to the bandgap for temperatures $<$ 20 K. For all samples the intensity maxima occur at 2.2 and 4.5 tesla when the magnetic field is applied perpendicular to the QW plane. Experiments in which the sample normal (z-axis) is tilted with respect to the applied magnetic field $B$ show that the PL intensity maxima positions depend on the magnetic field component $B_{z}$. Time-resolved PL comparison with GaAs QWs yields much longer recombination times for the InGaAs QWs. Furthermore, cross sectional scanning tunneling microscopy studies indicate the formation of Indium rich InGaAs clusters in the InGaAs QWs. We interpret the observed oscillations in terms of the Aharonov-Bohm effect and quasi-indirect excitons with ring-like trajectories of carriers. The oscillation period corresponds to orbits with radius equal to 24 nm. [Preview Abstract] |
Thursday, March 24, 2011 11:51AM - 12:03PM |
W11.00004: Optical two-dimensional Fourier transform spectroscopy of single GaAs quantum wells Yuri D. Glinka, Zheng Sun, Xiaoqin Li, Allan Bracker Optical two-dimensional Fourier transform spectroscopy is applied to study the coherent coupling between light-hole and heavy-hole excitons in single GaAs quantum wells instead of those consisted of ten or four periods of GaAs separated by Al$_{0.3}$Ga$_{0.7}$As barriers measured previously. The effect of the confinement energy as well as Coulomb and disorder correlation lengths on coherent coupling dynamics is discussed. The financial support from ARO, NSF, and Welch foundation is gratefully acknowledged. [Preview Abstract] |
Thursday, March 24, 2011 12:03PM - 12:15PM |
W11.00005: Origin of the Terahertz Absorption Peak in Single-Walled Carbon Nanotubes Qi Zhang, Lei Ren, H. Gojuki, E.H. Haroz, T. Arikawa, J. Kono, C.L. Pint, R.H. Hauge, A.K. Wojcik, A.A. Belyanin Single-walled carbon nanotubes (SWNTs) are promising for high-frequency electronics and terahertz (THz) applications, as well as for fundamental studies of finite-frequency dynamics of one-dimensional electronics. Previous studies of dynamic conductivities of various types of SWNTs have revealed a pronounced and broad absorption peak around 4 THz, whose origin has been a matter of controversy. Both the effects of curvature-induced band gaps and plasmonic absorption due to finite lengths have been proposed to be important, but a consensus has not emerged. We have studied 4THz peak in highly aligned and length-controlled SWNT films and metallically-enriched SWNT films through FTIR and THz time-domain spectroscopy. We provide evidence that this peak is observable only when the THz polarization is parallel to the nanotubes and only in metallic tubes. We will discuss the origin of this absorption peak in light of these new findings. [Preview Abstract] |
Thursday, March 24, 2011 12:15PM - 12:27PM |
W11.00006: UV light emission from ZnO nanostructures in SiO$_{2}$ synthesized by ion implantation and thermal annealing Bimal Pandey, Akhilesh Singh, Prakash Poudel, Arup Neogi, Duncan Weathers Zinc Oxide (ZnO) nanostructures were synthesized by the implantation of low energy (35 keV) ZnO molecular ions into thermally grown SiO$_{2}$ at a fluence of 5 $\times $ 10$^{16}$ ions/cm$^{2}$. Implanted samples were annealed in an oxygen environment to allow the growth of ZnO precipitates. X-ray photoelectron spectroscopy (XPS), Fourier transform spectroscopy (FTIR) and energy dispersive x-ray spectroscopy (EDS), confirm the formation of ZnO. High resolution transmission electron microscopy (HRTEM) shows the formation of nanostructures having diameters ranging from 2 nm to 5 nm in the SiO$_{2}$. Photoluminescence (PL) measurements show excitonic and band-edge emission in the ultraviolet region at temperatures ranging from 4 K - 300 K. Time-resolved PL measurements performed at 4K showed an electron-hole recombination lifetime on the order of a few hundred picoseconds. [Preview Abstract] |
Thursday, March 24, 2011 12:27PM - 12:39PM |
W11.00007: Ab initio calculation of indirect light absorption by free carriers in transparent conducting oxides Hartwin Peelaers, Emmanouil Kioupakis, Chris G. Van de Walle Phonon-assisted absorption of light by free carriers is an important optical process in many materials and a challenging problem for computational condensed-matter physics. As transparent conducting oxides play an important role as contacts to light emitters and photovoltaic devices, it is important to consider not only the optical absorption across the band gap, but also the absorption by free carriers due to indirect processes mediated by phonons and defects. Here we calculate these losses using a full ab initio methodology as opposed to a phenomenological Drude model containing empirical parameters. These calculations involve the electron-phonon coupling matrix elements, which are dominated by the longitudinal optical phonon modes. We also compare with the Fr\"ohlich model, which describes the electron-phonon matrix elements in the long-wavelength limit. [Preview Abstract] |
Thursday, March 24, 2011 12:39PM - 12:51PM |
W11.00008: Optical absorption of light carrying orbital angular momentum by semiconductors: free-particle quantum kinetics P.I. Tamborenea, G.F. Quinteiro We develop a free-carrier theory of the optical absorption of light carrying orbital angular momentum (twisted light) by bulk and quasi-two-dimensional semiconductors. We obtain the optical transition matrix elements for Bessel-mode twisted light and use them to calculate the wave function of photo-excited electrons to first-order in the vector potential of the laser [1]. We then pose the problem of the quantum kinetics of interband transitions in terms of the Heisenberg equations of motion of the electron populations, and interband and intraband coherences [2]. We solve the equations of motion in the low-excitation regime, and obtain analytical expressions for the coherences and populations; with these, we calculate the orbital angular momentum transferred from the light to the electrons and the paramagnetic and diamagnetic electric current densities. \\[4pt] [1] G.\ F.\ Quinteiro and P.\ I.\ Tamborenea, EPL {\bf 85}, 47001 (2009).\\[0pt] [2] G.\ F.\ Quinteiro and P.\ I.\ Tamborenea, Phys.\ Rev.\ B {\bf 82}, 125207 (2010). [Preview Abstract] |
Thursday, March 24, 2011 12:51PM - 1:03PM |
W11.00009: Optical harmonics generation in semiconductor quantum dots: A tunable terahertz source Yan Xie, Weidong Chu, Suqing Duan, Wei Zhang The high-order harmonic generation (HHG) study have been extended to semiconductor quantum dots (QDs) and coupled QDs (CQDs), the so-called ``artificial atoms and molecules.'' One motivation of the study of the HHG in QDs is to find an efficient way of terahertz wave generation due to their controllable energy spectra and wave functions. With the help of Floquet theory, we show that the HHG in quantum dot structures can be changed from only odd orders to both odd and even orders by controlling the coupling parameters. The selection rules of the odd-even HHG in a noninversion-symmetric multilevel system are determined by the parity of emitted photon numbers during allowable virtual steps. On the other hand, by mapping the optical problem to a transport problem, we find that the terahertz generation efficiency is determined by the bandwidth of the quasienergy spectrum. Our studies are useful for engineering tunable terahertz sources based on semiconductor quantum dots. [Preview Abstract] |
Thursday, March 24, 2011 1:03PM - 1:15PM |
W11.00010: Ultrafast Quantum Control in Semiconductor Nanostructures using Twisted Light Guillermo Quinteiro, Pablo Tamborenea, Jamal Berakdar We investigate possible uses of twisted light (TL) ---or light carrying orbital angular momentum (OAM)--- as a tool to control semiconductor-based nanostructures. Two systems are considered, namely quantum dots (QD) and quantum rings (QR). For both structures we employ a simplified two-band model in the effective-mass approximation, having a conduction and a heavy-hole valence bands. In the case of disk-shaped QDs, we predict that the TL would allow to induce optical transitions which are normally regarded to be forbidden. The OAM $l$ and other parameters of the TL beam can be used to precisely control the final state of the electron. In the case of QRs, we study induced electric currents. We analyze the evolution of the system in terms of Heisenberg equations of motion. We find an analytical solution that resembles the standard Optical Bloch Equations. Using this solution, we find the evolution of the $z$-component of OAM and the electric current circulating the ring. Our results indicate that the electric current could be as large as $\mu$A, in the time-scale of pico-seconds. For an appropriate radius of the ring, the photo-induced magnetic field would be large enough to switch within picoseconds the magnetic moment of particles placed within the ring. [Preview Abstract] |
Thursday, March 24, 2011 1:15PM - 1:27PM |
W11.00011: Far-infrared absorption of PbSe nanorods Byung-Ryool Hyun, Adam Bartnik, Weon-kyu Koh, Nikolay Agladze, Jun Yang, Al Sievers, Christopher Murray, Frank Wise The far-infrared absorption spectra of PbSe nanodots and nanorods are measured as a function of aspect ratio, and show the expected splitting of the single Frohlich sphere mode in nanocrystals into two modes parallel and perpendicular to the nanorod axis. We analyze this splitting by modeling the dependence of the nanocrystal's local field factor on its shape. Excellent agreements is found with the features measured in experiment. We predict that this shape-dependent local field factor will cause a two-order of magnitude increase of the third-order susceptibility of long nanorods in the near-infrared. [Preview Abstract] |
Thursday, March 24, 2011 1:27PM - 1:39PM |
W11.00012: Bright spot pattern generation in GaAs/AlGaAs multiple quantum wells Angelo Mascarenhas, Brian Fluegel, D.W. Snoke Exciton photoluminescence pattern generation is investigated in multiple quantum wells. High-contrast outer rings and localized bright spots are generated using efficient field-assisted upconversion of laser light whose photon energy lies below the energy of the luminescing quantum well transition. Time-resolved images of the bright spot reveal that the ring transients are driven by carrier diffusion both from the laser excitation spot as well as from the bright spot. These dynamics are not explained simply by two-dimensional rate equations for generation and diffusion. The behavior must be understood as a result of three-dimensional transport in the vertically extended samples. [Preview Abstract] |
Thursday, March 24, 2011 1:39PM - 1:51PM |
W11.00013: Nonlinear optical properties of polychlorotriphenylmethyl radicals: a computational study Claudia Cardoso, Bruce Forbes Milne, Fernando Nogueira The special interest in molecular design for the development of novel second-order nonlinear optical (SONLO) materials is driven by their potential applications in new optoelectronic technologies. Various strategies are currently employed to enhance the molecular SONLO activity, and studies revealed that species having open-shell electronic states exhibit larger $\beta$ values than analogous closed-shell systems. Their open-shell electronic structure leads to accessible low-lying charge transfer electronic states which enhance the $\beta$ values with respect to their closed-shell counterparts. We present the results of a Density Functional Theory calculations of a series of substituted polychlorinated triphenylmethyl radicals. This family of radicals has been measured by Ratera et al using Hyper-Rayleigh Scattering and showed to have enhanced $\beta$ values. The present study compares results obtained within LDA and several hybrid functionals, namely long-range corrected functionals. The effect of solvents was also considered through the use of the polarizable continuum model. [Preview Abstract] |
Thursday, March 24, 2011 1:51PM - 2:03PM |
W11.00014: Binding energies of indirect excitons in double quantum well systems Alex Rossokhaty, Stefan Schmult, Werner Dietsche, Klaus von Klitzing, Igor Kukushkin A prerequisite towards Bose-Einstein condensation is a cold and dense system of bosons. Indirect excitons in double GaAs/AlGaAs quantum wells (DQWs) are believed to be suitable candidates. Indirect excitons are formed in asymmetric DQW structures by mass filtering, a method which does not require external electric fields. The exciton density and the electron-hole balance can be tuned optically. Binding energies are measured by a resonant microwave absorption technique. Our results show that screening of the indirect excitons becomes already relevant at densities as low as ~5$\times$10$^{9}$cm$^{-2}$ and results in their destruction. [Preview Abstract] |
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