Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session P32: Optical Properties and Dynamics of Quantum Dots and Quantum Wells |
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Sponsoring Units: DCMP Chair: Sam Carter, Naval Research Laboratory Room: C144 |
Wednesday, March 23, 2011 8:00AM - 8:12AM |
P32.00001: Sub micron scale patterning of material optical response through focused ion beam induced InAs/GaAs quantum dot nucleation Timothy Saucer, Jieun Lee, Andrew Martin, Deborah Tien, Joanna Mirecki-Millunchick, Vanessa Sih We report on the technique of using a focused ion beam to produce preferential sites for InAs/GaAs quantum dot nucleation. We mill an array of holes in the GaAs substrate and then deposit a thin layer of InAs below the critical thickness for dot formation in unpatterned areas. The array of holes on the substrate act as preferential nucleation sites and induce quantum dot formation only in the patterned regions. We conduct photoluminescence spectroscopy in a templated multilayer quantum dot sample at temperatures down to 10K and for various patterning conditions. We find that outside of our patterning regions we have no quantum dot luminescence, indicating that the patterning modifies the optical response of the material. We find that we can control this quantum dot formation down to array spacings of 250nm, showing excellent potential for this technique to be used for sub micron spatial control of a material's optical properties. [Preview Abstract] |
Wednesday, March 23, 2011 8:12AM - 8:24AM |
P32.00002: Photoluminescence imaging of Focused-Ion-Beam induced individual quantum dots Jieun Lee, Timothy Saucer, Andrew Martin, Deborah Tien, Joanna Millunchick, Vanessa Sih Quantum dots are nanostructures that confine electrons in 3 spatial dimensions. Due to their discrete atom-like energy levels, a wide variety of applications related to the optical properties of dots are possible. One such application is to integrate quantum dots in optical nanocavities for the enhanced interaction between electrons and photons. However, self-assembled dots typically nucleate at random locations, hindering the accurate coupling between the dot and cavity. Therefore, spatial control on self-assembled dots at the fabrication level is highly desirable. Here, we report on optical measurements conducted on InAs quantum dots that are prepatterned in a square array by a focused-ion-beam. Using scanning confocal microscopy, we spatially map the photoluminescence of individual quantum dots. Single dot luminescence with 160 $\mu $eV linewidth is observed indicating good optical quality and statistical analysis over 16 array sites show reasonable placement accuracy and emission inhomogeneity. [Preview Abstract] |
Wednesday, March 23, 2011 8:24AM - 8:36AM |
P32.00003: Strongly confined excitons in self-assembled InGaAs quantum dot clusters Megan Creasey, Xiaoqin Li, Jihoon Lee, Zhiming Wang, Gregory Salamo Quantum dot clusters (QDCs) consisting of regular geometric patterns of six InGaAs quantum dots (QD) are grown on a GaAs substrate using a hybrid growth method that combines droplet homoepitaxy and Stranski-Krastonov growth. These novel structures have potential applications as tunable single photon sources, entangled photon sources, or error corrected qubits - devices critical to the fields of secure optical communications and quantum computing We study the photoluminescence arising from a single cluster using both continuous wave and ultrafast spectroscopic techniques with variations in the sample temperature and excitation power. Our results suggest excitons (bound electron-hole pairs) are strongly confined within the individual QDs rather than loosely confined throughout the entire QDC. The work at Texas is supported financially by NSF, ARO, AFOSR, ONR, the Welch Foundation, and the Alfred Sloan Foundation. The work at Arkansas is supported by the NSF. [Preview Abstract] |
Wednesday, March 23, 2011 8:36AM - 8:48AM |
P32.00004: Selective control of polarized luminescence from GaN/AlN self-assembled quantum dots Daniel Rich, Ofer Moshe, Benjamin Damilano, Jean Massies GaN/AlN self-assembled quantum dots (QDs) were grown by the Stranski-Krastanov method on Si(111) using molecular beam epitaxy. During the subsequent cooling from growth temperatures, the thermal expansion coefficient mismatch between the Si substrate and GaN/AlN film containing vertically stacked QDs leads to an additional biaxial tensile stress at the Si/III-Nitride interface. We have modified the thermal stress in the QD layers by etching stripes of varying widths using inductively coupled Cl/Ar plasma reactive ion etching. The results show that a suitable choice of stripe width and orientation can create regions of in-plane uniaxial stress ranging from 20-30 kbar which enables a selective and local control of polarized emission from the QDs. Localized cathodoluminescence (CL) spectroscopy of the QDs exhibits emissions from both the ground and excited states, whose relative contributions depend on the level of excitation and temperature. We have studied these emissions using time- and polarization-resolved CL for ensembles of QDs. The effects of screening of the polarization field in the QD, state-filling, changes in the polarization anisotropy and lifetime with varying excitation were studied experimentally and modeled with a self-consistent 6x6 k.p calculation method. [Preview Abstract] |
Wednesday, March 23, 2011 8:48AM - 9:00AM |
P32.00005: Polarization states of charged excitons in coupled InAs/GaAs quantum dot molecules Ramana Thota, Swati Ramanathan, Kushal Wijesundara, Eric Stinaff, Allan Bracker, Dan Gammon The polarization state of charged excitons in coupled InAs/GaAs dots can reveal useful information about the spin state of its charge carriers. In this study, we examine the complete polarization state through Stokes parameter measurements to relate the polarization parameters of the luminescence to the spin configurations of the various charged excitons they originate from. We demonstrate that this method is a useful tool to identify and possibly create spin states for quantum computation applications. [Preview Abstract] |
Wednesday, March 23, 2011 9:00AM - 9:12AM |
P32.00006: Resonant Fluorescence from Quantum Dot Molecular Excitonic Transitions Mark Kerfoot, Allan Bracker, Daniel Gammon, Michael Scheibner Quantum dot molecules formed by two vertically stacked quantum dots are a rich testing ground for basic concepts regarding the measurement and control of quantum states. The well defined geometry is ideal for studying interaction mechanisms, such as the interaction of two dipoles each located in one of the quantum dots of the quantum dot molecule. A prerequisite for doing so is the ability to detect the interaction mediated changes in the properties of the individual, uncoupled quantum dots. Here we use resonant fluorescence to study exciton transitions in quantum dot molecules. We measure the photoluminescence of the same transition we optically excite with a narrow band laser. With this method, features on the scale of the homogeneous line width of the intradot exciton transition are well resolved. This enables us to study the fine-structure of different charge and spin configurations with high sensitivity. [Preview Abstract] |
Wednesday, March 23, 2011 9:12AM - 9:24AM |
P32.00007: Exchange-controlled spin dynamics in coupled quantum dots Eric A. Stinaff, Kushal C. Wijesundara, Allan Bracker, Dan Gammon We measure circular polarization memory of neutral exciton states with polarization dependent photoluminescence spectra. As a consequence of anisotropic exchange interaction a low degree of circular polarization memory was observed in the spatially direct and indirect excitons where they anticross. With applied electric field as we tune the excitonic emission from intra-dot to inter-dot the electron-hole wave function overlap reduces and we observe an increase in polarization memory due to reduced exchange interaction. We observe a sudden unexpected dip in circular polarization memory of the spatially indirect exciton state that is coincident with the applied field where the single hole level resonance is observed. Possible mechanisms for this loss of circular polarization memory will be presented. [Preview Abstract] |
Wednesday, March 23, 2011 9:24AM - 9:36AM |
P32.00008: Two-body and three-body interactions in phonon-assisted exciton energy transfer between quantum dots Kaijie Xu, Carlo Piermarocchi We theoretically study the dynamics of exciton energy transfer between semiconductor quantum dots. Phonons play a critical role in the exciton energy transfer process when the energy of the dots involved in the process is different. We find that the phonon-assisted energy transfer cannot be correctly described by two-body exciton-photon and exciton-phonon interactions if each dot is modeled as a single-level exciton system. Higher excited levels of the exciton state have to be included to properly describe the phonon-assisted process. However, excited states can be traced out by introducing a single-level exciton model with an effective three-body exciton-photon-phonon coupling term. The three-body term describes a change in the exciton-photon dipolar coupling due to phonon-induced deformations of the ground exciton wavefunction. The multi-level exciton model with two-body interactions and the single-level exciton model with three-body interaction reproduce the same exciton transfer rates to the leading order contributions of perturbation theory. [Preview Abstract] |
Wednesday, March 23, 2011 9:36AM - 9:48AM |
P32.00009: ABSTRACT WITHDRAWN |
Wednesday, March 23, 2011 9:48AM - 10:00AM |
P32.00010: Optical lattices for electrons in semiconductors Carlo Piermarocchi, Michael G. Moore, Martin J. A. Schuetz, Monique Combescot We theoretically investigate the trapping of electrons in a semiconductor using counter-propagating laser beams. We consider two different physical mechanisms that can lead to an efficient electron trapping: (a) Pauli blocking between the electron and a virtual exciton coupled to the laser field, and (b) the virtual excitation of a three-body Coulomb resonance corresponding to a bound charged exciton state (a trion). Both processes induce a momentum transfer between photon and electron, and lead to a sinusoidal trap for electrons with a period determined by the laser beam modulation. The depth of the potential is proportional to the laser intensity and inversely proportional to the exciton-photon detuning. Competing effects such as laser heating, phonons, and disorder are analyzed. [Preview Abstract] |
Wednesday, March 23, 2011 10:00AM - 10:12AM |
P32.00011: Many-body two-quantum coherences in 2DFT spectra of semiconductors Denis Karaiskaj, A. Bristow, X. Dai, L. Yang, S. Mukamel, R. Mirin, S. Cundiff Investigating the correlations of multiple excitons in semiconductors is a challenging many-body problem that has drawn considerable experimental and theoretical attention over the last two decades. Nonlinear four-wave mixing (FWM) experiments have long been known to provide direct probes for the many-body effects in the ultrafast dynamics of excitons in quantum wells. However, it is very difficult to separate the different contributions such as excitation induced dephasing, excitation induced shift, local field effects, and multiple exciton correlations. With the advent of two-dimentional Fourier-transform (2DFT) spectroscopy, the biexcitonic contributions could be isolated and the many-body contributions could be identified. Phase-resolved 2DFT spectra for the negative delay FWM signal will be presented which show interesting diagonal and off-diagonal peaks. The energy positions, line shapes, and the complexity of the 2D peaks indicate significant many-body coherences and reinforce the ability of 2DFT to disentangle two-quantum transitions (D. Karaiskaj, \textit{et al}., Phys. Rev. Lett. \textbf{104}, 117401 (2010)). [Preview Abstract] |
Wednesday, March 23, 2011 10:12AM - 10:24AM |
P32.00012: Electron transfer and relaxation dynamics in heterovalent ZnSe/GaAs quantum well structures Amit Dongol, Hans Peter Wagner We investigate the electron transfer and relaxation dynamics in heterovalent ZnMgSe/ZnSe quantum wells (QW's) grown on GaAs using the nonlinear optical method of three-beam degenerate four-wave-mixing (FWM). We use ultra-short (90 fs) laser pulses with non-collinear wave-vectors \textbf{k}$_{1}$, \textbf{k}$_{2}$ and \textbf{k}$_{3}$ at a center wavelength of 441 nm ($\sim $2.81 eV) which is resonantly tuned to the heavy hole exciton transition energy at 25 K. In the experiment the time coincident strong pump pulses \textbf{k}$_{1}$ and \textbf{k}$_{2}$ creates both an exciton density grating in the QW and an electron-hole pair grating in the GaAs while the delayed weak pulse \textbf{k}$_{3}$ simultaneously probes the exciton lifetime T$_{1}$ as well as the electron grating injection time T$_{t}$ from the substrate into the QW. Intensity dependent experiments reveal that the diffraction efficiency due to the electron grating increases faster with increasing \textbf{k}$_{1}$ and \textbf{k}$_{2}$ pulse intensities than the FWM efficiency due to the generated exciton density grating. This behavior which is attributed to exciton bleaching at high intensities enables the discrimination of times T$_{1}$ and T$_{t}$, both being in the order of a few tens of picoseconds. [Preview Abstract] |
Wednesday, March 23, 2011 10:24AM - 10:36AM |
P32.00013: Excitons in moving lattices Jason Leonard, Alexander Winbow, Mikas Remeika, Yuliya Kuznetsova, Alexander High, Aaron Hammack, Leonid Butov, Joseph Wilkes, Alrun Guenther, Alexander Ivanov, Micah Hanson, Arthur Gossard We report on the study of indirect excitons in moving lattices--conveyers--created by a set of AC voltages applied to the electrodes on the sample surface. The wavelength of this moving lattice is set by the electrode periodicity, the amplitude is controlled by the applied voltage and the speed is controlled by the AC frequency. We probed the conveyer speeds from well below to well above the sound velocity. We observed the dynamical localization-delocalization transitions for excitons in the conveyers and measured its dependence on the exciton density and conveyer speed and amplitude. We also developed the theory of exciton transport via conveyers. [Preview Abstract] |
Wednesday, March 23, 2011 10:36AM - 10:48AM |
P32.00014: Effect of growth kinetics on intersubband transitions in GaN/AlN multiple quantum wells J. Yang, S.D. Carnevale, T.F. Kent, M.R. Brenner, R.C. Myers The large conduction band offset of nearly 2 eV between GaN and AlN provides very large electron confinement that could be useful for ultrafast intersubband--based photonics operating at telecommunications wavelengths. However, it is difficult to control interface roughness and compositional profiles with monolayer precision, which is crucial for engineering sublevels for quantum cascaded intersubband photonics. Here we examine the effect of Ga-rich and N-rich growth conditions of highly-confined GaN/AlN multiple quantum wells prepared by plasma-assisted molecular beam epitaxy. Structural quality is examined through high-resolution x-ray diffraction and atomic force microscopy. The efficiency of intersubband and interband transitions in these heterostructures is measured using temperature dependent absorption and photoluminescence spectroscopy. [Preview Abstract] |
Wednesday, March 23, 2011 10:48AM - 11:00AM |
P32.00015: Trions and quatrons in Semiconductor Coupled Quantum Wells Roman Ya. Kezerashvili, Oleg L. Berman The three-body restricted problem for trions, when a spatially separated exciton and electron or hole are located in the parallel quantum wells (QW), is reduced to the 2D two body problem for the exciton and the projection of the electron or hole on the plane of the excitonic QW. In the limit of a large spatial separation of the QWs the eigenfunctions and energy spectrum for the trions are obtained analytically. It is shown that the Schr\"odinger equation for the trion can be reduced to the 2D two-body problem with Coulomb electron-hole interaction for the 2D direct exciton and the Schr\"odinger equation for the 2D harmonic oscillator for the relative motion of the exciton and the image of the projection of the electron or hole on the plane of the quantum well with the exciton. The 2D Wigner crystallization of the trions in the coupled QWs is discussed. The four-body restricted problem for spatially separated exciton and electron and hole, located in the in three parallel QWs, is reduced to the 2D three body problem for the exciton and the projection of the electron and hole on the plane of the excitonic QW. In the limit of a large spatial separation of the QWs the eigenfunctions and energy spectrum for quatron formed by the exciton and electron and a hole are obtained analytically. The 2D superfluidity and Kosterlitz-Thouless phase transition in the dilute Bose gas of quatrons is discussed. [Preview Abstract] |
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