Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session A9: Focus Session: Optics of Nanostructures -- Quantum Dots I |
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Sponsoring Units: DMP Room: A105 |
Monday, March 15, 2010 8:00AM - 8:36AM |
A9.00001: Entangled photons via dressed states Invited Speaker: |
Monday, March 15, 2010 8:36AM - 8:48AM |
A9.00002: Heterostructure quantum wires as an ideal source of entangled photon pairs Ranber Singh, Gabriel Bester We show [1] that based on symmetry arguments, {\it idealised} quantum nanostructures grown along the [111] direction, such as self-assembled quantum dots or heterostructure wires must have a vanishing FSS. We confirm this prediction by million-atom empirical pseudopotential many-body calculations of {\it realistic} structures. We use experimentally realised shapes, compositions and sizes and find that the vanishing FSS must be present in experimentally realized structures. We further study how robust the results are against deformations and conclude that [111] grown structures, especially heterostructure wires, are ideal candidates for the generation of entangled photon pairs. Through the control of size, shape and composition they would emit at the optical fibre communication wavelength (conventional C band) of 1.55 $\mu$m (0.8 eV). The suggested structures, and their atomistic symmetry, are tolerant of imperfections in their interfaces, unlike [001] grown structures, which should boost their attractiveness. [1] Phys. Rev. Lett. {\bf 103}, 063601 (2009) [Preview Abstract] |
Monday, March 15, 2010 8:48AM - 9:00AM |
A9.00003: Output Polarization Characteristics of an Electrically Injected Quantum Dot Spin Polarized Single Photon Source Ayan Das, Junseok Heo, Debashish Basu, Wei Guo, Pallab Bhattacharya The fine structure of the excitons in a quantum dot, its dependence on the confinement symmetry of the dot and on externally applied electric or magnetic field have been a subject of interest in recent years. We have studied these effects in an electrically injected quantum dot spin polarized single photon source. The GaAs-based microcavity diode consists of a single InAs/GaAs self organized quantum dot as the single photon source and a MnAs/Al$_{0.1}$Ga$_{0.9}$As Schottky tunnel barrier as the spin injector. Single photon emission from the device is confirmed by antibunching behavior in photon correlation measurements. The output circular polarization of the exciton emission, measured in the Faraday geometry, remains zero upto a field of $\sim $ 0.9 Tesla and then increases to 8{\%}. The output linear polarization shows a complementary trend. The results are explained on the basis of the observed in-plane anisotropy of the quantum dots due to epitaxy-related effects and the Zeeman interaction of electron and hole spins and their effect on the optical transitions of excitons involving mixed and pure states. [Preview Abstract] |
Monday, March 15, 2010 9:00AM - 9:12AM |
A9.00004: Heterogeneous confinement in lateral quantum dot molecules Jie Peng, Gabriel Bester We show that by the combination of an atomistic many-body approach, knowledge of the detailed morphology of the QDMs, and PL experiments, we reach a thorough understanding of the underlying processes involved in the optical experiment. We highlight the importance of electronic coupling in lateral dot molecules fostered by the presence of an In-poor basin connecting the dots from below. This leads to a peculiar confinement situation with regions of type I band alignment close to a coupling region on the verge to type II. Beyond the static picture we find strong evidence, backed up by PL measurements, for a dynamical model ensuing from the lack of potential barrier felt by the electron in opposition to the decoupled holes. This model leads to a qualitatively different behavior for absorption and emission processes under non-resonant excitation and represent an interesting type of nano- laboratory. Having the ability to manipulate the position of the significantly coupled electrons within the QDM leaves it not only as a wavelength tunable single-photon source but also as a potential building block for quantum gates based on single charges and spins. In order to achieve this, regimes of coherent coupling within the QDM have to be investigated, such as the resonant tunneling of an electron. [Preview Abstract] |
Monday, March 15, 2010 9:12AM - 9:48AM |
A9.00005: Defects in diamond: Quantum computing and atomic magnetometry Invited Speaker: Owing to their remarkable stability, colour centers in diamond have already found an application in quantum cryptography. In this talk I will discuss recent progress regarding spin-based quantum information processing and atomic magnetometry. In particular, it is possible to place single impurity atoms into isotopically modified diamond with high degree of control\footnote{G. Balasubramanian et al., Nat Mater 8, 383 (May, 2009).}\footnote{N. Mizuochi et al., Phys. Rev. B 80, 041201(R), (2009).}. We demonstrate the generation and precise tomography of bi- and tripartite nuclear and electron nuclear spin entanglement, for the first time in solid state system\footnote{P. Neumann et al., Science 320, 1326 (Jun 6, 2008).}. It proves that the quality of entanglement is excellent fostering their potential use in quantum applications like quantum teleportation protocols. We show that scalable quantum logic elements can be realized by exploring long range magnetic dipolar coupling between individually addressable single electron spins\footnote{P. e. a. Neumann, Nature Physics, in press, (2010).}. Furthermore, we experimentally demonstrate an ideal quantum non-demolition measurement of the nitrogen nuclear spin associated with nitrogen-vacancy centre. [Preview Abstract] |
Monday, March 15, 2010 9:48AM - 10:00AM |
A9.00006: Electrostatic conveyer for excitons A.G. Winbow, J.R. Leonard, M. Remeika, A.A. High, E. Green, A.T. Hammack, L.V. Butov, M. Hanson, A.C. Gossard We report on the realization of electrostatic conveyers for indirect excitons in GaAs/AlGaAs coupled quantum wells. The conveyer is a laterally moving lattice potential for excitons. Its amplitude and speed can be controlled by laterally modulated oscillating electrode voltages. We observed the exciton transport via the conveyer over several tens of microns and studied the transport as a function of the conveyer amplitude, exciton density, and exciton lifetime. We observed a \textit{dynamical} localization-delocalization transition for the excitons in the conveyer with varying exciton density and conveyer amplitude: In the localization regime, excitons are moved by the conveyer, following the moving lattice potential, while in the delocalized regime, excitons do not follow the conveyer motion. [Preview Abstract] |
Monday, March 15, 2010 10:00AM - 10:12AM |
A9.00007: Dark Exciton States in PbSe Nanocrystals Richard Schaller, David Bussian, Jin Joo, Jeffrey Pietryga, Victor Klimov, Scott Crooker Colloidal semiconductor nanocrystals (NCs) offer a size-tunable energy gap and unique physical processes that are of interest for optoelectronic applications. Single excitons in PbSe NCs recombine at room temperature slowly (hundreds of nanoseconds) relative to other NC compositions. Large dielectric screening effects may control the lifetime, but theoretical work indicates that the lowest energy state is an optically passive dark exciton. We measure photoluminescence lifetimes of single excitons in multiple NC sizes as a function of temperature and magnetic field in an attempt to understand the detailed electronic structure. Our measurements indicate that the single exciton lifetime increases from $\sim $800 ns at 298 K to $\sim $5 $\mu $s at 1.6 K whereas application of magnetic fields at low temperature reduces lifetime. Such behavior is indicative of thermally activated emission from a lower-energy dark state with an exchange splitting of order 1 to 10 meV. This data suggests that relaxation in PbSe NCs at room temperature is dominated by dielectric screening. [Preview Abstract] |
Monday, March 15, 2010 10:12AM - 10:24AM |
A9.00008: Coupling an epitaxial quantum dot a fiber-based external-mirror microcavity Andreas Muller, Edward Flagg, Michael Metcalfe, John Lawall, Glenn Solomon Single InAs semiconductor quantum dots were coupled to an open optical microcavity. It consists of a Bragg reflector grown underneath the quantum dot layer, and a micromirror external to the semiconductor sample. This cavity is fully spectrally tunable with a piezoelectric actuator and a single quantum dot can be conveniently positioned at an antinode of the cavity field. Fabrication of the micromirror at the tip of a single-mode fiber permits efficient light collection into a well-defined collection mode. [Preview Abstract] |
Monday, March 15, 2010 10:24AM - 10:36AM |
A9.00009: Radiative and nonradiative pathways in multiexciton recombination in giant nanocrystal quantum dots Anton Malko, Siddharth Sampat, Han Htoon, Javier Vela-Becerra, Yongfen Chen, Jennifer Hollingsworth, Victor Klimov Recently,\footnote{Y. Chen et al., \textit{JACS} \textbf{130}, 5026 (2008)} we developed ``giant'' nanocrystal quantum dots (g-NQDs), in which a small emitting core of CdSe is overcoated with a thick shell of a wider-gap CdS. We conduct room-temp measurements of photoluminescence (PL) lifetimes in such g-NQDs as a function of excitation power and a number of shell monolayers. At low pump levels, corresponding to excitation of less than 1 exciton per dot on average ($\langle $\textit{${\rm N}$}$\rangle <$ 1), we observed excitonic radiative lifetimes of $\sim $100 ns and a linear scaling of the PL signal with pump intensity. At powers corresponding to $\langle $\textit{${\rm N}$}$\rangle >$1, fast ($\sim $1ns) PL component appeared, accompanied by a transition to a sub-linear scaling of PL intensity with $\langle $\textit{${\rm N}$}$\rangle $.~Our findings indicate that while g-NQDs indeed produce suppression of nonradiative Auger recombination,\footnote{F. Garcia-Santamaria et al., \textit{Nanoletters} \textbf{9}, 3482 (2009)} this suppression is incomplete. We conduct systematic studies of relative efficiencies of nonradiative and radiative processes in these nanostructures. [Preview Abstract] |
Monday, March 15, 2010 10:36AM - 10:48AM |
A9.00010: Resonant Subband Landau Level Coupling in the Wide Quantum Well L.-C. Tung, X.-G. Wu, L. N. Pfeiffer, K. W. West, R. R. Du, Y.-J. Wang Subband energies and intersubband couplings in an ultra high-mobility GaAs/Al{\_}{\{}0.24{\}}Ga{\_}{\{}0.76{\}}As wide quantum well have been investigated by magneto-infrared spectroscopy. In this ultra clean and symmetric wide quantum well, couplings between the 1st and up to the 4th electric subbands have been observed, including a symmetry-forbidden coupling between the 1st and 3rd subbands. The results can be interpreted in terms of the magnetoplasma coupling in a quasi-three-dimensional system or the resonant subband Landau level coupling with a slab plasma frequency dependent depolarization shift in a two-dimensional system. The latter appears to be a better interpretation for the observed anticrossings in this system. [Preview Abstract] |
Monday, March 15, 2010 10:48AM - 11:00AM |
A9.00011: Stark effect, polarizability and electroabsorption in silicon nanocrystals Ceyhun Bulutay, Mustafa Kulakci, Rasit Turan The experimental quantum-confined Stark effect data in embedded Si nanocrystals (NCs) is unambiguously explained with an atomistic pseudopotential theory. The majority of the Stark shift comes from the valence states which undergo a level crossing with respect to the applied field. The nonmonotonic behavior of the experimental data as well as the peak intensity appearing around 0.5~MV/cm and the subsequent fall off are well reproduced by the theory. However, for small Stark fields, the 300~K emission comes out to be stronger in the theoretical estimation due to thermally-activated nonradiative processes not accounted in the theory that possibly degrade the emission rate at higher temperatures. The polarizability of embedded Si NCs is obtained over a diameter range of 2.5--6.5~nm which unveils the importance of Coulombic effects for larger NCs. The intraband electroabsorption analysis asserts that p-doped Si NCs will benefit from a wider voltage tunability. Finally, the spin-orbit interaction is considered as a function of NC size which begins to have an impact on the results for large Stark fields. [Preview Abstract] |
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