Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session W14: Focus Session: Optics of Nanostructures -- Quantum Dots II |
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Sponsoring Units: DMP Chair: Carlo Piermarocchi, Michigan State University Room: B113 |
Thursday, March 18, 2010 11:15AM - 11:27AM |
W14.00001: Effect of Photoexcitation and Population Relaxation on Carrier Multiplication Efficiency in Semiconductor Nanocrystals Kirill Velizhanin, Andrei Piryatinski, Victor Klimov The carrier multiplication (CM) is the process of production of more than one electron-hole pair (exciton) per single photon absorbed. It has important implications in developing new photovoltaic devices. To theoretically describe such processes we have developed an exciton scattering model which treats contributions of different biexciton photogeneration channels and the population relaxation to the CM efficiency on equal footing [1]. Furthermore, the model allows one to study CM processes in both bulk and nanocrystalline semiconductor materials. The model is applied to study CM efficiency in bulk and nanocrystals of PbSe. The model parameters, i.e., single- and bi-exciton energies and wavefunctions as well as transition dipoles and Coulomb scattering amplitudes, are obtained within the effective mass envelope function formalism. Analysis of interfering bi-exciton photogeneration channels, effect of population relaxation, and the comparison with experiment will be provided. [1] A. Piryatinsky and K. A. Velizhanin, arXiv:0911.1139v [Preview Abstract] |
Thursday, March 18, 2010 11:27AM - 11:39AM |
W14.00002: Effect of Quantum Confinement on Radiative and Nonradiative Decay Rates in Ge Nanocrystals Istvan Robel, Doh C. Lee, Scott A. Crooker, Jeffrey M. Pietryga, Richard D. Schaller, Victor I. Klimov We study the effects of quantum confinement on radiative and nonradiative Auger recombination in colloidal germanium nanocrystals (NCs). The observed temperature- (1.7-300K) and magnetic field-dependence (0-15T) of the photoluminescence lifetimes indicates the presence of an optically dark (dipole-forbidden) lowest excited state closely separated from an optically active bright state. Thermal activation of the bright state and magnetic-field-induced mixing of the dark and bright states independently yield a consistent energy scale of the splitting of about 1 meV. Further, we investigate the effect of spatial confinement on nonradiative Auger recombination. An interesting result of these studies is that Auger rates in Ge NCs are close to those in similarly sized NCs of direct gap semiconductors despite a large (orders of magnitude) difference in their respective bulk Auger constants. This observation indicates that, as in the case of radiative decay, spatial confinement eliminates the need for the momentum-conserving phonon in Auger recombination. [Preview Abstract] |
Thursday, March 18, 2010 11:39AM - 11:51AM |
W14.00003: Independent tuning of quantum dot frequencies in a photonic crystal cavity Susanna Thon, Hyochul Kim, Pierre Petroff, Dirk Bouwmeester One of the main obstacles to coupling multiple quantum dots (QDs) to a single nanocavity mode in a cavity quantum electrodynamics system is the ability to independently tune the quantum dot frequencies. We demonstrate that in a specially designed GaAs photonic crystal membrane structure with two embedded QD layers, the QD emission frequencies of one layer can be tuned independently of the other by applying a voltage across only one of the QD layers. Initial results on coupling two QDs to a single cavity mode are discussed. [Preview Abstract] |
Thursday, March 18, 2010 11:51AM - 12:03PM |
W14.00004: Multiple recombination centers model of fluorescence intermittency of single colloidal semiconductor quantum dots Pavel Frantsuzov, Sandor Volkan-Kacso, Boldizsar Janko We present a new physical model resolving a long-standing mystery of the power-law distributions of the blinking times in single colloidal quantum dot fluorescence. The model considers the non-radiative relaxation of the exciton through multiple recombination centers. Each center is allowed to switch between two quasi-stationary states. We point out that the conventional threshold analysis method used to extract the exponents of the distributions for the on-times and off-times has a serious flaw: The qualitative properties of the distributions strongly depend on the threshold value chosen for separating the on and off states. Our new model explains naturally this threshold dependence, as well as other key experimental features of the single quantum dot fluorescence trajectories, such as the power-law power spectrum (1/f noise). [Preview Abstract] |
Thursday, March 18, 2010 12:03PM - 12:15PM |
W14.00005: Origin of one-photon and two-photon absorption peaks in PbSe nanocrystals from atomistic pseudopotential calculations Alberto Franceschetti, Jun-Wei Luo, Alex Zunger PbSe nanocrystals represent the paradigm nanoscale system exhibiting carrier multiplication upon light absorption, yet the origin of the absorption peaks remains elusive. The second peak in the one-photon absorption spectrum has been often attributed to dipole-forbidden P-S and S-P inter-band transitions. Recent two-photon photo-luminescence excitation experiments have shown that the S-P and P-S transitions, which are selectively excited by two-photon absorption, are indeed close in energy to the second one-photon absorption peak. Here we report atomistic pseudopotential calculations of the one-photon and two-photon absorption spectra of PbSe nanocrystals 30.6 {\AA} in radius. We find that the main two-photon absorption peak indeed originates from P-S and S-P transitions, and is blue-shifted by 36 meV with respect to the second one-photon absorption peak, in excellent agreement with experiment. However, our calculations show unequivocally that, contrary to previous interpretations, the second one-photon absorption peak originates from dipole-allowed P-P transitions. [Preview Abstract] |
Thursday, March 18, 2010 12:15PM - 12:27PM |
W14.00006: Calculations of strong coupling between a single quantum dot and a plasmonic nanoresonator Xiaohua Wu, Jason Montgomery, Stephen Gray, Matthew Pelton Quantum-mechanical strong coupling between an optical cavity and a single solid-state emitter offers a robust, scalable platform for quantum information processing. Several experiments have demonstrated strong coupling between single quantum dots and dielectric microcavities. Although plasmonic cavities, composed of metal nanostructures, have fast loss rates compared to dielectric cavities, they also have much smaller mode volumes, and therefore stronger dot-cavity coupling strengths; they thus have the potential to serve as a fully nanoscale system with controllable strong coupling. In this work, we present simulation results for a quantum dot between two silver nanoparticles. Although the calculated absorption cross-section shows a double-peak structure only in the strong-coupling regime, the scattering spectrum can show two peaks even in the weak-coupling regime. Since scattering dominates the extinction spectra that are typically measured, this has important implications for experimental verification of strong coupling. [Preview Abstract] |
Thursday, March 18, 2010 12:27PM - 1:03PM |
W14.00007: Radiative Cascades in Charged and Neutral Semiconductor Quantum Dots Invited Speaker: We measured, for the first time, two photon radiative cascades due to sequential recombination of quantum dot confined electron hole pairs in the presence of an additional spectator charge carrier. We identified direct, all optical cascades involving spin blockaded intermediate states, and indirect cascades, in which non radiative relaxation precedes the second radiative recombination. I will discuss the potential of semiconductor quantum dots as reliable sources for polarization entangled photon pairs and the possibility to use them for entangling information carrying flying qubits (photons) with anchored matter qubits (charge carriers' spins). [Preview Abstract] |
Thursday, March 18, 2010 1:03PM - 1:15PM |
W14.00008: AFM assembly and optical characterization of individual hybrid nanostructures Daniel Ratchford, Farbod Shafiei, Suenne Kim, Xiaoqin Li Interactions between the different components of hybrid nanostructures allow for structures with versatile functionality. The properties of such hybrid structures sensitively depend on composition and geometry. Therefore, intrinsic properties can only be revealed in experiments performed on individual nanostructures with well controlled geometries. Our experiment uses AFM nanomanipulation to assemble individual hybrid nanostructures consisting of single CdSe/ZnS quantum dots near metallic nanoparticles. We then characterize the structures' optical decay rates with confocal microscopy. Metallic nanoparticles change the local photon density, and therefore, are expected to modify the quantum dot decay dynamics. In our preliminary results, we observe an increase in the total decay rate for single CdSe/ZnS dots coupled to silver nanoparticles. [Preview Abstract] |
Thursday, March 18, 2010 1:15PM - 1:27PM |
W14.00009: Probing carrier separation in a novel series of infrared type-II nanoheterostructures Jeffrey M. Pietryga, Doh C. Lee, Istvan Robel, Victor I. Klimov Three-dimensional confinement of carriers within semiconductor nanocrystals gives rise to novel characteristics such as size-controlled energy gaps, but also increased Coulomb interactions between charge carriers that can hamper spatial separation of electrons and holes, such as in photovoltaic devices. One approach to efficient ``splitting'' of photogenerated excitons is through introduction of appropriate energy gradients within a heterostructured nanocrystal. Specifically, a type-II staggered alignment of electronic states can provide nearly complete separation of electrons and holes between different spatial regions of a nanoheterostructure. In this work, we use static and transient photoluminescence spectroscopies to examine the evolution of type-II behavior in PbSe/CdSe/CdS core/shell/shell heterostructured nanocrystals. We observe a marked red shift and a dramatic increase in radiative lifetimes (to $>$10 $\mu $s) with increasing CdS shell thickness, indicative of type-II carrier separation, at effective band gaps of 0.95 eV, the narrowest band gap of any such system to date. We will compare these measurement to those of a homologous ``tetrapod'' system that exhibits surprisingly high emission quantum yields, to further elucidate the effect of structure on carrier separation. [Preview Abstract] |
Thursday, March 18, 2010 1:27PM - 1:39PM |
W14.00010: Fluorescence Resonance Energy Transfer near Metal Nanoparticle V.N. Pustovit, T.V. Shahbazyan We study theoretically fluorescence resonance energy transfer (FRET) between donor and acceptor molecules attached to a metal nanoparticle supporting localized surface plasmon. We obtain a general expression for energy transfer rate that incorporates the effects of plasmon enhancement and quenching by the nanoparticle. Specifically, we find that while the quenching prevails over the enhancement, the distance dependence of FRET has a pronounced maximum when the distance between molecules and nanoparticle surface is about the nanoparticle radius. We also find that FRET depends strongly on the angular positions of donor and acceptor molecules at the nanoparticle surface. [Preview Abstract] |
Thursday, March 18, 2010 1:39PM - 1:51PM |
W14.00011: Do ``giant'' nanocrystal quantum dots need to be giant to suppress Auger recombination? Florencio Garcia-Santamaria, Ranjani Viswanatha, Richard Schaller, Victor Klimov Nanocrystals quantum dots (NQDs) are attractive materials for various light-emitting applications including optical amplification and lasing. A complication associated with the multiexcitonic nature of light amplification in NQDs is the picosecond optical-gain decay induced by nonradiative Auger recombination. In this process, one exciton recombines by transferring the energy to the other. Recently, we observed that Auger recombination is very efficiently suppressed in a novel type of nanocrystals dubbed ``giant'' quantum dots (g-NQDs). These g-NQDs comprise an emitting core of CdSe overcoated with a thick shell (up to 20 monolayers) of wider-gap CdS so that the overall particle diameter is larger than 10 nm. Thanks to their long multi-exciton lifetimes these nanocrystals show unusually large optical-gain bandwidth and low amplified-spontaneous-emission thresholds. Here we discuss potential reasons for Auger recombination suppression in these quantum dots and provide evidence that the large size of the nanocrystal is not as important as the properties of the CdSe/CdS interface. Specifically, even dots with just 3 - 4 monolayers of CdS show very long Auger lifetimes after we apply a special treatment, which modifies the core-shell interface. [Preview Abstract] |
Thursday, March 18, 2010 1:51PM - 2:03PM |
W14.00012: Optical Properties of CdSe/Zns Quantum Dots Coupled to Gold Nanoparticles Shin Grace Chou, Hyeong Gon Kang, Matthew Clarke, Jeeseong Hwang, Rajasekhar Anumolu, Leonard Pease, Silvia Lacerda We look into the detailed photophysical interactions between isolated CdSe/ZnS quantum dot (QD) and neighboring gold nanoparicles (AuNP) in samples with both particles randomly distributed onto a glass coverslip and with QD-AuNP dimer pairs sorted using differential mobility analyzer. The coupled nanoparticles pairs were analyzed using a time-correlated confocal spectral-microscopy station, where the spectral diffusion, the blinking pattern, and the lifetime of the QD were monitored simultaneously. The AuNP has been found to enhance the fluorescence intensity of the QD while significantly altering the blinking statistics of the individual QD. As the sample was excited at the same spot by multiple laser sources that are on or off the peak plasmon resonance of the AuNP, detailed changes in the QD optical signal were analyzed in the context of exciton-plasmon coupling and photo-thermal effects. Overall, our$^{ }$results yield new insights that lead to a better understanding of the fundamental interaction mechanism of QD-metal nanostructures. [Preview Abstract] |
Thursday, March 18, 2010 2:03PM - 2:15PM |
W14.00013: Exciton Energy Transfer between Quantum Dots in Photon-Confined Systems Kaijie Xu, Carlo Piermarocchi We address fundamental theoretical aspects of the exciton energy transfer between two quantum dots embedded in dielectric structures that confine photonic modes. We focus on how the photon density of states in these structures can affect the energy transfer mechanism. We will consider the case of two quantum dots in a planar micro-cavity and in a one-dimensional waveguide. We compute the dynamics of excitons and photons during the transfer between the two dots, assuming that we have only one exciton in one dot as initial condition. We study the dependence of the transition rate on the inter-dot distance and compare our result to the Forster mechanism. The effect of dissipation by phonon emission in the energy transfer process will also be discussed. We show how the interplay of phonons and confined photons can affect the standard picture of the exciton energy transfer based on a dipolar interaction. [Preview Abstract] |
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