Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session J24: Focus Session: Optical Properties of Nanostructures IV: Quantum Dots |
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Sponsoring Units: DMP Chair: Ado Jorio, Universidade Federal de Minas Gerais Room: Morial Convention Center 216 |
Tuesday, March 11, 2008 11:15AM - 11:27AM |
J24.00001: Assessment of carrier multiplication in semiconductor nanocrystals by transient photoluminescence spectroscopy Gautham Nair, Scott Geyer, Moungi Bawendi The enhancement of carrier multiplication (CM) is an important aim that could increase solar cell performance and widen the range of materials suitable for future solar technologies. Pump-probe measurements have shown evidence of strongly enhanced CM in lead chalcogenide, InAs, and CdSe nanocrystals (NCs). However, the nature of the enhancement mechanism is not well understood. We have carried out an experimental assessment of CM yields in semiconductor NCs by carefully studying exciton and biexciton signatures in transient photoluminescence decays. In the case of CdSe NCs, though the technique is particularly sensitive due to the biexciton's relatively fast radiative rate, we have found no evidence for CM up to photon energies as high as 3.1 Eg. This result is strongly in disagreement with previous reports on CM in CdSe NCs. The implications of our findings on the efficiency and material dependance of CM are discussed within a general physical framework. [Preview Abstract] |
Tuesday, March 11, 2008 11:27AM - 11:39AM |
J24.00002: The shape and symmetry dependence of excitonic radiative decay in CdSe nanocrystals Qingzhong Zhao, Alberto Franceschetti, Peter A. Graf, Wesley B. Jones, Kwinseon Kim, Lin-Wang Wang For the last decade, the exciton recombination dynamics of nanocrytal quantum dots (NQDs) through radiative decay have been extensively studied by experimental and theoretical methods, because some NQDs, like CdSe and CdS/ZnS, show near unity quantum yield. Using atomistic, semiempirical pseudopotential calculations, we investigate the radiative decay of band-edge excitons in CdSe nanocrystals with perfect and imperfect shapes. While the lifetimes of bright excitons are in the nanosecond range and not sensitive to size and shape, we find that the radiative lifetimes of the ground state dark excitons are highly dependent on the surface shape and symmetry. The introduction of one [100] or [101] facet can drastically reduce the dark exciton lifetime from milliseconds to microseconds, and such faceting is observed by STEM. This provides an alternative to the explanation by spin-flip assisted or surface-assisted recombination mechanisms of the observed microsecond dark exciton lifetime. Our results highlight the importance of QD surface shape and broken symmetry in exciton dynamics. [Preview Abstract] |
Tuesday, March 11, 2008 11:39AM - 11:51AM |
J24.00003: Peculiar many-body effects revealed in the spectroscopy of highly charged quantum dots M. Ediger, Gabriel Bester, A. Badolato, P. Petroff, K. Karrai, A. Zunger, R. Warburton We have discovered new consequences of Coulomb interactions in self-assembled quantum dots by interpreting experimental spectra with results of atomistic pseudopotential calculations. The Coulomb effects are evident in the photon emission process and we can tune them in situ by controlling the quantum dot charge in the range from $+6e$ to $-6e$. We find two regimes in the same dot: $J \le \Delta E$ for electron charging yet $J \simeq \Delta E$ for hole charging. We discover a breakdown of the Aufbau principle for holes; clear proof of non-perturbative hole-hole interactions; promotion-demotion processes in the final state of the emission process; and pronounced configuration hybridizations in the initial state. The level of charge control and the energy scales result in Coulomb effects with no obvious analogues in atomic physics. [Preview Abstract] |
Tuesday, March 11, 2008 11:51AM - 12:03PM |
J24.00004: Anomalous Polarization Behavior of a Zeeman Doublet in CdSe Nanocrystal Quantum Dots H. Htoon, M. Furis, S. A. Crooker, Al. L. Efros, S. Jeong, V. I. Klimov It is well known that a Zeeman doublet observed in emission spectra of a degenerate quantum state in the case of detection along an applied magnetic field (B field) is characterized by left and right circular polarizations. However, our single nanocrystal quantum dot (NQD) studies conducted in B-fields up to 5 T indicate that the Zeeman doublet of some of the CdSe NQDs exhibits a completely different polarization behavior. Specifically, we observe that the lower-energy state of the doublet becomes increasingly circularly polarized with increasing B field, while the higher-energy state shows a zero degree of circular polarization (i.e remains linearly polarized).~ We explain this anomalous polarization behavior in terms of mixing between the Zeeman split levels derived from the low- and high-energy bright exciton states. This mixing relies on strong long-range electron-hole exchange interactions that are unique to ultrasmall nanocrystals. [Preview Abstract] |
Tuesday, March 11, 2008 12:03PM - 12:15PM |
J24.00005: Influence of the electronic structure and the multi-exciton spectral density on the multiple exciton generation in semiconductor nanocrystals Christophe Delerue, Guy Allan Several experimental works have reported that a single high-energy photon could generate multiple excitons in semiconductor nanocrystals and several theories are proposed to explain these results. We calculate the electronic structure of InAs, Si and PbSe nanocrystals and we investigate two models of the multiple exciton generation (MEG). We show that the impact ionization process is efficient at high energy, with lifetimes as small as 10 fs. We present simulations of the MEG showing that, in PbSe and Si nanocrystals, the impact ionization alone cannot explain the observed efficiencies, even without relaxation by electron-phonon scattering. We calculate the spectral densities of multi-exciton states and we evaluate the possibility of direct photo-generation of multi-excitons. We confirm the importance of the multi-exciton spectral densities because of their rapid variation over several orders of magnitude. The high MEG efficiencies in PbSe and Si nanocrystals imply a very efficient relaxation in multi-exciton states characterized by a negligible density. [Preview Abstract] |
Tuesday, March 11, 2008 12:15PM - 12:27PM |
J24.00006: State-resolved optical pumping and single exciton gain in CdSe quantum dots. Ryan Cooney, Patanjali Kambhampati, Samuel Sewall, D.M. Sagar Optical gain in semiconductor quantum dots has been under intense investigation. Optical gain has been difficult to produce unless special geometries were employed such as thin films of CdSe quantum dots, or more recently, type II CdS/ZnSe core /shell structures. The prototypical CdSe quantum dots in dispersion typically show small gain even at extremely high carrier concentrations. The key problem is induced absorptions due to multi-exciton interactions which result in losses that cancel the expected gain. Our recent state-selective approach can be used to prepare initial excitonic states, which has yielded much insight on exciton relaxation dynamics. This approach was used here to generate some of the largest gains ever measured, at the lowest thresholds, for all sizes of CdSe quantum dots in dispersion. These results show that gain in quantum dots is general, if the system is driven correctly. [Preview Abstract] |
Tuesday, March 11, 2008 12:27PM - 12:39PM |
J24.00007: Optical conditional gates in laterally coupled quantum dots: the role of electron-hole exchange interaction Sophia Economou, Thomas Reinecke We propose a fast, optically induced two-qubit C-PHASE gate in laterally coupled quantum dots. We use a model potential with two asymmetric local minima to account for the difference in size and composition of the two dots. By making use of the excited bound states of the total potential, which extend over both dots and which gives rise to an effective coupling between the two resident electron spins, we avoid the need for an external bias, such as that typically used in vertically coupled dots. The electron-hole exchange interaction is shown to play an important role in our proposal. By lowering the symmetry of the eigenstates, it allows for a simple design of a fast (about 50 ps) C-PHASE gate. The dissipative dynamics of the excited states have been taken into account in our numerical simulation of the fidelity. The calculated fidelity depends on the values of the decay rates. Our proposal is consistent with the single qubit rotations we proposed [Phys. Rev. Lett. \textbf{99}, 217401 (2007)], and the combination of the two allows for universal quantum gates. [Preview Abstract] |
Tuesday, March 11, 2008 12:39PM - 12:51PM |
J24.00008: Fine Structure of Charged Excitons and Multiexcitons in Self-Assembled InGaAs/GaAs Quantum Dots Vladan Mlinar, Jun-Wei Luo, Gabriel Bester, Alex Zunger As a quantum-dot is loaded with Ne electrons and Nh holes, complex charged excitons and multiexcitons are formed. Their fingerprint is the splitting of each exciton line into a set of multiplets separated by ``fine structure'' splitting, whose calculation [1] poses a serious test to many-body theories. Previously, such fine-structure splittings were calculated and measured for charged excitons with (Ne,Nh) of (2,1), (1,2), (3,1), and (1,3), demonstrating good agreement with experiment. Here, we extend the calculation to both charged and neutral multi-excitons with (Ne,Nh) of (2,2), (3,2), (2,3), (3,3), (4,3), (3,4), and (4,4). The energy splittings, oscillator strength, and polarizations of the optical emission are obtained from many body pseudopotential calculations. We present here predictions for the optical emission from negatively and positively charged biexcitons which reveal fine structure splittings in the order of 100 micro-eV, within the experimental accuracy of single dot micro photoluminescence. We will discuss the evolution of the patterns of multiplet lines, their spacings, and regularities vs. the number of particles (Ne,Nh). [1] M. Ediger, G. Bester, et al., Phys. Rev. Lett 98, 36808 (2007). [Preview Abstract] |
Tuesday, March 11, 2008 12:51PM - 1:03PM |
J24.00009: Absorption spectroscopy of quantum dot molecules Danny Kim, M.F. Doty, M. Bashkansky, M. Scheibner, A.S. Bracker, D. Gammon The energy levels of a vertically-coupled self-assembled InAs/GaAs quantum dot pair are probed using differential transmission spectroscopy. This technique offers very fine spectral resolution ($<0.1~\mu eV$) allowing us to resolve the linewidths and fine structure for the various energy levels found in the rich spectrum of coupled quantum dots. For example, we observe increased broadening of the neutral exciton as it approaches the anticrossing point, as a result of a non-zero tunneling term well away from resonance. Excitons particular to coupled dots ---i.e. a positive trion/biexciton, where the additional hole is on the spectator dot--- exhibit polarization and power-dependent behavior that is in marked contrast to their single dot counterparts. Finally, the occupation of these exciton states are manipulated by using a second laser that is resonant on a related energy level. These experiments are a crucial step in using these molecules for coherent nonlinear optical processes. [Preview Abstract] |
Tuesday, March 11, 2008 1:03PM - 1:15PM |
J24.00010: F\"{o}rster optical signatures in quantum dot molecule photoluminescence Juan E. Rolon, Sergio E. Ulloa We formulate a realistic model that predicts the optical signatures of the F\"{o}rster resonant energy transfer processes (FRET) in InAs/GaAs self-assembled quantum dot molecules (QDMs) in presence of laser illumination and electric fields. We study the time evolution of a multiexcitonic Hamiltonian and construct a map of its dressed spectrum, resulting in effective coupling of the different states under laser illumination. In addition to interdot hole- and electron-tunneling, FRET is found to be an important quantum coupling mechanism in QDMs. We find FRET optical signatures to be highly dependent on structural parameters and severely constrained by the narrow spectral overlap between excitonic transitions in the donor-acceptor pair. However, detailed analysis of the orbital character of the localized hole reveals that it is possible to obtain strong spectral overlap between the first excited single exciton level in the donor dot and the lowest energy single exciton level in the acceptor dot. Remarkably, although FRET occurs via a single pair of exciton levels, its effects are evident throughout the calculated dressed spectrum. We observe a redistribution of spectral weights of direct and Stark shifted exciton lines, and a set of anticrossings among exciton states not directly coupled by FRET. Our results suggest experimental schemes to quantify FRET in photoluminescence experiments. [Preview Abstract] |
Tuesday, March 11, 2008 1:15PM - 1:27PM |
J24.00011: Excited-state absorption and quantum confined Stark effect in embedded silicon and germanium nanocrystals Ceyhun Bulutay Realistic-sized Si and Ge nanocrystals (NCs) embedded in wide band-gap matrices are studied theoretically using an atomistic pseudopotential Hamiltonian. Based on this electronic structure, first the interband absorption is studied which shows the importance of surface polarization effects that significantly reduce the absorption when included. This reduction is found to increase with decreasing NC size or with increasing permittivity mismatch between the NC core and the host matrix. The intraconduction and intravalence band absorption coefficients are also obtained in the wavelength range from far-infrared to visible region. Next, excited-state absorption at three different optical pump wavelengths, 532 nm, 355 nm and 266 nm are studied for 3- and 4 nm-diameter NCs. This reveals strong absorption windows in the case of holes and a broad spectrum in the case of electrons which can especially be relevant for the discussions on achieving gain in these structures. Finally, the interband absorption of NCs is studied under high DC electric field causing the quantum-confined Stark effect. [Preview Abstract] |
Tuesday, March 11, 2008 1:27PM - 1:39PM |
J24.00012: Level Anti-Crossing Spectroscopy - Optically Mapping the Electronic Structure of Coupled Quantum Dots M. Scheibner, M. Yakes, A.S. Bracker, I.V. Ponomarev, M.F. Doty, C.S. Hellberg, L.J. Whitman, T.L. Reinecke, D. Gammon We introduce an all optical level anti-crossing spectroscopy (LACS) with which the ground \textit{and} the excited state energy levels of quantum dots (QDs) can be measured for a hole(electron) by itself and in the presence of other charges.$^{1}$ Analogies are drawn to the atomic shell-model and connections are made in an average way to structural STM measurements. An applied bias provides an electric field between two InAs/GaAs QDs which ``tilts'' the energy levels of both QDs relative to each other. Molecular resonances between energy levels of the two QDs are measured purely optical as sequences of anti-crossing patterns in the electric field dependent PL spectrum. These sequences provide in situ characterization of the level structure. We anticipate that such measurements will precede more sophisticated quantum control demonstrations, allow precise reverse engineering and boost detailed theoretical modeling of QD structures. This work is supported by NSA/ARO. [Preview Abstract] |
Tuesday, March 11, 2008 1:39PM - 1:51PM |
J24.00013: Charged Quantum Dots in High Quality Micropillar Cavities Matthew Rakher, Nick Stoltz, Larry Coldren, Pierre Petroff, Dirk Bouwmeester We report on nanodevices that for the first time allow for charge tuning of single InAs quantum dots located near the field maximum of high quality micropillar cavities. Through the innovation of a novel trench style cavity design, we are able to embed doped layers for electrical gating within a microcavity and obtain Q values greater than 50,000. Using these devices, we demonstrate record high single photon count rates with a capture efficiency of 38{\%} and a Purcell effect up to 8. We also show high frequency polarization modulation of single photons enabled by Stark shift tuning a charged quantum dot between two polarization modes of a slightly elliptical micropillar with frequencies up to 100 KHz. Furthermore, we demonstrate a charge tunable quantum dot coupled to a micropillar cavity mode, which is an important step in quantum communication protocols involving trapped single electrons or holes. This type of device enables a quick, non-destructive measurement of the spin state of the trapped charge. [Preview Abstract] |
Tuesday, March 11, 2008 1:51PM - 2:03PM |
J24.00014: Photoluminescence spectra of thin films containing CdSe/ZnS quantum dots irradiated by 532-nm laser radiation and gamma-rays Suresh Sharma, Jay Murphree, Tonmoy Chakraborty, Ajani Ross, Cecil Shive We have investigated temporal behavior of the photoluminescence (PL) spectra of thin films containing CdSe/ZnS quantum dots irradiated by 532 nm laser radiation and gamma-rays. Under $\sim $ 100 W/cm$^{2}$ laser radiation, the PL intensity (I$_{PL})$ increases with irradiation time upto about 500s and thereafter declines linearly. The wavelength of the PL emission ($\lambda _{peak} )$ exhibits a blue-shift with exposure time. Upon simultaneous irradiation by 100 W/cm$^{2}$ 532-nm laser, as well as 0.57 and 1.06 MeV gamma-rays, the temporal behaviors of both I$_{PL}$ and $\lambda _{peak} $ are significantly different; I$_{PL}$ increases to a saturation level, and the magnitude of the blue-shift in $\lambda _{peak} $ is reduced. We also present data on the effects of the density of the quantum dots on the temporal behavior of the PL spectra, as well as additional data on samples synthesized with CdSe/ZnS quantum dots embedded in conducting polymer films. We discuss possible mechanisms underlying our observations. [Preview Abstract] |
Tuesday, March 11, 2008 2:03PM - 2:15PM |
J24.00015: Control of sp-d exchange interactions in pseudo-type II Mn:ZnSe/CdSe core-shell nanocrystal quantum dots David Bussian, Ming Yin, Scott Crooker, Victor Klimov Dilute magnetic semiconductors (DMSs) have been the focus of considerable research due to their potential usability in spin-based electronic devices. Unpaired electrons of dopant atoms, such as Mn$^{2+}$, can couple strongly to electrons of the semiconductor (sp-d exchange interaction), which should allow for the manipulation of the spin degree of freedom using traditional microelectonic circuitry. We have developed a novel approach for manipulating sp-d interactions between the dopant and the semiconductor wherein Mn ions are incorporated into cores of ZnSe/CdSe core-shell semiconductor nanocrystal quantum dots (NQDs). These NCs represent quasi-type II hetero-structures that allow one to tune both the band edge transition energy and dopant-carrier wavefunction overlap by changing the size of the core and/or shell thickness. We will report our recent results from a set of doped heterostructures for which we demonstrate tunability of both the magnitude and the sign of the sp-d exchange interaction energy as a function of hetero-NQD geometry. [Preview Abstract] |
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