Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session Q14: Focus Session: Optics of Nanostructures: Quantum Dots and Nanomaterials |
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Sponsoring Units: DMP Chair: Scott Crooker, Los Alamos National Laboratory Room: B113 |
Wednesday, March 17, 2010 11:15AM - 11:51AM |
Q14.00001: Locking of the quantum dot electron-nuclear spin system to a resonant laser Invited Speaker: The electron-nuclear spin system in quantum dots is a paradigm for the study of the central spin problem - a localized spin coupled to a mesoscopic spin bath. We present our recent laser absorption measurements of the quantum dot electron-nuclear spin system. Our results demonstrate that quantum dot absorption spectra are strongly influenced by the nuclear spin physics. At moderate magnetic fields and for a wide range of system parameters the hyperfine coupled electron-nuclear system provides for active locking of quantum dot optical transitions to the laser resonance. The nuclear spin ensemble polarizes dynamically and ensures bi-directional tracking of the laser frequency detuning over tens of natural linewidths. Our analysis reveals that on-resonance locking is associated with a narrowing of the nuclear Overhauser field variance pointing towards increased electron spin coherence time in a controlled nuclear spin environment. [Preview Abstract] |
Wednesday, March 17, 2010 11:51AM - 12:03PM |
Q14.00002: Intrinsic bi-directional dynamic nuclear polarization by optically pumped trions in quantum dots Wen Yang, L. J. Sham We develop a microscopic theory for dynamic nuclear polarization (DNP) in a quantum dot through dipolar interaction between the nuclear spins and the hole spin in the optically generated trion in the Faraday configuration. In sharp contrast to other uni-directional DNP mechanisms where the resulting nuclear field always increases or decreases the electronic excitation energy, our theory shows that the DNP induced by the trion is bi-directional: for the laser frequency on the blue (red) side of the trion excitation, the nuclear field tends to increase (decrease) the trion excitation energy into resonance with the laser frequency.[1] As a result, the sharp Lorentzian trion absorption line is broadened nearly symmetrically into a round top with abrupt edges, where bistable states of the nuclear polarization manifest as hysteretic loops. This intrinsic bi-directional DNP mechanism serves as an alternative explanation to the recently observed bi-directional hysteretic enhancement of trion absorption in single InGaAs quantum dot [C. Latta et al., Nature Phys. 5, 758 (2009)], attributed to competition between two uni-directional DNP mechanisms. Our work is supported by NSF-PIF and ARO/IARPA. [1] Bi-directional DNP by the trion was first discovered and explained in a recent work in the Voigt configuration [X. D. Xu et al., Nature 459, 1105 (2009)]. [Preview Abstract] |
Wednesday, March 17, 2010 12:03PM - 12:15PM |
Q14.00003: Single Dot Optically Detected Resonance Spectroscopy of Interface Fluctuation Quantum Dots V.R. Whiteside, C.J. Meining, A.V. Stier, B.D. McCombe, J.G. Tischler, A.S. Bracker, D. Gammon We report magnetic field (0 -- 10T) single dot photoluminescence (PL) and optically detected resonance (ODR) experiments of interface fluctuation quantum dots (IFQDs) for a 2.8nm GaAs/AlGaAs quantum well (QW) that has been doped with Si donors (in the barriers) to allow creation of both neutral excitons and negatively charged excitons, a.k.a. trions. Each dot has a signature ODR line shape throughout the magnetic field range studied. Three different line shapes are observed; the interpretation of these line shapes will be discussed. Upon comparison of the single dot ODR to the ensemble ODR, we find the single dots' relative ODR signal follows the ensemble's but is three times larger. Combining the single dot ODR measurements with the diamagnetic shift and Zeeman splitting results, it is possible to assign an observed spectral line to either a charged or neutral exciton. Our results indicate that the diamagnetic shift of the trion is smaller than that of the neutral exciton. This is attributed to the larger spatial extent of the trion wavefunction. [Preview Abstract] |
Wednesday, March 17, 2010 12:15PM - 12:27PM |
Q14.00004: Polarization Control of Optical Bistability in Coupled Microdisks B.B. Buckley, D.D. Awschalom, S.N. Ghosh, C.G.L. Ferri, Y.K. Verma, S. Ghosh, X. Li, N. Samarth Semiconductor microcavities are powerful systems both for the study of fundamental light-matter interactions and for applications in photonics. Microdisks composed of GaAs/(Al,GaAs) containing interface-fluctuation quantum dots for gain have been shown to offer low threshold lasing and cavity-induced electron spin coherence modulation\footnote{S. Ghosh et al. Nature Materials 5, 261 (2006).}. More recently, the control of lasing emission bistability by means of pump polarization was measured to occur under non-uniform illumination in elliptical microdisk pairs coupled evanescently along their semi-major axis \footnote{S. N. Ghosh, et al., submitted (2009).}. Hysteretic bistability is present when the pump light polarization is perpendicular to the evanescent coupling axis and disappears when the pump polarization is parallel to the coupling axis. This lasing bistability control is a unique functionality which may find use in optical logic devices. [Preview Abstract] |
Wednesday, March 17, 2010 12:27PM - 12:39PM |
Q14.00005: Theoretical explanation of correlations between on- and off-events in quantum dot fluorescence intermittency Sandor Volkan-Kacso, Pavel Frantsuzov, Boldizsar Janko Clear correlations have been found recently between the on- and off-events in the intensity trajectories of single colloidal quantum dots. These so-called memory effects have been found by the application of the commonly used threshold analysis. We propose, for the first time, a theoretical explanation for these correlations by using the theoretical framework of multiple recombination centers for fluorescence intermittency (blinking). The correlations are found to be threshold dependent and the Pearson correlation coefficient is shown to be both positive (on-on and off-off) or negative (on-off). We demonstrate that the model of multiple recombination centers can easily reproduce both the correct trends and signs of the correlations. The long correlations, as opposed to other models of blinking, are intrinsic to our model. [Preview Abstract] |
Wednesday, March 17, 2010 12:39PM - 12:51PM |
Q14.00006: Many-body exciton states in self-assembled quantum dots coupled to a Fermi sea P.M. Koenraad, N.A.J.M. Kleemans, J. van Bree, A.O. Govorov, G.J. Hamhuis, R. Notzel, A.Yu. Silov Using voltage dependent photoluminescence spectroscopy we have studied the coupling between QD states and the continuum of states of a Fermi sea of electrons in the close proximity of a self-assembled InAs quantum dot embedded in GaAs. This coupling gives rise to new optical transitions, manifesting the formation of many-body exciton states. The lines in the photoluminescence spectra can be well explained within the Anderson and Mahan exciton models. The presence of Mahan excitons originates from the Coulomb interaction between electrons in the Fermi sea and the hole(s) in the QD whereas a the second type of many-body exciton is due to a hybridized exciton originating from the tunnel interaction between the continuum of states in the Fermi sea and the localized state in the QD. Our study demonstrates the possibility to investigate a variety of many-body states in QDs coupled to a Fermi sea and opens the way to investigate optically the Kondo effect and related spin phenomena in these systems. [Preview Abstract] |
Wednesday, March 17, 2010 12:51PM - 1:03PM |
Q14.00007: Variation of Wurtzite InP Nanowire Photoluminescence With Diameter Saranga Perera, K. Pemasiri, A. Wade, L.M. Smith, H.E. Jackson, J.M. Yarrison-Rice, S. Paiman, Q. Gao, H. Tan, C. Jagadish We use time-resolved photoluminescence (PL) spectroscopy to study the PL variations of nanowires (NWs) at 12 K with nominal diameters of 5, 20, 30, 50, 100,and 150nm. The NWs were prepared by Au catalyst-assisted MOCVD growth with 420C growth temperature and V/III ratio of 700. A pulsed Titanium-Sapphire laser (780nm) was used to excite the nanowire sample. We observed time-resolved PL from single NWs from each of the NW samples. In 5, 20, 30nm diameter NW samples, we observe a PL peak around 810-820 nm at early times which decays rapidly leaving a long-lived defect-related emission peak. In the 100 nm and 150nm samples at early times we observe a PL peak around 825nm that evolves at later times to a long-lived defect emission line at 850nm. Some quantum confinement should be expected as the NW radius becomes smaller than the Bohr radius particularly for the 5 nm and 20 nm samples. We report on possible evidence for quantum confinement of the excitons. [Preview Abstract] |
Wednesday, March 17, 2010 1:03PM - 1:15PM |
Q14.00008: Photoluminescence of Highly Strained GaAs/GaP Core-Shell Nanowires M.A. Fickenscher, M. Montazeri, L.M. Smith, H.E. Jackson, J.M. Yarrison-Rice, J.H. Kang, Q. Gao, H.H. Tan, C. Jagadish, Y. Guo, J. Zou, M.E. Pistol, C.E. Pryor We present low temperature photoluminescence (PL) and time-resolved PL spectra from highly strained GaAs/GaP core-shell nanowires (NWs). Theoretical modeling predicts that the band structure of the NWs can be tuned by changing the ratio of the core radius to total NW radius. For this study, the ratio was changed by altering either the thickness of the GaP shell or the GaAs core radius with the other held fixed. Cross-sectional TEM is used to measure the range of core and shell radii. The PL from both methods confirms that the band gap can be shifted to dramatically higher energies from the 1.515eV GaAs free exciton peak and is consistent with the theoretical predictions as well as direct Raman measurements of the strain. [Preview Abstract] |
Wednesday, March 17, 2010 1:15PM - 1:27PM |
Q14.00009: Raman measurements of strain in GaAs/GaP core-shell nanowires Mohammad Montazeri, M.A. Fickenscher, L.M. Smith, H.E. Jackson, J.M. Yarrison-Rice, J. H. Kang, Q. Gao, H.H. Tan, C. Jagadish, Y. Guo, J. Zou, M.E. Pistol, C.E. Pryor We present results of Raman scattering from as--grown highly strained GaAs/GaP core-shell nanowires with 50 nm diameter GaAs cores and 25 nm GaP shells. We show that the degree of compressive hydrostatic and shear strain of the GaAs core can be separately determined using Raman scattering. Analysis of the shift and splitting of the TO-mode Raman spectra shows that the GaAs core has a -1.2 {\%} compressive hydrostatic strain and a -0.7 {\%} shear strain. Our measurements are consistent with 8-band k.p calculations and predict a 260 meV increase of the GaAs core band gap and a $\sim $100 meV heavy hole-light hole splitting of the valence band. These results open up new opportunities to strain engineering of the band structure by varying the nanowire core/shell ratio. [Preview Abstract] |
Wednesday, March 17, 2010 1:27PM - 1:39PM |
Q14.00010: Physical origin of the Drude-Smith model for the terahertz conductivity of nanomaterials D. Baillie, T.L. Cocker, M. Buruma, L.V. Titova, R. Sydora, F. Marsiglio, F.A. Hegmann While the Drude-Smith model has been shown to describe the terahertz (THz) conductivity in a variety of nanomaterials,$^{1,2}$ its validity and physicality have been questioned. We present a refinement to the backscattering interpretation of the Drude-Smith model, which is shown to emerge analytically from a structurally confined Drude gas of electrons. In addition, the modified Drude-Smith model agrees with our Monte Carlo simulations of Brownian particles confined to a box and driven by an external THz field. This simple, classical model for electron transport in nanomaterials not only provides an excellent fit to the observed THz conductivity in films of nanogranular VO$_{2}$, nanostructured gold, and Si nanocrystals, but also allows for the extraction of meaningful, physically-relevant parameters. Finally, the modified Drude-Smith model as a new effective medium theory is discussed. [1] D G Cooke et al., \textit{Phys. Rev. B }\textbf{73}, 193311 (2006). [2] M Walther et al., \textit{Phys. Rev. B }\textbf{76}, 125408 (2007). [Preview Abstract] |
Wednesday, March 17, 2010 1:39PM - 1:51PM |
Q14.00011: Percolation and localization dynamics in silicon nanocrystal films L.V. Titova, T.L. Cocker, X.Y. Wang, D.G. Cooke, A. Meldrum, F.A. Hegmann We apply time-resolved THz spectroscopy [1] to probe the time progression of the ac-conductivity in optically excited Si nanocrystal (NC) films with varying Si vol {\%}, NC sizes and separations. A percolation transition is observed at 38 $\pm $ 1 vol {\%} Si. Above this threshold, we observe a transition form initial ($<$50 ps) long-range percolative inter-NC transport characterized by a non-zero DC conductivity to eventual localization of carriers at individual NCs. Below percolation threshold, early-time ($<$25 ps) inter-NC tunneling conduction is observed in films with sub-nm separations, followed by the final localization of the photoexcited carriers in the largest NCs. In the films with larger ($>$ 1 nm) inter-NC spacing, long-range transport is suppressed suggesting strong photoexcited carrier localization. Comparison of the observed dynamics to Monte Carlo simulations will be discussed. [1] D. G. Cooke et al, Phys. Rev. B \textbf{73}, 193311~(2006). [Preview Abstract] |
Wednesday, March 17, 2010 1:51PM - 2:03PM |
Q14.00012: Formation of Phonoritons in Organic-Semiconductor Heterostructures Que Huong Nguyen We theoretically study the formation of polaritons and phonoritons in hybridization structure of a quantum dot system embedded in an organic material. The Frenkel exciton in the organic material is coupling with the Wannier Mott exciton in semiconductor quantum dots when the two excitons are in resonance to form an organic-inorganic hybrid exciton. While a new kind of polariton is formed by coupling of hybrid exciton with photon, a strong laser pump could lead to the coupling of the polariton with phonon field to form phonoriton, a new quasiparticle as a coherent superposition of exciton, photon and phonon. As the Frenkel and Wannier excitons have complimentary properties in interaction with photon and phonon fields, the hybridization of these two components will enhance phonoriton formation. [Preview Abstract] |
Wednesday, March 17, 2010 2:03PM - 2:15PM |
Q14.00013: Enhanced luminescence in Er-doped Si nanoparticles Tuan Hoang, Kevin Mantey, Munir Nayfeh We have studied the effect of Er ions on the luminescence of the 1-nm and 2.9-nm Si particles in solution. Under UV illumination, the 1-nm and 2.9-nm Si particles are known to have broadband luminescence in the blue and red region, respectively. We observed an enhancement in the luminescence of the particles with increasing concentration of Er ions. We discuss the result in terms of doping of the nanoparticles with Er ions. Preliminary DFT calculation shows that the ion can form a stable state just inside the particle. In this state, the electric field of the Er ion changes the bond length of the dimers, which are believed to be responsible for the optical activity of the particles. [Preview Abstract] |
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