Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session A36: Quantum Dots, Quantum Wells, and Metamaterials: Optical Characterization and Applications |
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Sponsoring Units: DCMP DMP Chair: Brennan Pursley, Naval Research Laboratory Room: 299 |
Monday, March 13, 2017 8:00AM - 8:12AM |
A36.00001: Spectral broadening of optical transitions in InAs/GaAs coupled quantum dot pairs P. Kumar, C. Czarnocki, C. Jennings, J. Casara, A. L. Monteros, N. Zahbihi, M. Scheibner, S. E. Economou, A. S. Bracker, B. C. Pursley, D. Gammon, S. G. Carter The optical transitions in InAs/GaAs coupled quantum dot (CQD) pairs are investigated experimentally. These coupled dot systems provide new means to study the interaction of quantum states with the mechanical modes of the crystal environment.$^{\mathrm{i}}$ Here, the line width and line shape of CQD optical transitions are analyzed in detail as a function of temperature, excitation power, excitation energy, and tunnel coupling strength. A significant line broadening, up to 25 times the typical lifetime-limited linewidth of single-dot excitons, is being observed at level anti-crossings where the coherent tunnel coupling between spatially direct and indirect exciton states is considerable. The experimental observations are compared with theoretical predictions where linewidth broadening at anti-crossings is attributed to the phonon assisted transitions, and found to be strongly dependent on the energy splitting of the two exciton branches.$^{\mathrm{ii}}$ This work focuses on understanding the linewidth broadening due to the pure dephasing, and fundamental aspects of the interaction of these systems with the local environment. i.) M. Kerfoot et al., Nature Commun. 5, 3299 (2014); ii.) J. M. Daniels et al., Phy Rev B 88, 205307 (2013) [Preview Abstract] |
Monday, March 13, 2017 8:12AM - 8:24AM |
A36.00002: Quantum Dots' Photo-luminescence Line Shape Modeling Muchuan Hua, Ricardo Decca Two usual phenomena observed in quantum dots (QDs) photo-luminescence (PL) spectra are line broadening and energy shift between absorption and emission peaks. They have been attributed to electron-phonon coupling and surface trapping during the PL process. Although many qualitative work describing these phenomena has been carried out, quantitative results are far less common. In this work, a semi-empirical model is introduced to simulate steady state QDs' PL processes at room temperature. It was assumed that the vast majority of radiative recombination happens from surface trapped states. Consequently, the PL line shape should be highly modulated by transition rates between states in the conduction band and between them and surface trapping states. CdSe/ZnS (core/shell) colloidal QD samples with different sizes were used to examine the model. The model was able to successfully reproduce the PL spectra of these samples even when the excitation happens within the emission spectra, giving raise to up-conversion events. This model might help understand and make more precise predictions of QDs' PL spectra and could also aid on the design of QDs' optical devices. [Preview Abstract] |
Monday, March 13, 2017 8:24AM - 8:36AM |
A36.00003: Renormalization of Optical Transition Strengths in Semiconductor Quantum Dots due to Band Mixing Kirill Velizhanin Strength of electron–photon coupling determines such important quantum dot (QD) characteristics as the radiative lifetime and absorption cross section. This strength is often assumed to be fully encoded by the so called Kane momentum matrix element. This parameter, however, pertains to a bulk semiconductor material and, as such, is not sensitive to the quantum confinement effects in a QD. In this work we demonstrate that the quantum confinement, via the so called band mixing, can result in a significant suppression of the strength of electron interaction with electromagnetic field. Within the envelope function formalism, we show how this suppression can be described by introducing an effective energy-dependent Kane momentum. Then, the effect of band mixing on the efficiencies of various photoinduced processes can be fully captured by the conventional formulae (e.g., spontaneous emission rate), once the conventional Kane momentum is substituted with the renormalized energy-dependent Kane momentum introduced here. As an example, we evaluate the energy-dependent Kane momentum for PbSe and PbS QDs and show that neglecting band mixing in these systems can result in the overestimation of absorption cross sections and emission rates by a factor of ~2. [Preview Abstract] |
Monday, March 13, 2017 8:36AM - 8:48AM |
A36.00004: Effect of mesogenic ligands on short and long-term spectral stability of CdSe/ZnS quantum dots Jose Amaral, Edwin Betady, Makiko Quint, Denzal Martin, Sheida Riahinasab, Linda Hirst, Sayantani Ghosh Surface modification of chemically synthesized CdSe/ZnS quantum dots (QDs) by performing a ligand-exchange can improve the optical properties, including short- and long-term photo-stability. Using a custom-designed mesogenic ligand, we significantly and advantageously alter the photophysical properties of CdSe/ZnS core-shell QDs. Our investigation is two-fold, as we follow the effect of ligand exchange on (1) the static and dynamic photoluminescence (PL) properties of QDs under continuous illumination, and (2) the temperature dependence of PL. We find that a reduction in Forster resonance energy transfer due to the ligand exchange process results in stabilizing both recombination lifetimes and emission intensity for over an hour of high power photo-excitation. Our temperature-dependent PL studies indicate thermally activated PL recovery at higher temperatures, and a lack of emission enhancement at low temperatures resulting from greater charge separation by the mesogenic ligands. We conclude that this process improves photoluminescence stability and sample longevity of QD films whose applications require long term resistance to photobleaching. [Preview Abstract] |
Monday, March 13, 2017 8:48AM - 9:00AM |
A36.00005: Measurement of two-photon absorption cross section of silicon nanocrystals in colloidal suspension Brandon Furey, Michael Downer, Dorothy Silbaugh, Adrien Guillaussier, Yixuan Yu, Brian Korgel Two-photon absorption (2PA) in liquid suspensions of ligand-passivated silicon nanocrystals (ncSi) is difficult to measure directly because of their low absorption cross section and competing nonlinear optical processes at high light intensity. Here we overcome these difficulties by measuring background-free, 2PA-induced photoluminescence (PL) as a function of the intensity of ultrashort 800 nm excitation pulses and then calibrating the response by measuring PL induced by one-photon absorption. Using this indirect method, 2PA cross sections of ncSi with diameters 2.2 and 2.7 nm suspended in toluene were 0.505 $\pm$ 0.005 and 2.24 $\pm$ 0.02 E-50 cm$^4$ s / photon, respectively. This procedure was validated using rhodamine B in ethanol for which the 2PA cross section was 21.8 $\pm$ 0.1 E-50 cm$^4$ s / photon, which agrees with direct measurements.\footnote{A. Nag, D. Goswami, J. Photochem. and Photobio. A: Chem. 206 (2-3), 188 (2009)} The size dependence of 2PA cross sections for ncSi and comparisons with other reference samples will be discussed. Water-dispersible ncSi may be suitable for bio-imaging.\footnote{C.M. Hessel, J. Wei, B. Korgel et al., Chem. Mater. 24 (2), 393 (2012)} This application is demonstrated using 2PA confocal microscopy of ncSi-incubated mouse tissue. [Preview Abstract] |
Monday, March 13, 2017 9:00AM - 9:12AM |
A36.00006: Abstract Withdrawn
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Monday, March 13, 2017 9:12AM - 9:24AM |
A36.00007: Optical defect detection of semiconductor thin film by sub-micron resolution second harmonic technique Farbod Shafiei, Tommaso Orzali, Alexey Vert, P Y Hung, Man Hoi Wong, Gennadi Bersuker, Michael Downer Epitaxial semiconductor III-V film such as GaAs or InP are strong candidate for electron transport and opto-electronic devices due to higher mobility of carriers in these films. Growth of such polar materials over mismatch and nonpolar substrate such as Si leave us with variety of defects such as treading dislocation. As these defects act as sink for charge carriers and scattering point for mobile charges, detection and control of such a defects became very important for semiconductor community. For the first time a noninvasive nonlinear optical technique has been used to map the localization of the light by these defects. A nonlinear nearfield scanning optical microscope (NSOM) is used to reveal the hotspot looking localization of the light due to presence of the defects. Films with variety of dislocation defects density has been studied which their nonlinear second harmonic optical maps distinguish such a defect density. [Preview Abstract] |
Monday, March 13, 2017 9:24AM - 9:36AM |
A36.00008: Design, fabrication and characterization of rugged, high-performance quantum dot photocathodes Jeffrey Pietryga, Istvan Robel, Nikolay Makarov, Jaehoon Lim, Qianglu Lin, John Lewellen, Nathan Moody Semiconductor nanocrystal quantum dots (QDs) are bright, tunable fluorophores used as, e.g., biolabels and downcoverting phosphors. Such applications make use of over three decades in advances in techniques for overcoming the natural tendency of these materials toward losing photoexcited carriers to surface defect states or to ionization. Ironically, QDs first gained attention as a material class for use in photocatalysis, which uses QD photoionization to drive redox reactions. Here, we explore the use of QDs in an alternative application that also exploits photoionization, namely within photocathodes for the electron guns that will enable next-generation light sources. We evaluate the efficiency of electron photoemission of conductive, solution-cast QD films of a variety of compositions in a typical electron gun configuration. By quantifying photocurrent as a function of excitation photon energy, excitation intensity and pulse duration, we demonstrate efficiencies superior to standard copper cathodes in films that are more robust against oxidation. Finally, we establish the dominant mechanism responsible for electron emission in the multi-photon excitation regime, which suggests numerous pathways for further enhancements. [Preview Abstract] |
Monday, March 13, 2017 9:36AM - 9:48AM |
A36.00009: Imaging potential energy landscapes with quantum dots Laura Kinnischtzke, Nick Vamivakas Electric and magnetic field control of InAs quantum dots has been leveraged for sensing in recent years. Self-assembled quantum dots have been used for charge sensing, magnetometry, and thermometry.\footnote{A. N. Vamivakas et. al., Phy. Rev. Lett., 107, 166802 (2011)}$^{,}$\footnote{F. Haupt et. al., Phy. Rev. Applied, 2, 024001 (2014)}$^{,}$\footnote{F. Seilmeier et. al. Phy. Rev. Applied, 2, 024002 (2014)} We extend this sensing platform to measuring potential landscapes in proximal metals using a charge-tunable quantum dot device architecture. The voltage profile is extracted by identifying variations in the charging plateau through non-resonant photoluminescence spectroscopy, and compared with finite element analysis simulations. [Preview Abstract] |
Monday, March 13, 2017 9:48AM - 10:00AM |
A36.00010: Interaction of Phenyldithiocarbamates and Hydrides With CdSe Quantum Dots and its Effect on their Optical Response Svetlana Kilina Using DFT and TDDFT, we study the effect of surface anionic ligands, such as hydrides and phenyldithiocarbamates (PTC), on the photophysics of quantum dots (QDs). Hydrides strongly enhance the emission of CdSe QDs, although the mechanism of such enhancement is unclear. Our simulations show that H- removes surface Se ions responsible for trap states, which explains increased emission after hydride treatment. PTC are commonly used ligands for passivating QDs that enhance their conductivity and redshift optical spectra, while no reports have been focused on the ligand exchange chemistry itself. Our calculations demonstrate that PTC ligand exchange with CdSe QDs is a more complex process than was initially thought, in which ligand decomposition competes with ligand exchange. In addition, we found that solvent polarity governs PTC deprotonation. In nonpolar solvents, some part PTC loos their protons, which bound to the Se sites. However, and no such deprotonation is observed in polar solvents. Fully protonated PTC bound to the QD surface lead to appearance of trap states with low optical activities. In contrast, partial deprotonation of PTC result in the lowest energy optically active transitions leading to improved emission of QDs. [Preview Abstract] |
Monday, March 13, 2017 10:00AM - 10:12AM |
A36.00011: Improvement of quantum dot IR photodetector performance due to selective bipolar doping Xiang Zhang, Vladimir Mitin, Andrei Sergeev, Kimberly Sablon, Michael Yakimov, Serge Oktyzbrsky Unipolar inter-dot doping creates charge redistribution and nanoscale potential barriers, which exponentially increase the photoelectron lifetime and IR photoresponse. However, the doping also increases the dark current and noise current. For independent optimization of signal and noise characteristics we propose and study quantum dot (QD) structures with selective bipolar doping, i.e. the n-doping of inter-dot space and p-doping of QD layers. The bipolar doping allows us to obtain large potential barriers around QDs at any electron fillings. This provides decoupled control of photocarrier lifetime and the dark current in quantum dot IR photodetectors (QDIP). Here we report experimental results, which demonstrate increase of the photoresponse and suppression of the noise current due to the selective bipolar doping. [Preview Abstract] |
Monday, March 13, 2017 10:12AM - 10:24AM |
A36.00012: Power and temperature dependent photoluminescence investigation of the linear polarization at normal and inverted interface transitions in InP/InAlAs and InGaAsP/InAlAs QW structures Hamidreza Esmaielpour, Vincent R. Whiteside, Louise C. Hirst, David V. Forbes, Robert J. Walters, Ian R. Sellers We present an investigation of the interface effects for InGaAsP/InAlAs QW and InP/InAlAs QW structures capped with an InP layer. Continuous wave photoluminescence (PL) spectroscopy of these samples at 4 K shows features associated with the interfaces of an InAlAs layer grown on an InP layer (normal interface) and an InP layer grown on an InAlAs material (inverted interface). Power dependent PL of the InGaAsP QW indicates that there are two features related to the inverted interface, whereby the linear polarization of one increases and for the other decreases. In addition, a temperature dependent study of this sample shows that as the temperature increases: the linear polarization for both features decreases; at room temperature, there is negligible polarization effect. A power dependent PL study of the InP QW structure shows both normal and inverted interface transitions have opposing trends in linear polarization. Notably, the temperature dependent PL investigation displays a reduction of polarization degree for the inverted interface: as expected; while an increase of polarization for the normal interface was observed. In addition, power and temperature dependence of peak energy of the interface transitions for both samples will be presented. [Preview Abstract] |
Monday, March 13, 2017 10:24AM - 10:36AM |
A36.00013: Radiative Recombination Rates in Narrow AlGaN Quantum Wells Sergey Rudin, Greg Rupper, Gregory Garrett, Chelsea Haughn Polar Al$_{\mathrm{x}}$Ga$_{\mathrm{1-x}}$N narrow quantum wells are important elements of deep-ultraviolet light emitting devices, and electron-hole radiative recombination rates are important characteristics of these nanostructures. We evaluated these rates for a set of AlGaN/AlN wells with 60{\%} Al concentration, with well widths from 0.6 nm to 2 nm, and n-doped barriers. We obtained the dependence on carrier density, lattice temperature from 10 K to 300 K, and well width and compared our theoretical results with the experimentally determined radiative rates. The polarization fields and density dependent screening of the polarization fields were included in the model. We employ a Green's function formalism with self-energies evaluated in the self-consistent T-matrix approximation, for a two-band model, with the bands determined in the \textbf{k\textbullet p} approximation. This formalism models the Coulomb correlations sufficiently to include the effect of excitons. The results are applicable in a wide range from low densities of carriers, through the Mott transition, up to relatively high densities, over a wide range of temperatures. The recombination coefficient was obtained from the integrated photo-luminescence. We then model density decay by $\partial $n$_{\mathrm{h}}$/$\partial $t $=$ An$_{\mathrm{h}} \quad +$ n$_{\mathrm{e}}$n$_{\mathrm{h}}$B(n$_{\mathrm{e}}$, n$_{\mathrm{h}})$, where An$_{\mathrm{h}}$ is the non-radiative decay rate and n$_{\mathrm{e}}$n$_{\mathrm{h}}$B(n$_{\mathrm{e}}$, n$_{\mathrm{h}})$ is the radiative recombination rate from our model. The photo-luminescence decay is compared to experimental results obtained using time-resolved photoluminescence. [Preview Abstract] |
Monday, March 13, 2017 10:36AM - 10:48AM |
A36.00014: Light radiating-manipulation in toroidal metamaterial by the gain in quantum dots Jie Li, Zhenggao Dong Toroidal dipolar response in a metallic metastructure, composed of double flat rings, is utilized to manipulate the radiation pattern of a single dipolar emitter (e.g., florescent molecule/atom or quantum dot). Strong Fano-type radiation spectrum can be obtained when these two coupling dipoles are spatially overlapped, leading to significant radiation suppression (so-called nonradiating source) attributed to the dipolar destructive interference. Moreover, this nonradiating configuration will become a directionally super-radiating nanoantenna after a radial displacement of the emitter with respect to the toroidal flat-ring geometry, which emits linearly polarized radiation with orders of power enhancement in a particular orientation. Furthermore, via surface plasmon amplification with the assistance of the gain medium of PbS quantum dots, not only toroidal dipole response can be greatly strengthened but also the directional super-radiating intensity also obtains strong enhancement. Our results are promising in manipulating the radiation power and direction of a single emitter, such as fluorescent molecule/atom and quantum dot, by utilizing the intriguing toroidal dipolar response based on the proposed flat-ring metastructure. [Preview Abstract] |
Monday, March 13, 2017 10:48AM - 11:00AM |
A36.00015: The Electronic and Optical Properties of Nanoscale Meta-lattice Made by High Pressure CVD ZhaoHui Huang, Vincent Crespi Meta-lattice can be defined as an artificial 3D superlattice with periodic structural modulation occurred at ~10nm scale. One viable route to synthesize can be as follows: A template is first prepared by close-packed nanoscale silica spheres, then Si/Ge or a binary semiconductor is infiltrated into voids by high pressure chemical vapor deposition (CVD). Later silica spheres can be removed by chemical method, and voids in the inverse meta-latice offer a second opportunity for infiltration. Due to the characteristic length of voids, meta-lattice provides a platform to test novel mesoscopic electronic and optical phenomena. More specifically, a meta-lattice solid can be taken as a collection of molecular clusters connected by thin and narrow metabonds. Electronic properties are expected to share both characteristics of Bloch electrons and molecular states, for example, localized optical transition. Since a significan portion of atoms are located on the surface, the structural details may play a critical role. Here we employ large scale tight-binding calculations and non-equilibrium Green's function method to investigate the electronic (including electronic transport) and optical properties for Si meta-lattices. [Preview Abstract] |
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