Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session U36: Focus Session: Optical Properties of Nano-Dots, Holes, and Wires |
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Sponsoring Units: DMP Chair: Connie Li, Naval Research Laboratory Room: Baltimore Convention Center 339 |
Thursday, March 16, 2006 8:00AM - 8:12AM |
U36.00001: Geometrical scale invariance of the enhanced transmission spectra of subwavelength hole arrays Sinan Selcuk, Kwangje Woo, David B. Tanner, Arthur F. Hebard There is at present a lack of consensus on the relative strength of contributions from surface plasmon polaritons (SPPs) and composite diffracted evanescent waves (CDEWs) to the mechanism responsible for enhanced transmission through subwavelength hole arrays. For regular square hole arrays with small open area fraction $d^{2}/a^{2}$ ($a$ = lattice constant and $d^{2}$ = square hole area), both theories predict that the wavelength $\lambda =\lambda _{m}$ at which maximum transmission occurs scales linearly with $a$. The two interpretations diverge however when the open area fraction increases and the distance between two adjacent hole edges decreases. We test these ideas by comparing transmission spectra of sets of arrays where each set has a fixed open area fraction but is scaled to different sizes by changing $a$. We find in our preliminary data that for a factor of 10 change in $a$, from 1 $\mu $m to 10 $\mu $m, the transmission peaks, when plotted as a function of $\lambda $/$a$, collapse onto the same scaling curve. This collapse is independent of hole size $d$ and is in good agreement with SPP theory when the wavelength-dependent refractive indices of the substrates (quartz and ZnSe) are taken into account. [Preview Abstract] |
Thursday, March 16, 2006 8:12AM - 8:24AM |
U36.00002: Polarization and hole-shape dependence of the transmission of sub-wavelength hole arrays Kwangje Woo, Sinan Selcuk, Arthur F. Hebard, David B. Tanner We have measured the infrared and visible transmission of arrays of holes in silver films, to study the effects of hole shape and hole spacing as a function of the polarization of the light. The anomalous transmission enhancement of sub-wavelength hole arrays in metal films has been attributed to surface plasmon polaritons (SPPs) but this picture is not enough to explain the dependence of hole shape on the transmission due to the long wavelength approximation. We have measured the transmission of arrays of square holes, rectangular holes, and slits in a silver film. We studied the effect of different hole shapes on the enhanced transmission as a function of the polarization angle of the light and found a strong dependence on the hole shape and the polarization angle. In addition, transmission spectra of an array of square holes on a rectangular grid will be presented. [Preview Abstract] |
Thursday, March 16, 2006 8:24AM - 8:36AM |
U36.00003: Tunneling, dipole interactions and coherent Rabi oscillations in quantum dot molecules. Juan E. Rolon, Jose M. Villas-Boas, Sergio E. Ulloa Quantum dot molecules (QDMs) - coupled quantum dot systems - have proved to be a promising optoelectronic circuit element for future implementation of quantum computation at the nanoscale. Here we investigate theoretically the coherent manipulation between exciton states in a single QDM. In particular, we study Rabi oscillations induced via strong laser pumping and their dependence on the interdot quantum coupling strength, including particle interdot tunneling and Coulomb interactions. The dynamics of the system is extracted by solving a quantum master equation using a multilevel density matrix that considers direct and indirect excitons in a rotating wave approximation. Possible decoherence mechanisms, such as coupling to wetting layer states and non-radiative recombination, are incorporated into the master equation using a Lindblad formulation. Careful control of the interdot coupling strength, laser detuning, and intensity, results in different population level dynamics. These are found to be critical for the entanglement between exciton states and the ultimate realization of Bell states for potential quantum information processing. [Preview Abstract] |
Thursday, March 16, 2006 8:36AM - 8:48AM |
U36.00004: Microscopic Models of Hybrid Nanocrystal Superstructures: photonic properties Alexander O. Govorov, Garnett W. Bryant, Wei Zhang, Timur Skeini, Jaebeom Lee, Nicholas A. Kotov We investigate the optical properties of hybrid superstructures composed of metal and semiconductor nanoparticles (NPs), and bio-linkers/polymers. Our study is inspired by recent experiments on bio-conjugated semiconductor-metal NP complexes and their potential applications as sensors. Metal NPs can quench semiconductor NP photoluminescence (PL). However, a plasmon enhancement can be achieved by organizing many Au NPs into a spherical or cylindrical shell around a CdTe NP. We compute electromagnetic fields induced in NP superstructures using a multipole expansion approach to describe the optical response of these complexes. Enhancement of CdTe emission can result from plasmon mediated enhancement of the excitation (Ag structures) or enhancement of the emission process (Au structures). The resultant optical response comes from a complex interplay of this enhancement and quenching and determines the potential applications of these superstructures. [Preview Abstract] |
Thursday, March 16, 2006 8:48AM - 9:00AM |
U36.00005: Spectroscopy of Charged Quantum Dot Molecules E.A. Stinaff, M. Scheibner, A.S. Bracker, I.V. Ponomarev, M.E. Ware, M.F. Doty, T.L. Reinecke, D. Gammon, V.L. Korenev Spins of single charges in quantum dots are attractive for many quantum information and spintronic proposals. Scalable quantum information applications require the ability to entangle and operate on multiple spins in coupled quantum dots (CQDs). To further the understanding of these systems, we present detailed spectroscopic studies of InAs CQDs with control of the discrete electron or hole charging of the system. The optical spectrum reveals a pattern of energy anticrossings and crossings in the photoluminescence as a function of applied electric field. These features can be understood as a superposition of charge and spin configurations of the two dots and represent clear signatures of quantum mechanical coupling. The molecular resonance leading to these anticrossings is achieved at different electric fields for the optically excited (trion) states and the ground (hole) states allowing for the possibility of using the excited states for optically induced coupling of the qubits. [Preview Abstract] |
Thursday, March 16, 2006 9:00AM - 9:12AM |
U36.00006: Multi-Excitonic Quantum Dot Molecules M. Scheibner, E.A. Stinaff, M.F. Doty, M.E. Ware, A.S. Bracker, D. Gammon, I.V. Ponomarev, T.L. Reinecke, V.L. Korenev With the ability to create coupled pairs of quantum dots, the next step towards the realization of semiconductor based quantum information processing devices can be taken. However, so far little knowledge has been gained on these artificial molecules. Our photoluminescence experiments on single InAs/GaAs quantum dot molecules provide the systematics of coupled quantum dots by delineating the spectroscopic features of several key charge configurations in such quantum systems, including X, X$^{+}$,X$^{2+}$, XX, XX$^{+ }$(with X being the neutral exciton). We extract general rules which determine the formation of molecular states of coupled quantum dots. These include the fact that quantum dot molecules provide the possibility to realize various spin configurations and to switch the electron hole exchange interaction on and off by shifting charges inside the molecule. This knowledge will be valuable in developing implementations for quantum information processing. [Preview Abstract] |
Thursday, March 16, 2006 9:12AM - 9:24AM |
U36.00007: Spin interactions in InAs quantum dots M.F. Doty, M.E. Ware, E.A. Stinaff, M. Scheibner, A.S. Bracker, D. Gammon, I.V. Ponomarev, T.L. Reinecke, V.L. Korenev Fine structure splittings in optical spectra of self-assembled InAs quantum dots (QDs) generally arise from spin interactions between particles confined in the dots. We present experimental studies of the fine structure that arises from multiple charges confined in a single dot [1] or in molecular orbitals of coupled pairs of dots. To probe the underlying spin interactions we inject particles with a known spin orientation (by using polarized light to perform photoluminescence excitation spectroscopy experiments) or use a magnetic field to orient and/or mix the spin states. We develop a model of the spin interactions that aids in the development of quantum information processing applications based on controllable interactions between spins confined to QDs. [1] Polarized Fine Structure in the Photoluminescence Excitation Spectrum of a Negatively Charged Quantum Dot, Phys. Rev. Lett. 95, 177403 (2005) [Preview Abstract] |
Thursday, March 16, 2006 9:24AM - 9:36AM |
U36.00008: Theory of Charged Quantum Dot Molecules I.V. Ponomarev, M. Scheibner, E.A. Stinaff, A.S. Bracker, M.F. Doty, M.E. Ware, D. Gammon, T.L. Reinecke, V.L. Korenev Recent optical spectroscopy of excitonic molecules in coupled quantum dots (CQDs) tuned by electric field reveal a richer diversity in spectral line patterns than in their single quantum dot counterparts. We developed a theoretical model that allows us to classify energies and intensities of various PL transitions. In this approach the electric field induced resonance tunneling of the electron and hole states occurs at different biases due to the inherent asymmetry of CQDs. The truncated many-body basis configurations for each molecule are constructed from antisymmetrized products of single-particle states, where the electron occupies only one ground state level in single QD and the hole can occupy two lowest levels of CQD system. The Coulomb interaction between particles is treated with perturbation theory. As a result the observed PL spectral lines can be described with a small number of parameters. The theoretical predictions account well for recent experiments. [Preview Abstract] |
Thursday, March 16, 2006 9:36AM - 9:48AM |
U36.00009: Ultrafast dynamics of surface plasmon polaritons in subwavelength nanohole array on metallic film A.S. Kirakosyan, T.V. Shahbazyan, M. Tong, Z.V. Vardeny The ultrafast dynamics of surface plasmon polaritons (SPP) photogenerated on the surfaces of an Al film perforated with 2D subwavelength hole array ($\sim$300 nm lattice constant) was studied by the pump-probe correlation spectroscopy. Following an instantaneous rise at the onset of the impinging pulse, the transient differential transmission exhibits a fast rise with characteristic time constant of $\sim$300 fs reaching a plateau at $\sim$2 ps, followed by a slower decay with characteristic time of $\sim$40 ps. The observed dynamics can be explained by a fast energy transfer in the Al film from the electron gas to the lattice, with subsequent cooling of the Al film by heat transfer to the glass substrate. The fast dynamics is accompanied by a blue shift of the SPP band due to the increase in the Al lattice temperature. The obtained fast lattice temperature rise is caused by the strong electron-phonon interaction in Al, which makes the electron-lattice energy transfer rate comparable to the rate of nonequilibrium electrons thermalization via electron-electron interactions. A theoretical model based on the Boltzmann equation for nonequilibrium electron gas interacting with quasi-equilibrium phonons was developed, and is in good agreement with the data. [Preview Abstract] |
Thursday, March 16, 2006 9:48AM - 10:00AM |
U36.00010: Internal and external polarization memory loss in single quantum dots Q.Q. Wang, A. Muller, M.T. Cheng, H.J. Zhou, P. Bianucci, C.K. Shih Exciton spin relaxation counts among the most basic features of quantum dot (QD) ground-state dynamics and is intimately connected to the ubiquitous fine-structure doublet anisotropy. Numerous resonant measurements on QD ensembles support a spin relaxation frozen on the exciton lifetime, in agreement with theoretical expectations. Recent investigations, however, question this breakdown based on strictly non-resonant experiments on single QDs, pointing to possible variations among QDs. By using non-linear resonant control of single QDs we examine spin relaxation under different environments and excitation conditions. Data from dots with different dipole moments reveals two distinctive channels for polarization memory loss: (i) an external pathway due to carrier escape and capture to and from the wetting layer that is responsible for memory loss increasing with intensity; and (ii) an internal loss channel, independent of external excitation, due to intrinsic spin relaxation. The values obtained for the latter rule out a universal freezing of exciton spin relaxation in self-assembled QDs. [Preview Abstract] |
Thursday, March 16, 2006 10:00AM - 10:12AM |
U36.00011: Polarized Mid-Infrared Surface Emission from InAs Quantum Dots D. Wasserman, S.A. Lyon, C. Gmachl, E.A. Shaner Polarized mid-infrared surface electroluminescence from self-assembled InAs quantum dots has been observed at 77K. A graded AlGaAs injector is used to inject electrons into excited states in the quantum dot layer. A superlattice electron filter prevents direct electron tunneling out of the quantum dot excited states, increasing the probability of optical transitions to lower energy dot states. Two mid-infrared peaks are seen in the electrically pumped surface emission spectra of the device, one at 100meV, the other at 170meV. The emission peaks are orthogonally polarized within the growth plane, indicating photon emission from intersublevel electron transitions within anisotropically shaped quantums dots. This work suggests the feasibility of using quantum dot mid-infrared emission to study both the morphology of, and intersublevel transitions within, self-assembled quantum dots. [Preview Abstract] |
Thursday, March 16, 2006 10:12AM - 10:24AM |
U36.00012: Optical properties of semiconductor-metal nanocrystal molecules: Exciton-plasmon interactions Wei Zhang, Alexander O. Govorov, Garnett W. Bryant Motivated by recent experiments on bio-conjugated semiconductor-metal hybrid nanocrystal superstructures, we develop a theory to describe a system composed of a semiconductor quantum dot (QD) and a metal nanoparticle (NP) in the presence of external electric fields. The interaction between exciton (in QD) and plasmon (in NP) leads to interesting optical properties. We explore both the linear regime (for weak external field) and the non-linear regime (for strong external field). The interference between the external field and the induced internal field results in strong enhancement of energy absorption (compared with the energy absorption of QD in the absence of a metal NP) and also leads to an asymmetric peak and valley in the total energy absorption (Fano-like shape). We also consider Rayleigh scattering which also reveals this type of behavior. Our theory is useful for understanding present experimental results and can give guidance for future experiments, which may have important applications. [Preview Abstract] |
Thursday, March 16, 2006 10:24AM - 10:36AM |
U36.00013: The Optical Properties of Aluminum Oxide Templated Nanostructures Mahnaz El-Kouedi, Brandy Broglin, Jerry Heath, J. Tres Brazell The optical properties of aluminum oxide templated nanostructures has been investigated with the specific goal of using the associated structures for enhanced transmission of light, and Surface Enhanced Raman Spectroscopy (SERS). We will present fabrication and characterization data for three different nanostructures based on the synthesis of anodic aluminum oxide (AAO) templates. These structures include self-assembled metal coated masks with sub-wavelength apertures for the enhanced transmission of light, striped Au/Ag nanowire arrays and nano-textured aluminum surfaces for SERS studies. Characterization using electron and atomic force microscopies, as well as absorbance, reflectance and Raman spectroscopy will be presented. [Preview Abstract] |
Thursday, March 16, 2006 10:36AM - 10:48AM |
U36.00014: First-Principles Optical Cross-Sections of Ultrathin ZnO Nanowires Shelly Elizondo, John Mintmire One-dimensional nanostructures such as inorganic nanowires and nanotubes represent potential materials for key components of future electronic, optoelectronic, and nanoelectromechanical systems. They will also serve as important model systems to demonstrate quantum-size effects in nanostructured materials. We examine the electronic and optical properties of ZnO nanowires with different geometrical configurations within a first-principles, all-electron self-consistent local density functional (LDF) approach. Orientations along different growth directions are taken into account, with the preferred $\pm$ [0001] direction as the primary focus. The ultrathin nanowires considered here range in diameter from approximately 0.50 nm to 3 nm. We discuss trends in electronic properties and resulting optical properties as a function of nanowire axis orientation and diameter. This work was supported by the US Office of Naval Research, the DoD HPCMO CHSSI program through the Naval Research Laboratory, and the NSF IGERT program. [Preview Abstract] |
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