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Semiconductor Physics Tuesday Cavity QED concepts stimulated a tremendous technological development towards solid-state based, compact, and scalable cavity QED systems. In this contribution we report on a strongly coupled cavity QED system consisting of a CdSe nanocrystal coupled to a single photon mode of a polymer microsphere. The strong exciton-photon coupling is manifested by the observation of a cavity mode splitting of ¯hω ≈ 37µeV and photon lifetime measurements of the coupled exciton-photon state. The single photon mode is isolated by lifting the mode degeneracy in a slightly deformed microsphere and addressing it by high-resolution imaging spectroscopy. This cavity mode is coupled to a localized exciton of an anisotropically shaped CdSe nanocrystal on the microsphere surface that emits highly polarized light in resonance to the mode. With colloidal CdSe NRs we add a new material class for which solid-state based cavity QED was implemented. Keynote Talk HL 18.3 Tue 16:00 HSZ 01 Deutsch-Jozsa Algorithm using Triggered Single Photons from a Single Quantum Dot — •Oliver Benson 1 , Matthias Scholz 1 , Thomas Aichele 2 , and Sven Ramelow 1 — 1 Humboldt-Universität zu Berlin, Institut für Physik, Hausvogteiplatz 5-7, 10117 Berlin, Germany — 2 CEA/Université J. Fourier, Laboratoire Spectrométrie, Grenoble, France Recently, wide attention has been drawn to the implementation of quantum algorithms by solely using linear optics. Previous experimental demonstrations along this line focused on coherent photon states from attenuated laser pulses [1] or spontaneous parametric down-conversion [2] in order to simulate simple quantum algorithms or to demonstrate concepts of noise resistant quantum computation [3]. We realize the on-demand operation of the two-qubit Deutsch-Jozsa algorithm using a triggered single-photon source. Our experimental setup resembles a classical Mach-Zehnder interferometer that is combined with a single-photon source realized by an exciton transition in a single quantum dot [4]. A variation of our experimental setup enables us to implement ideas of the concept of decoherence-free subspaces [5] in a triggered quantum algorithm on the single-photon level. [1] S. Takeuchi, Phys. Rev. A 62, 032301 (2000) [2] M. Bourennane et al., Phys. Rev. Lett. 92, 107901 (2004) [3] M. Mohseni et al., Phys. Rev. Lett. 91, 187903 (2003) [4] V. Zwiller et al., Appl. Phys. Lett. 82, 1509 (2003) [5] P. Zanardi and M. Rasetti, Phys. Rev. Lett. 79, 3306 (1997) Keynote Talk HL 18.4 Tue 16:30 HSZ 01 Imaging the Local Density of Photonic States in Photonic Crystal Nanocavities — •Michael Kaniber, Felix Hofbauer, Simon Grimminger, Max Bichler, Gerhard Abstreiter, and Jonathan J. Finley — Walter Schottky Institut, Am Coulombwall 3, 85748 Garching We present investigations of the coupling of InGaAs quantum dots (QDs) to both extended and strongly localised optical modes in 2D photonic crystal (PC) nanostructures. The samples consist of a 180nm thick GaAs membrane into which a PC is formed by etching a triangular lattice of air holes. By measuring the local QD spontaneous emission rate (Rspon) we ”image”the photonic DOS at frequencies throughout the photonic bandgap (PBG) and close to localised modes at single missing hole defects (Q∼10000, Vmode < 0.5(λ/n) 3 ). For QDs emitting into the PBG but detuned from the cavity mode, we observe a strong suppression of Rspon compared to its value in a homogenous photonic environment (R0/Rspon=30±6) due to the reduced photon DOS. In contrast, for QDs coupled to the cavity modes we measure 1/Rspon ∼ 50ps, corresponding to a large Purcell enhancement (Rcavity/R0=18x). Single dot measurements reveal clear photon anti-bunching when the emission frequency is detuned from the cavity mode and enhanced photon extraction efficiency (∼30%) due to the PBG which suppresses inplane emission. Most surprisingly, anti-bunching is not observed for QDs coupled to the cavity modes possibly due to the onset of low threshold lasing. Supported financially via Sonderforschungsbereich-631 Keynote Talk HL 18.5 Tue 16:45 HSZ 01 Theory of optical properties for quantum dots in microcavities — •Frank Jahnke, Jan Wiersig, Norman Baer, and Christopher Gies — Institute for Theoretical Physics, University of Bremen Semiconductor quantum dots are of strong current interest due to their application potential in light-emitting devices and single photon sources. The emission properties can be controlled to a high degree by embedding the quantum dots in a semiconductor microcavity. We present a microscopic theory for this system. In addition to carrier-photon interaction, also carrier-carrier and carrier-phonon interaction are included, which are the sources of scattering and dephasing processes. We investigate the photoluminescence dynamics for weak excitation and the laser regime for elevated pumping. The influence of the Purcell effect and of large spontaneous emission coupling on the optical properties is demonstrated. Signatures of carrier and photon correlations due to various interaction processes are analyzed. HL 19 Quantum dots and wires: Optical properties II Time: Tuesday 17:15–19:30 Room: HSZ 01 HL 19.1 Tue 17:15 HSZ 01 Influence of doping on the electronic and optical properties of Si nanocrystallites — •Luis Ramos 1 , Elena Degoli 2 , Stefano Ossicini 2 , Jürgen Furthmüller 1 , and Friedhelm Bechstedt 1 — 1 Institut für Festkörpertheorie und -optik, Friedrich-Schiller-Universität Jena, Max-Wien-Platz 1, D-07743 Jena, Germany — 2 Università di Modena e Reggio Emilia, via Fogliani, I-42100 Reggio Emilia, Italy Silicon nanocrystallites (NCs) have been intensively studied in the last years, since they can confine holes and electrons and circumvent the indirect-gap character of the lowest-energy optical transitions of Si bulk. Besides quantum confinement, oxidation, oxygen-related defects,[1] and doping have been investigated. Recently, an increase of the photo luminescence (PL) intensity was observed for Si NCs doped with both group- III and group-V species. Since measurements for single NCs are difficult, ab initio theoretical investigations become important to suggest dopants and to clarify the mechanisms of PL in Si NCs. Our calculations are based on the density-functional theory, the generalized-gradient approximation, the projector-augmented wave method, and the pseudopotential approximation. The electronic structure and optical absorption spectra of free-standing doped Si NCs of different sizes and shapes are investigated in simple-cubic supercells. Besides the influence of shape and size on the impurity formation energies, bond lengths, and radiative lifetimes, significant changes in the optical absorption spectra are predicted for Si NCs doped with group-V impurities. [1] L.E. Ramos, J. Furthmüller, and F. Bechstedt, Appl. Phys. Lett. 87, 143113 (2005); Phys. Rev. B 71, 035328 (2005) HL 19.2 Tue 17:30 HSZ 01 Temperature dependent fluorescence quantum efficiency of cascaded energy transfer nanocrystal structures — •S. Rohrmoser, T. Franzl, T.A. Klar, A.L. Rogach, and J. Feldmann — Photonics and Optoelectronics Group, Physics Department and CeNS, Ludwig- Maximilians-Universität München We present temperature dependent fluorescence studies of cascaded energy transfer (CET) structures made of CdTe nanocrystals. Funnel like band gap profiles are realized by applying layer-by-layer assembly to CdTe nanocrystals of distinct sizes. For high-energetic excitation, the CET structure comprising only one layer of red-emitting nanocrystals emits 4 times more red light than a reference sample of equal absorbance consisting of only red emitting nanocrystals, hence increasing the final excitation density by a factor of 28. To investigate the underlying process in more detail, temperature dependent and time resolved measurements have been performed. The results reveal an activation barrier involved in the energy transfer process and help to understand the long-lived feeding of the central layer. 1. T. Franzl, T.A.Klar, S. Schietinger, A.L. Rogach, J. Feldmann, ”Exciton recycling in graded gap nanocrystal structures” Nano Letters, 4, 1599 (2004)
Semiconductor Physics Tuesday HL 19.3 Tue 17:45 HSZ 01 Structural investigations of MBE-grown InN Nano-Whiskers — •Ratan Debnath 1 , Toma Stoica 1,2 , Ralph Meijers 1 , Thomas Richter 1 , Raffaella Calarco 1 , and Hans Lüth 1 — 1 Institute of Thin Films and Interfaces (ISG1) and CNI - Centre of Nanoelectronic Systems for Information Technology, Research Center Jülich,52425 Jülich, Germany — 2 INCDFM, Magurele, POB Mg7, Bucharest, Romania Nanowires are intensively studied for future device applications of lowdimensional systems. GaN nanowhiskers were investigated in great detail showing for instance high crystalline quality and efficient luminescence. Investigations of InN nanowhiskers have been staying behind since it is more difficult to produce InN of good quality. However InN has interesting properties like high electron mobility, low bandgap and non-toxicity. InN was grown by plasma-assisted molecular beam epitaxy (PAMBE) on Si(111) substrates under N-rich conditions resulting in columnar morphology of the grown layers. Substrate temperature was considerably lower compared to GaN growth. The samples were investigated using scanning electron microscopy (SEM) as well as Photo- and Cathodoluminescence (PL and CL) spectroscopy. The growth was optimized to obtain uniform columns of good crystalline quality. An optical bandgap was found in the range 0.73-0.82eV and electron concentrations between 8 × 10 17 and 6 × 10 18 cm −3 were determined. HL 19.4 Tue 18:00 HSZ 01 Storage of excitons in elongated semiconductor nanocrystals — •Robert M. Kraus 1 , Pavlos G. Lagoudakis 1 , Andrey L. Rogach 1 , John M. Lupton 1 , Jochen Feldmann 1 , Dmitry Talapin 2 , and Horst Weller 2 — 1 Photonics and Optoelectronics Group, Physics Department and CeNS, Ludwig-Maximilians-Universität München, Germany — 2 Institute of Physical Chemistry, University of Hamburg, Germany Spherical CdSe nanocrystals capped by a CdS rod-like shell, referred to as nanorods, exhibit interesting spectral dynamics on the single particle level.[1,2] However, for the purpose of applications, the ensemble properties of nanorods are most interesting. We are especially interested in the behaviour of an ensemble of nanorods under the influence of an electric field, as this bears great relevance for future devices. We show here that by applying an electric field to an ensemble of nanorods in a vertical sample geometry a linear quantum-confined Stark shift of the order of 60 meV can be observed in the emission energy. During the application of the electric field the excitons are effectively hindered from radiative recombination and can be stored coulombically for up to 100 µs. Furthermore, modulation of the electric field leads to a modulation in both the wavelength and the spectral width of the nanoparticle emission. [1] J. Müller et al., Phys. Rev. Lett. 93, 167402 (2004) [2] J. Müller et al., Nanoletters 5, 2044 (2005) HL 19.5 Tue 18:15 HSZ 01 Electric field induced photoluminescence quenching in CdSe nanocrystal doped SiO2 on Si — •Helmut Karl, Alexander Achtstein, and Bernd Stritzker — Institut für Physik, Universität Augsburg, D-86135 Augsburg, Germany Buried CdSe nanocrystals were synthesized by sequential ion implantation of Cd and Se in 500 nm thick thermally grown SiO2 on p-doped silicon. The formation of the CdSe nanoclusters was initiated by a postimplantation thermal annealing step. Then an optically semitransparent thin Au gate electrode was evaporated on top of the SiO2 forming a MOS capacitor structure. The embedded surface near CdSe nanocrystals show efficient steady state CdSe bandedge photoluminescence when excited by a cw-HeCd laser at a wavelength of 442 nm at room temperature. The silicon substrate was electrically contacted by indiffusion of an evaporated Al thin film. With this structure we observe strong electric field induced photoluminescence quenching when a voltage is applied between the Au gate electrode and the silicon substrate for temperatures between 10 K and room temperature. PL quenching of more than 80 % was found for an electric field variation between 0 and +/- 4x10 7 V/m. CV-characteristics in conjunction with the electric field dependence of the PL quenching will be discussed. HL 19.6 Tue 18:30 HSZ 01 Size dependence of the dynamics of the Mn 3d 5 luminescence in wire-like arrangements of (Zn,Mn)S nanoparticles — •L. Chen 1 , P.J. Klar 1 , W. Heimbrodt 1 , F.J. Brieler 2 , and M. Fröba 2 — 1 Dept. Physics and WZMW, Philipps-University of Marburg, Germany — 2 Institute of Inorganic and Analytical Chemistry, Justus-Liebig- University of Gießen, Germany (Zn,Mn)S nanoparticles with Mn concentrations ranging from 1% to 30% and in a wire-like arrangement were formed inside mesoporous SiO2 matrices of various pore diameters. The nanoparticles were characterised using photoluminescence and excitation spectroscopy. It is found, that the Mn 2+ ions are incorporated on cation lattice sites replacing Zn. The decay times of the internal Mn 2+ (3d 5 ) luminescence are studied in detail by time resolved spectroscopy over more than 5 orders of magnitude in intensity. A concentration and a remarkable size dependence of the time behaviour has been observed indicating a geometry dependence of the energy-transfer processes within the Mn system. HL 19.7 Tue 18:45 HSZ 01 Optical properties of implanted single ZnO nanowires — •Daniel Stichtenoth 1 , Sven Müller 1 , Carsten Ronning 1 , Lars Wischmeier 2 , Chegnui Bekeny 2 , and Tobias Voss 2 — 1 II. Institute of Physics, University of Göttingen, Germany — 2 Institute for Solid State Physics, University of Bremen, Germany Doping of semiconductor nanostructures via ion implantation processes offers the advantage of precise control of the doping concentration in both lateral and depth direction beyond any solubility limit. In this study, single crystalline ZnO nanobelts and -wires were synthesized according to the VLS mechanism and subsequently dispersed on top of Si substrates. The nanowires were implanted either with 14 N, 31 P, 14 N & 31 P, or 20 Ne ions. Nitrogen and Phosphorous are potential acceptors in ZnO; whereas, the objective of the Ne-implantation was to monitor the implantation induced damage. The range of the ions, set by the ion energy, matched the diameter of the nanowires, and post-implantation annealing procedures were done under vacuum conditions in order to remove the introduced damage. The treated nanowires were individually investigated by temperature-dependent µ-PL measurements; correlations between the optical spectra and the implanted species as well as the implantation parameters are discussed. HL 19.8 Tue 19:00 HSZ 01 Nitrogen implanted ZnO nanowires — •Sven Müller, Daniel Stichtenoth, Daniel Schwen, and Carsten Ronning — 2nd Institute of Physics, University Göttingen, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany Single crystalline ZnO nanowires were grown via a chemical vapor deposition process: ZnO powder was placed into a horizontal tube furnace and heated up to 1350 ◦ C. The vapour was transported by an Ar gas flow to the substrates in a temperature zone between 1000 - 1180 ◦ C. Prior growth the Si substrates were covered with a thin gold layer, which acts as a catalyst for the vapour-liquid-solid (VLS) mechanism. The wurtzite ZnO nanowires grew along the c-axis and had a belt-like shape in the nanometer range. These ZnO nanowires were implanted with 50 keV nitrogen ions (as a potential acceptor) in order to change the electrical and optical properties. Directly after the implantation process, the properties were dominated by the radiation damage, which was subsequently healed by annealing in vacuum or in an oxygen atmosphere. The nitrogen implantation generated three new luminescence transitions at energies of 3.35 eV, 3.32, and 3.235 eV. The origin of these features will be discussed in respect to their temperature- and power-dependencies. HL 19.9 Tue 19:15 HSZ 01 Optical Spectroscopy on Silicon Nanoparticles — •Stephan Lüttjohann 1 , Cedrik Meier 1 , Andreas Gondorf 1 , Axel Lorke 1 , and Hartmut Wiggers 2 — 1 Laboratorium für Festkörperphysik, Universität Duisburg–Essen, 47048 Duisburg — 2 Institut für Verbrennung und Gasdynamik, Universität Duisburg–Essen, 47048 Duisburg The optical properties of silicon nanoparticles have been studied by photoluminescence and Raman spectroscopy. The particles are fabricated in a low pressure microwave reactor by decomposition of silane. We have investigated particles in a size range of between d=4.2nm and 60nm. For particles with diameters smaller than 30nm, quantum effects become relevant and are observed in Raman spectra as well as in photoluminescence spectra. The Raman spectra show the phonon confinement
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