14 Externe wissenschaftliche Einrichtungen

Permanent URI for this collectionhttps://elib.uni-stuttgart.de/handle/11682/15

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Open Access
Coupling single quantum emitters and plasmonic structures
(2013) Pfeiffer, Markus; Lippitz, Markus (Prof. Dr.)
In this work the interaction of plasmons in metallic nanostructures and excitations in single epitaxially grown semiconductor quantum dots is studied. The enhancement of the electromagnetic field close to metallic structures is used to modify the emission properties of single quantum dots positioned in these regions. The near-field enhancement especially for optical nanoantennas varies on a length scale much shorter than the wavelength. As a consequence, the individual nanostructures have to be placed with a precision of a few nanometers. To have full control over the coupling of a quantum dot to a plasmonic nanostructure one needs the ability to determine and prepare all parameters which influence the coupling. For precisely fabricated nanostructures consisting of epitaxially grown GaAs/AlGaAs quantum dots and different gold nanostructures the near-field coupling is studied here by the characterization of the photoluminescence in the far-field. In the first experimental chapter (chapter 4) of this thesis the optical properties of metallic nanostructures on semiconductor substrates are described. The different modes of plasmonic structures are characterized. The influence of the substrate on the optical properties of nanoantennas are also discussed. The findings will be transferred in the following chapters to nanoantennas with resonance frequencies at the excitation or emission frequency of the quantum dots. In chapter 5 I will present nano-positioning of metallic nanostructures on single quantum dots with an atomic force microscope. The plasmon resonance is used here to achieve an enhancement of the excitation efficiency. The spectral dependence of the field enhancement of the nanoantennas is characterized by varying the excitation of the quantum dots. We observe a clear shift of the near-field enhancement to lower energies, which can be partially explained by the spectral differences of the near- and far-fields of a radiating dipole. In chapter 6 a positioning method based on electron beam lithography is described. With that method nanostructures can be positioned with an accuracy of about 9 nm and oriented arbitrarily. The positioning method is used to place resonant nanoantennas close to single epitaxially grown quantum dots. We achieve for the first time controlled coupling of epitaxial semiconductor quantum dots to resonant metallic nanostructures. The dependence of coupling of quantum dot excitons with plasmons in the nanoantenna is investigated as a function of the relative position of the two. The resonant character of the coupling is studied by tuning the antenna resonance. The coupling of single quantum dots to metallic wires is also investigated. The emission of the quantum dots is modified by the additional emission possibility in the form of propagating plasmons along the wire. In the last chapter the coupling of single quantum dots to extendend periodic plasmonic structures is investigated. The emission of the quantum dots is here influenced by near- and far-field coupling to collective excitations of the antenna arrays. This work describes the controlled investigation of the near-field coupling of embedded quantum dots to metallic nanostructures. The findings pave the way for semiconductor based plasmonic quantum circuits on the micrometer scale.
Open Access
Optische Eigenschaften und Dynamik von photonisch gekoppelten Metall-Partikel-Plasmonen
(2006) Zentgraf, Thomas; Giessen, Harald (Prof. Dr.)
In der vorliegenden Arbeit wird die resonante Kopplung zwischen plasmonischen und photonischen Anregungen in periodisch angeordneten Metallstrukturen mit Hilfe optisch-spektroskopischer Verfahren untersucht. Es werden speziell Gold-Nanopartikel und Nanodraht Strukturen in Kombination mit einem dielektrischen Schichtwellenleiter aus Indium-Zinn-Oxid bzw. Tantal-Dioxid betrachtet. Die photonische Kopplung der periodisch angeordneten Partikel-Plasmon-Resonanzen führt, zusammen mit den Moden des Schichtwellenleiters, zur Ausbildung eines polaritonischen Zustandes. Zunächst wird der Einfluss einer periodisch strukturierten Einheitszelle untersucht. In dieser Supergittergeometrie ergibt sich, dass die Anregungseffizienzen der Moden durch den Strukturfaktor der Einheitszelle bestimmt sind. Eine anschauliche Beschreibung kann mit dem Modell der "Leere-Gitter-Näherung" und der Fouriertransformation der Gitterstruktur erreicht werden. Es zeigt sich, dass durch Veränderung des Strukturfaktors die Kopplung zwischen den Resonanzen gezielt verändert werden kann. Damit ergibt sich die Möglichkeit, die photonische Bandstruktur des Polaritons zu beeinflussen, sowie die Bandaufspaltung verringern oder erhöhen zu können. Im zweiten Teil der Arbeit wird mittels kohärenter zeitaufgelöster Spektroskopie der Einfluss der Kopplung zwischen Plasmonen und Wellenleitermoden auf die Phasenkohärenzzeit der kollektiven Elektronenoszillation in solchen Systemen untersucht. Auf Grundlage eines einfachen Modells wird die zeitliche Dynamik des entstehenden Polaritons beschrieben und durch Vergleich mit den experimentellen Daten die Dephasierungszeit des Polaritons bestimmt. Durch die Kopplung der Gold-Nanostrukturen kommt es zu einer veränderten photonischen Zustandsdichte des Gesamtsystems. Der strahlende Zerfall der Plasmonen, als einer der Hauptdämpfungsmechanismen, kann durch geeignete periodische Strukturierung gezielt verändert werden. Für bestimmte Perioden führt dies zu einer deutlich verlängerten Phasenkohärenzzeit.
Open Access
Ultrafast spectroscopy of single quantum dots
(2012) Wolpert, Christian; Lippitz, Markus (Juniorprofessor Dr.)
In this thesis, the coherent interaction of single semiconductor quantum dots and ultrafast optical pulses is studied. Under certain conditions, localized exciton transitions in quantum dots can be seen as semi-isolated two-level systems. While this description is sufficient for the explanation of some observations in coherent experiments, it is sometimes necessary to explicitly consider coupling of the discreet quantum states confined to the dot with the environment. We start out from simple, classical examples of coherent spectroscopy and then turn towards experiments where the interaction with the vicinity of the dot becomes an important factor. First, a novel method for transient differential reflectivity spectroscopy of single quantum systems is introduced. It is a pure far-field optical technique which does not require any sophisticated sample preparation steps which makes it applicable to a broad range of structures. Pump pulses excite the sample structure and probe pulses read out the pump-induced changes in the system after a variable delay time. In the case of a single dipole, the signal is given in the form of the spectral inteferogram between the backscattered wave from the particle and the probe light which is reflected at the sample surface. This form of homodyne detection amplifies the weak scattered wave from the particle and thus makes this kind of spectroscopy for single quantum dots feasible. In the remainder of this thesis our spectroscopic method is applied to either characterize the coherent properties of single quantum dots, to prepare and read-out a desired quantum state or to deliberately manipulate them. Coherence times and oscillator strengths are determined for localized exciton transitions. Arbitrary population states can be written by driving coherent population oscillations using resonant pulses, while entangled superpositions of two exciton states in a single dot are investigated by quantum beats on transient differential spectra. We finally exploit the interaction between the dot and a nearby absorbing layer to switch the dot's absorption spectrum on ultrafast timescales via light-induced transient electric fields.