Universität Stuttgart
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Item 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.Item Open Access Long-term stability of capped and buffered palladium-nickel thin films and nanostructures for plasmonic hydrogen sensing applications(2013) Strohfeldt, Nikolai; Tittl, Andreas; Giessen, HaraldOne of the main challenges in optical hydrogen sensing is the stability of the sensor material. We found and studied an optimized material combination for fast and reliable optical palladium-based hydrogen sensing devices. It consists of a palladium-nickel alloy that is buffered by calcium fluoride and capped with a very thin layer of platinum. Our system shows response times below 10 s and almost no short-term aging effects. Furthermore, we successfully incorporated this optimized material system into plasmonic nanostructures, laying the foundation for a stable and sensitive hydrogen detector.Item Open Access Modulationsdynamik von rot oberflächenemittierenden Halbleiterlasern(2007) Ballmann, Tabitha; Schweizer, Heinz (Prof. Dr.)Zusammenfassung: Oberflächenemittierende Laser mit Vertikalresonator (VCSEL), die bei einer Wellenlänge von 650-670 nm emittieren, sind insbesondere für optische Datenverbindungen mit Plastikfasern geeignet, die ein Absorptionsminimum bei dieser Wellenlänge besitzen. Hier wird das Bauteildesign, die Herstellung und die Charakterisierung im stationären und modulierten Betrieb von selektiv oxidierten VCSEL beschrieben. Ein Herstellungsprozeß mit parasitätsarmem Bauteildesign wurde entwickelt. Die VCSEL-Geometrie wurde im Hinblick auf eine hohe optische Leistung, Betrieb bis zu hohen Temperaturen und eine schnelle Modulation untersucht. Wärmeerzeugung und -abfuhr und Ladungsträgertransport sollen dabei verstanden werden. Die Absorption bzw. Photonenlebensdauer im VCSEL kann direkt aus den Meßwerten des externen Quantenwirkungsgrads extrahiert werden. Für Aperturen >13 µm ergibt sich eine Absorption von 13 cm-1, was hauptsächlich der Lichtabsorption durch freie Ladungsträger der Dotieratome zugeordnet werden kann. Zu kleineren Aperturen hin ergeben sich kürzere Photonenlebensdauern (statt 2.59 ps nur 1.52 ps bei einer Apertur von 3.5 µm). Zusätzliche optische Verluste treten auf, indem die Ausläufer der Gaußmode an einer kleinen Apertur gestreut werden. Im Gleichstrombetrieb wurde die Temperatur im VCSEL-Inneren und die optische Ausgangsleistung abhängig von den Betriebsbedingungen (zugeführter Strom, Außentemperatur) und der Bauteilgeometrie (Mesa-, Aperturbreite) gemessen und mit einem Temperaturbilanzmodell rechnerisch nachvollzogen. Die Degradation der Stromschwelle und nicht die Degradation der Quantenausbeute legt den Wert in der Licht-Strom Kennlinie fest, an dem der Laser ausgeht. Durch die spektrale Verschiebung der Emissionswellenlänge ist die Temperatur im VCSEL-Inneren bekannt. Sie steigt für kleine Verhältnisse von Apertur- zu Mesabreite am wenigsten mit der Stromdichte an. Das Mesahalbleitermaterial über der engen Oxidapertur sorgt für eine Querverteilung der Wärme und des Stroms. Gleichzeitig hält die Apertur den als Heizquelle wirkenden Pumpschwellstrom klein. Für maximale optische Leistung ist dagegen eine mittlere Aperturgröße am besten. Zu kleinen Aperturen hin begrenzt die schlechtere Wärmeabfuhr über die thermische Leitfähigkeit die Ausgangsleistung. Zu großen Aperturen und damit auch großen Pumpströmen hin dominiert die dissipierte elektrische Leistung mit ihrer Wärmeerzeugung durch den elektrischen Widerstand. Im gepulsten Betrieb erhält man eine maximal mögliche Umgebungstemperatur von 150°C für das Materialsystem des 670 nm VCSEL GaInP/AlGaInP mit einer Banddiskontinuität von ca. 400 meV. Bei höheren Temperaturen gehen zu viele Elektronen den Quantenfilmen verloren. Dieser Wert entspricht den Innentemperaturwerten, bis zu denen im Gleichstrombetrieb Laseremission zu sehen war. Mißt man die VCSEL-Antwort auf eine Kleinsignalmodulation der Stromamplitude und paßt eine Drei-Pol-Transferfunktion aus den Laserratengleichungen an, ist es möglich, die relative Wichtigkeit der vier bandbreitenlimitierenden Effekte in einem Halbleiterlaser zu bestimmen. Das ist die intrinsische Dämpfung der Resonanzspitze (0.17 ns K-Faktor -> 52 GHz Bandbreite), die thermische Sättigung der Resonanzfrequenz, das parasitäre und das transportbedingte parasitätsähnliche Absinken der Antwortfunktion (33 ps diffusive Transportzeit der Ladungsträger über die Einbettungs- und Barrierenschicht der 1-lambda-cavity -> 18 GHz Bandbreite). Durch eine dickere Passivierungsschicht reduzierten wir die Kontaktflächenkapazität und damit das parasitäre RC-Produkt und erreichen Modulationsbandbreiten von 4 GHz für einen 650 nm VCSEL. Beseitigt man das parasitäre Abfallen der Antwort zu hohen Frequenzen hin, ist das eigentliche Limit im roten VCSEL ein thermisches Limit - wie im Gleichstrombetrieb. Mit höherem Arbeitsstrom nimmt die Bauteilerwärmung zu und Photonendichte und Bandbreite sättigen. Der kleine Apertur-VCSEL mit dem besseren Temperaturbudget erreicht deutlich höhere Resonanzfrequenzen und zwar 6.3 GHz bei 4.5 mA mit einer Apertur von 3.5 µm (bei 657.9 nm). Aus den Modulationsmessungen läßt sich zudem über die Verstärkungskompression die lokale Einfangzeit von der Barrierenregion in den Quantenfilm als maximal 2 ps lang abschätzen. Die digitale Großsignalantwort des VCSELs ist durch Ein- und Ausschaltverzögerungen weiter begrenzt. Die numerische Simulation der Antwort liefert für die Ladungsträgerlebensdauer an der Schwelle 0.39 ns (Apertur 7 µm) (wie auch aus der Schwellstromdichte des stationären Betriebs und aus der Kleinsignalmodulation). Bei einem Vorstrom über der Schwelle wird die Einschaltverzögerung mit steigender Kleinsignal-Resonanzfrequenz kürzer. Aber auch die RC-Aufladekurve durch die dünne Oxidschicht beeinflußt die Einschaltverzögerung noch. Es wurde ein Augendiagramm bei einer Datenrate von 1.25 Gb/s mit dem 650 nm VCSEL aufgenommen.Item Open Access Ultrafast near- and mid-infrared laser sources for linear and nonlinear spectroscopy(2016) Steinle, Tobias; Giessen, Harald (Prof. Dr.)Item Open Access Surface- and tip-enhanced resonant Raman scattering from CdSe nanocrystals(2015) Sheremet, Evgeniya; Milekhin, Alexander G.; Rodriguez, Raul D.; Weiss, Thomas; Nesterov, Maxim; Rodyakina, Ekaterina E.; Gordan, Ovidiu D.; Sveshnikova, Larisa L.; Duda, Tatyana A.; Gridchin, Victor A.; Dzhagan, Volodymyr M.; Hietschold, Michael; Zahn, Dietrich R. T.Surface- and tip-enhanced resonant Raman scattering (resonant SERS and TERS) by optical phonons in a monolayer of CdSe quantum dots (QDs) is demonstrated. The SERS enhancement was achieved by employing plasmonically active substrates consisting of gold arrays with varying nanocluster diameters prepared by electron-beam lithography. The magnitude of the SERS enhancement depends on the localized surface plasmon resonance (LSPR) energy, which is determined by the structural parameters. The LSPR positions as a function of nanocluster diameter were experimentally determined from spectroscopic micro-ellipsometry, and compared to numerical simulations showing good qualitative agreement. The monolayer of CdSe QDs was deposited by the Langmuir–Blodgett-based technique on the SERS substrates. By tuning the excitation energy close to the band gap of the CdSe QDs and to the LSPR energy, resonant SERS by longitudinal optical (LO) phonons of CdSe QDs was realized. A SERS enhancement factor of 2 × 10 3 was achieved. This allowed the detection of higher order LO modes of CdSe QDs, evidencing the high crystalline quality of QDs. The dependence of LO phonon mode intensity on the size of Au nanoclusters reveals a resonant character, suggesting that the electromagnetic mechanism of the SERS enhancement is dominant. Finally, the resonant TERS spectrum from CdSe QDs was obtained using electrochemically etched gold tips providing an enhancement on the order of 10 4 . This is an important step towards the detection of the phonon spectrum from a single QD.Item Open Access Coupling strength of complex plasmonic structures in the multiple dipole approximation(2011) Langguth, Lutz; Giessen, HaraldWe present a simple model to calculate the spatial dependence of the interaction strength between two plasmonic objects. Our approach is based on a multiple dipole approximation and utilizes the current distributions at the resonances in single objects. To obtain the interaction strength, we compute the potential energy of discrete weighted dipoles associated with the current distributions of the plasmonic modes in the scattered fields of their mutual partners. We investigate in detail coupled stacked plasmonic wires, stereometamaterials and plasmon-induced transparency materials. Our calculation scheme includes retardation and can be carried out in seconds on a standard PC.Item Open Access Chiral plasmonic near-field sources : control of chiral electromagnetic fields for chiroptical spectroscopies(2016) Schäferling, Martin; Giessen, Harald (Prof. Dr.)This thesis investigates the chiral near-field response of plasmonic nanostructures. The chiral properties of electromagnetic fields can be quantified by the so-called optical chirality, which is a figure that can be directly calculated from basic field properties. The larger the optical chirality is, the stronger the respective field will interact with chiral molecules. In principle, electromagnetic fields with high optical chirality enable the detection of the handedness of chiral molecules with enhanced sensitivity. This is of major importance in biochemistry and pharmaceutics because biological processes essentially depend on the handedness of the involved molecules. We introduce the concept of chiral plasmonic near-field sources to aid the respective chiroptical spectroscopy techniques. We cover two main topics in our systematic analysis: Firstly, what are the conditions and mechanisms to generate and enhance chiral near-fields? And secondly, which requirements for chiral plasmonic near-field sources exist and how can they be fulfilled? For this, we group the near-field sources regarding the chiral symmetry properties of both the nanostructure as well as the incident light. Both of these constituents influence the properties of the resulting chiral plasmonic near-field sources. Chiral nanostructures offer the possibility to enhance the optical chirality of the incident light. We show that planar chirality can lead to regions with chiral near-fields of uniform handedness. Regions with opposite handedness are clearly separated by the structure plane. Three-dimensionally chiral structures can exhibit chiral hot-spots where particularly strong optical chirality can be found. Based on our investigations of chiral structures, we introduce the concept of plasmonic racemates, which are mixtures of both handednesses of the chiral nanostructure. The local interaction with the chiral building blocks allows for the generation of chiral near-fields although the achiral superstructure exhibits no chiroptical far-field response. This enables chiroptical spectroscopy without additional contributions to the signal due to the presence of a chiral structure. Furthermore, we present a concept for metasurfaces that facilitate plasmonic racemates with particularly high integration density. The combination of an achiral linear plasmonic nanoantenna with linearly polarized light demonstrates that chiral near-fields can be formed locally in systems without structural chirality. This can be attributed to interference between incident and scattered light, as shown in our analysis. Based on this finding, we propose a chiroptical spectroscopy method that utilizes linearly polarized light instead of the circular polarization that is commonly used. Furthermore, we demonstrate that the eigenmodes of chiral systems can lead to particularly strong and extended chiral near-fields. The most efficient way to excite these modes is linearly polarized light. We obtained the best results from a design consisting of four intertwined helices. Chiral near-fields have been found in the whole volume surrounded by the structure. In addition, we discuss a configuration of slanted slits on top of a mirror. This design is easy to fabricate and enables chiroptical spectroscopy via reflection measurements. In summary, we provide fundamental insights into the functioning as well as the properties of chiral plasmonic near-field sources. We show that this concept can be used for highly sensitive enantiomer discrimination and how this can be accomplished. Furthermore, we provide a theoretical basis to optimize chiral plasmonic near-field sources.Item Open Access High-power broadband femtosecond near- and mid-infrared sources based on optical parametric oscillators and difference frequency generation at 40 MHz repetition rates(2014) Hegenbarth, Robin; Gießen, Harald (Prof. Dr.)The goal of this dissertation was the generation of femtosecond sources in the near- and mid-infrared spectral range with repetition rates in the 40 MHz range. The approach was based on nonlinear frequency conversion based on an Yb:KGW laser oscillator with up to 7.4 W output power as a pump source. Output powers on the Watt level for the near-infrared and in the 100 mW region in the mid-infrared around 5 µm, as well as several mW between 10 and 20 µm, were the goal. With the Yb:KGW laser oscillator an optical parametric oscillator (OPO) based on a 1 mm long magnesium-oxide-doped periodically poled lithium niobate (MgO:PPLN) crystal has been pumped. The signal of this OPO is almost gap-free tunable between 1446 and 1896 nm with up to 1.7 W average signal output power at femtosecond pulse durations. At wavelengths close to the point at which the intracavity group-delay dispersion equals zero, dual-signal-wavelength operation occurs due to equal group delay at two wavelengths. Due to the high pump power available, very high signal output coupling rates in excess of 50% are beneficial. In order to obtain a spectrally broadband mid-infrared (mid-IR) source, the difference frequency between the two OPO signals was generated outside the cavity by means of either a gallium selenide (GaSe) or silver gallium diselenide (AgGaSe2) crystal. The specifications of this system were characterized with a selection of different crystal lengths. This system generates up to 4.3 mW of average mid-IR power. Its spectra can be tuned between 10.5 and 16.5 µm (952 – 606 cm-1) with more than 50 cm-1 spectral width. Power and spectra are temporally very stable. Thus, it was demonstrated that it is feasible to combine this system with a scattering-type scanning near-field optical microscope. Even more power in the mid-IR spectral region was obtained with an AgGaSe2 OPO that was synchronously pumped by the MgO:PPLN OPO at single-signal-wavelength operation. With this device up to 113 mW average idler output power at 4857 nm (2059 cm-1) were obtained. By adjusting cavity length, phase-matching angle, and pump wavelength, its idler wavelength could be tuned between 4570 and 5121 nm (2188 – 1953 cm-1) with more than 40 cm-1 spectral width.Item Open Access Short-range surface plasmonics: localized electron emission dynamics from a 60-nm spot on an atomically flat single-crystalline gold surface(2017) Frank, Bettina; Kahl, Philip; Podbiel, Daniel; Spektor, Grisha; Orenstein, Meir; Fu, Liwei; Weiss, Thomas; Horn-von Hoegen, Michael; Davis, Timothy J.; Meyer zu Heringdorf, Frank-Joachim; Giessen, HaraldItem Open Access Hydrogen in metal nanoparticles : understanding and applying thermodynamic properties of metal-hydrogen nanostructures(2017) Strohfeldt, Nikolai; Giessen, Harald (Prof. Dr.)The mobility sector is undergoing a fundamental change from fossil fuels through electricity to hydrogen. However, for hydrogen technology to be successful, the storage devices need to be pushed forward. Currently, the most promising path is to employ nanotechnology in metal hydride storage systems. This thesis presents different methods and material systems exploring the interaction of metallic nanoparticles and hydrogen. It aims to expand the limited literature knowledge about size dependent effects on thermodynamic and optical properties at the nanoscale. Several analytical and numerical models are developed and compared to own experimental data as well as existing literature. The experimentally investigated structures are palladium square patches, palladium-gold disk stacks, and yttrium nanorods. All structures throughout the thesis are characterized using plasmonic extinction spectroscopy, an optical measurement technique employing localized oscillations of the conduction electrons as a sensitive tool for structural and electronic changes in nanoparticles. The palladium square-patch investigations show a hydrogen loading pressure that is increasing with nanoparticle size, whereas the hydrogen induced in-plane expansion is decreasing with size. In the yttrium rod antenna studies, a drastic but reversible hydrogen induced elimination of the plasmonic resonance is observed, rendering the structure a highly interesting plasmonic switch. A sensitive plasmonic gas sensor is realized combining palladium nanoparticles with gold antennas. Through palladium-gold disk nanostacks that plasmonically behave as one superstructure, large hydrogen induced peak shifts of comparatively narrow resonances are demonstrated. Complementing the experimental findings, analytical models are developed for the isotherms of palladium nanoparticles and the plasmonic resonances of square nanopatches. The isotherm model reveals a coherent loading mechanism of palladium nanoparticles. In contrast, the unloading mechanism and the general bulk behavior follow incoherent transitions with a reduced hysteresis. The developed plasmon resonance model illustrates a method for obtaining broadband dielectric data of nanoparticles without prior knowledge of any material properties besides the particle geometry and the plasmon resonance wavelength. The findings presented in this thesis will be helpful to develop more efficient energy storage systems and powerful hydrogen sensors through well designed nanostructured devices.