08 Fakultät Mathematik und Physik
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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 Perturbation and manipulation of leaky modes in photonic crystal fibers(2020) Upendar, Swaathi; Weiss, Thomas (Apl. Prof. Dr.)Optical fibers guide light in a central core surrounded by a cladding. The most common fibers are step-index fibers, which guide light using total internal reflection in the fiber core. Recently, a new class of fibers, with a microstructured cladding, which also include photonic crystal fibers have been developed. The photonic crystal fibers have a periodic refractive index profile in the cladding and guide light using a bandgap effect or modified total internal reflection. Photonic crystal fibers promise to surpass the guiding properties of the traditional step-index fiber and are being studied extensively. However, these new fibers support leaky modes in contrast to the perfectly guided or bound modes of the conventional step-index fiber. Leaky modes are solutions to Maxwell’s equations that radiate energy in the transverse direction of the fiber. This energy leakage leads to growing fields in the homogeneous exterior. Due to these growing fields in the exterior, the normalization of leaky modes has been a long standing challenge. The normalization for bound modes, which have exponentially decaying fields as we move away from the fiber core, is achieved using an integral of the time-averaged Poynting vector over the xy plane. However, this expression diverges for the case of leaky modes. In this thesis, we derive a general analytical normalization for leaky and bound modes in fiber structures that is independent of the region of integration as long as it encloses all spatial inhomogeneities. Using this analytical normalization, which is an essential factor in any perturbation theory, we develop perturbation theories for interior and exterior perturbations in fiber geometries supporting leaky modes. The perturbations are considered to be changes in the permittivity and permeability tensors of the fiber, which also extend to the axial, i.e., the translationally invariant direction. We formulate the exterior perturbation theory to also treat wavelength as a perturbation. This is highly useful to obtain important fiber quantites such as group velocity as a simple post processing step instead of repeatedly solving Maxwell’s equations for different wavelengths. We demonstrate the accuracy of both perturbation theories on analytically solvable capillary fibers and the more complicated photonic crystal fibers. We also demonstrate the usefulness of a perturbation theory in studying disorder, which involves averaging over many realizations. Furthermore, we present a theoretical study of a novel design to reduce the confinement loss of the fundamental core mode in photonic bandgap fibers with high index strands. This is done by modifying the radius of specific strands, which we call “corner strands”, in the core surround. We demonstrate the usefulness of the analytical normalization in optimizing the fiber design by providing a physically meaningful way of comparing field confinement for different fiber structures. As fundamental working principle, we show that varying the radius of the corner strands leads to backscattering of light back to the core. By using an optimal radius for these corner strands in each transmission window, the losses are decreased by orders of magnitude in comparison to the unmodified cladding structure. We do a parametric analysis of this phenomenon by varying different structural properties such as radius, pitch and the radius-to-pitch ratios to find the optimal design. Thus, we generalize the previously studied case of missing corner strands which only works for certain radius-to-pitch ratios in the first bandgap. This design can be adapted to any photonic bandgap fiber including hollow core photonic crystal fibers and light cage structures.Item Open Access Functional complex plasmonics : understanding and realizing chiral and active plasmonic systems(2016) Yin, Xinghui; Giessen, Harald (Prof. Dr.)The present thesis concerns itself with the theoretical study and experimental realization of complex plasmonic systems for highly integrated nanophotonic devices and enhanced chiroptical spectroscopy. In particular, the two broad topics of active metasurfaces and chiral plasmonic systems are investigated to this end. In this context, the chalcogenide phase change material GeSbTe is utilized to demonstrate, for the first time, metasurface based beam steering and varifocal lensing devices. The versatility of this approach to lending active functionality to plasmonic systems is further evidenced through our realization of a chiral plasmonic system that both exhibits a wavelength tunable and handedness switchable chiroptical response. Furthermore, in order to enable a systematic study of plasmon- enhanced chiroptical spectroscopy, we rst establish and analyze canonical chiral plasmonic building blocks, in particular, the loop wire and chiral dimer structure. The results from this undertaking lead to fundamental insights for understanding complex chiral plas- monic systems. Finally, we implement chiral media in the commercial electromagnetic full- field solver Comsol Multiphysics to carry out rigorous numerical studies of the macroscopic electrodynamic processes involved in plasmon-enhanced circular dichroism spectroscopy revealing both substantial enhancement due to near-field effects as well as upper boundaries to the magnitude of such enhancements.Item Open Access Imaging microspectroscopy of functional nanoplasmonic systems(2020) Sterl, Florian; Giessen, Harald (Prof. Dr.)Item Open Access Nonlinear optics in hybrid plasmon-fiber cavities(2021) Ai, Qi; Giessen, Harald (Prof. Dr.)Item Open Access Optical antennas : nanoscale radiation engineering and enhanced light-matter interaction(2014) Drégely, Daniel; Giessen, Harald (Prof. Dr.)This thesis studies optical nanoantennas from the near-infrared to the mid-infrared region. Nanoantennas are key components in the emerging field of nanophotonics. They exhibit strong interaction with the optical radiation field because of the excitation of plasmonic resonance, which leads to high near-field intensities, deep subwavelength energy confinement, and strongly enhanced radiation. This thesis addresses the key questions of how these properties can be used to enhance light-matter interaction and how to engineer optical radiation on the nanoscale by tailoring the antenna geometries. We demonstrate that radiofrequency antenna geometries can be scaled to the optical regime by experimental realization of optical Yagi-Uda nanoantennas. A Yagi-Uda antenna has unidirectional radiation properties, which means light incident from one direction is efficiently confined to a deep subwavelength volume while that incident from the other directions is not. We assess the near-field of a planar plasmonic Yagi-Uda nanoantenna with scanning near-field optical microscopy. We record phase and amplitude in order to identify the optical modes and demonstrate directional receiving of light at a wavelength of 1064 nm. We then fabricate three-dimensional Yagi-Uda nanoantenna arrays, which exhibit very high directivities out of the substrate plane. Since the antenna array is completely embedded in a dielectric matrix, scanning near-field optical microscopy cannot be used for optical characterization. Instead, we use Fourier transform infrared spectroscopy combined with near-field simulations to study the directional antenna array, which receives out of plane radiation at a wavelength of 1500 nm. Furthermore, we show by simulation how to use our nanoantenna array for beamsteering. In order to solve the challenge of mapping the near-field intensity of three-dimensional nanoantennas, we develop a novel field-mapping technique based on surface enhanced vibrational spectroscopy. The high near-field intensities generated by plasmonic structures are used to enhance vibrational transitions in molecules, which occur in the infrared spectral region. We position molecules at specific locations close to plasmonic antennas, which are designed to be in resonance with the vibrational band around 4400 nm, and measure the extinction spectrum of the coupled antenna-molecule system. We observe that the measured vibrational signal scales with the local near-field intensity, which is applied to map the plasmonic near-field intensity. This method maps the field in the infrared region and provides subwavelength resolution. We finally demonstrate that our technique is able to assess near-field intensities of plasmonic structures with three-dimensional complexity. Furthermore, we demonstrate for the first time optical power transfer by nanoantennas. We realize in experiment a wireless point-to-point link between a transmitter and a receiver nanoantenna at a wavelength of 785 nm. By fluorescence microscopy, we measure the radiation pattern and show that the transmission of the wireless link follows the inverse square power law of free space propagation. This enables low-loss power transfer across large distances at the nanoscale. In addition, we experimentally demonstrate beamsteering over a broad angular range by adjusting the wavefront of the incident optical field on the transmitter. In our experiment we show that the transmitter can address different receivers by effective beamsteering. The low-loss power transfer combined with the beamsteering functionality comprises a significant advancement compared to state-of-the-art waveguide connections. Our reconfigurable nanoantenna link may lead to technology breakthrough in information transfer between nanoscale devices and objects.Item Open Access Hybrid materials for nonlinear optics(2018) Albrecht, Gelon; Giessen, Harald (Prof. Dr.)The goals of this thesis are to find new and more efficient material systems as well as concepts for nonlinear optics on the nanoscale. Nonlinear optical effects are mainly limited in such systems by the low nonlinear susceptibility and low photo stability of the used materials. To improve the low nonlinear susceptibility, plasmonic materials have been used for several years. These systems use the near-field enhancement of the plasmonic resonance to increase the nonlinear conversion efficiency. The efficiency can additionally be increased by using the evanescent plasmonic near-field in the vicinity of the plasmonic nanostructure. Therefore, a highly nonlinear organic polymer is deposited on the plasmonic nanostructures, creating a hybrid organic plasmonic material. Several organic materials are particularly suited due to their high nonlinear susceptibility and their simple and reproducible handling. Combined with high photo stability, these are the key requirements for a suitable polymer. However, several tested polymers did not meet these requirements. Notably, the photo stability is too low. Furthermore, for the first time it could be unambiguously proven that these hybrid materials can be improved due to an increased overall nonlinear susceptibility. Many other concepts for hybrid materials only utilize the modified near-field distribution and cannot benefit from the surrounding nonlinear medium or cannot exclude this influence. The presented layout can easily be improved by replacing the used polymer with other existing polymers that exhibit larger nonlinear susceptibilities. The hybrid plasmonic structures use gold as plasmonic material. Even if it is more photo stable than polymers, gold does not withstand high illumination intensities due to its low dimensional stability. This is a major drawback since most applications require a stable plasmon resonance. To overcome this issue a simple but effective way to significantly increase the thermal stability as well as the photo stability of gold nanostructures is presented. The improved properties are due to an alumina protective coating. The alumina coating can be as thin as 4 nm maintaining access to the enhanced near-field of the plasmonic nanostructure. With this concept a platform for nonlinear optics and high temperature applications is available that is stable in air at temperatures up to 900°C and still has excellent optical properties. Moreover this system withstands laser intensities at least up to 10 GW/cm² , one order of magnitude more than usually used intensities for nonlinear spectroscopy on gold nanostructures. Finally, common and more uncommon plasmonic materials are surveyed to determine their linear and nonlinear optical properties. Furthermore, the thermal and chemical stability with and without a protective alumina coating is investigated. Based on the collected data silver, gold, copper, magnesium, and aluminum could be identified and confirmed to be suitable materials for nonlinear applications. Moreover, nickel, palladium, platinum, germanium, and YH2 are investigated for their plasmonic and thermal properties, however suitable nonlinear properties have not been observed. Based on this survey a comparison of the presented materials is possible, which surprisingly did not exist until this survey. Bi2Te2Se is investigated as an unusual plasmonic material that exhibits edge state plasmons. These edge state plasmons arise from the topological properties of the material. Up to now these edge state plasmons have only been observed via electron excitation. To reveal the predicted localized modes nanostructures are fabricated by several methods and dark field spectroscopy is applied. However, no optical plasmonic response could be identified, most likely due to the small scattering rate of the material.Item Open Access Hybrid plasmonic devices for sensing and thermal imaging(2015) Tittl, Andreas; Giessen, Harald (Prof. Dr.)Plasmonics is an emerging field in nanooptics, which focuses on the optical properties of resonant subwavelength metal nanoparticles. Historically, such geometries commonly employed noble metal nanoparticles to achieve a variety of effects ranging from nanofocusing of light to negative refraction. Building on these concepts, this thesis investigates hybrid nanoplasmonic devices, which combine passive noble metal nanostructures with chemically reactive or actively tunable materials to obtain novel functionalities. Utilizing various complex plasmonic geometries, this work pursues two complementary threads of research, covering the technological scale from fundamental science to device applications. On the one hand, it utilizes chemically synthesized hybrid plasmonic "smart dust" nanoprobes to detect progressively lower reagent concentrations. Starting from silica shell-isolated gold nanoparticles, which are used to map the catalytic reactions in adjacent extended palladium thin films, DNA-assembled bimetallic plasmonic nanosensors are investigated to resolve changes in sub-5nm Pd nanocrystals on the single antenna level, pushing the lower limit of chemical detection volume. On the other hand, it studies plasmonic perfect absorber structures, optical elements designed to absorb all radiation of a certain wavelength, which have shown promise for a variety of technological applications. Here, the focus is on both developing a theoretical model for the optical behavior of plasmonic perfect absorber structures, especially at large incident angles, as well as on the experimental realization of efficient gas sensors and active mid-infrared imaging devices.Item Open Access Optical properties of aperiodic metallic photonic crystal structures(2013) Bauer, Christina; Giessen, Harald (Prof. Dr.)In this thesis the linear optical properties of aperiodic metallic photonic crystals are studied. All structures consist of a metal grating on top of a waveguide material. The incident light can excite plasmonic modes in the metal as well as photonic modes in the waveguide underneath. These resonances are coupled to each other. In the first part of the thesis, the samples with a one-dimensional metal grating are studied. The structural arrangement of the metal wires on top of the dielectric waveguide layer is disordered, quasicrystalline, or fractal. For the disorder samples, the experimentally obtained coupling constant shows reduced values for larger disorder amounts. Additionally, the calculated coupling constants are compared to the experimentally obtained Urbach energies. It is found that the relation between these two parameters is dependent on the disorder model as well as on the average grating period. The optical properties of the samples with the quasicrystalline and fractal metal wire arrangement are analyzed with respect to their long, short, and average wire distances. The next part of the thesis deals with two-dimensional metal gratings. The metal disks are arranged in a quasicrystalline fashion with the disks being elliptically shaped and rotated with respect to the sample x axis. It is found that the optical properties of such structures are dependent on the eccentricity of the metal disks as well as on the rotation angle between the short main axis and the sample x axis. Afterwards, a theoretical model is developed in order to describe the optical properties of such structures. With the theoretical approach it is possible to calculate the normal incidence spectra as well as the oblique light incidence spectra. This model is used to predict the absorption enhancement of plasmonic solar cells.