Browsing by Author "Giessen, Harald (Prof. Dr.)"

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Open Access
3D printed micro-optics: materials, methods and applications
(2022) Weber, Ksenia; Giessen, Harald (Prof. Dr.)
Open Access
3D printing of sub-micrometer accurate ultra-compact free-form optics
(2016) Gissibl, Timo; Giessen, Harald (Prof. Dr.)
Additive manufacturing enables novel and unprecedented engineering and production possibilities, which are predicted to have an enormous impact in the 21st century. The technology allows for the straightforward three-dimensional printing of volumetric objects as designed. In this thesis, we present a novel concept in optics, which overcomes many difficulties in the fabrication of micro-optics and opens the new field of 3D printed micro- and nano-optics with complex lens designs. Our work is just at the interface between micro- and nano-optics and represents a paradigm shift for micro-optics. It takes only a few hours from lens design, to production, testing, and the final working optical device. Using dip-in femtosecond two-photon direct laser writing, our method goes far beyond state-of-the art attempts to manufacture simple micro-lenses by lithography. We prove the versatility of this method by writing different optics. Collimation optics, toric lenses, free-form surfaces with polynomials of up to 10th order for intensity beam shaping, as well as chiral photonic crystals for circular polarization filtering, all aligned onto the core of single mode fibers are shown. In addition, we show that three-dimensional direct laser writing is a suitable tool for the fabrication of complex multi-lens optical systems that show high quality optical imaging, beam shaping performance, and tremendous compactness with sizes below 300 µm. We determine the accuracy of our optics by analyzing the imaging and beam shaping quality as well as characterizing the surfaces by atomic force microscope measurements and interferometric measurements. The method yields high fabrication accuracy and allows to manufacture of lenses with a rms (root mean square) surface roughness of less than 15 nm. The surfaces deviate from their designs by less than ±1 µm. Our 3D printed compound lenses feature resolving powers of up to 500 line pairs per millimeter. Our printed micro-optical elements can thus achieve sufficient performance in order to enable compound lenses for high quality imaging. In addition, we show the performance of diffractive optical elements with diameters of just 4.4 µm, which enable beam shaping at the end facet of an optical fiber. The intensity is shaped into a uniform or into a donut-shaped intensity distribution. For this purpose, the diffractive optics are directly fabricated onto the end facet of the optical fiber and show unprecedented performance for optical beam shaping. Our method allows for a plethora of novel applications with tremendous impact on optical trapping of atoms and in-vivo imaging in the human body. In addition, applications for imaging and illumination in endoscopy, multiple sensors, and eyes for micro-robots can be realized.
Open Access
Advanced numerical and semi-analytical scattering matrix calculations for modern nano-optics
(2011) Weiss, Thomas; Giessen, Harald (Prof. Dr.)
The optical properties of nanostructures such as photonic crystals and metamaterials have drawn a lot of attention in recent years. The numerical derivation of these properties, however, turned out to be quite complicated, especially in the case of metallo-dielectric structures with plasmonic resonances. Hence, advanced numerical methods as well as semi-analytical models are required. In this work, we will show that the scattering matrix formalism can provide both. The scattering matrix approach is a very general concept in physics. In the case of periodic grating structures, the scattering matrix can be derived by the Fourier modal method. For an accurate description of non-trivial planar geometries, we have extended the Fourier modal method by the concept of matched coordinates, in which we introduce a new coordinate system that contains the material interfaces as surfaces of constant coordinates. In combination with adaptive spatial resolution, we can achieve a tremendously improved convergence behavior which allows us to calculate complex metallic shapes efficiently. Using the scattering matrix, it is not only possible to obtain the optical properties for far field incidence, such as transmission, reflection, absorption, and near field distributions, but also to solve the emission from objects inside a structure and to calculate the optical resonances of a system. In this work, we provide an efficient method for the ab initio derivation of three-dimensional optical resonances from the scattering matrix. Knowing the resonances in a single system, it is in addition possible to obtain approximated resonance positions for stacked systems using our method of the resonant mode coupling. The method allows describing both near field and far field regime for stacked two-layer systems, including the strong coupling to Fabry-Perot resonances. Thus, we can study the mutual coupling in such systems efficiently. The work will provide the reader with a basic understanding of the scattering matrix formalism and the Fourier modal method. Furthermore, we will describe in detail our extensions to these methods and show their validity for several examples.
Open Access
Characterization of the coherence of ultra-cold atoms with nonlinear matter wave optics methods
(2006) Meiser, Dominic; Giessen, Harald (Prof. Dr.)
In this dissertation we make use of the many analogies between quantum optical and ultra-cold atomic and molecular systems in order to study the coherence properties of the latter with methods of non-linear optics. We adapt the XFROG method that has first been developed for the characterization of ultra-short laser pulses, to the problem of reconstructing both amplitude and phase of the condensate wavefunction of a Bose-Einstein-condensate (BEC). Using the example of a vortex state we study the dependence of the reconstruction quality on the number of measurements and different sources of noise and we find that the method is feasible with available experimental technology. Exploiting the similarity between the coherent formation of ultra-cold molecules and optical sum frequency generation we devise a scheme for measuring second-order correlations of atoms through density measurements of molecules. We use perturbation theory in the cases of weak and strong coupling between atoms and molecules to calculate the momentum distribution of the molecules for the cases where the molecules are formed from a BEC, a normal Fermi gas and a Fermi gas with superfluidity in a Bardeen-Cooper-Schrieffer (BCS) state. These calculations are supplemented by exact integrations of Schroedinger's equation in the single mode approximation for the molecules. Atoms in a BEC are collectively transformed into molecules with a narrow momentum distribution reflecting the long coherence length of atoms in the BEC. For the normal Fermi gas molecules are formed non-collectively and their momentum distribution is much wider. The momentum distribution of molecules from a BCS state looks similar to the BEC case: The superfluid component leads to collectively formed molecules with a very narrow momentum distribution and the unpaired fraction gives rise to non-collectively formed molecules with a much wider momentum distribution similar to the normal Fermi gas case. The counting statistics of the molecules from a BEC is that of a coherent state, from a normal Fermi gas it is that of a thermal state and the BCS case interpolates between the two.
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.
Open Access
Compact and efficient femtosecond supercontinuum sources based on diode-pumped solid-state lasers
(2009) Hoos, Felix; Giessen, Harald (Prof. Dr.)
This thesis deals with the realization of compact and multi-Watt femtosecond supercontinuum sources based on diode-pumped laser oscillators and highly nonlinear glass fibers. The main aspect of this work focuses on the development of directly diode-pumped sub-200 fs laser oscillators which are suitable as pump lasers for those fibers. In addition, a comprehensive study of the properties of supercontinua generated with pump wavelengths around 1 µm is presented. Micro-structured fibers, either tapered fibers or photonic crystal fibers~(PCF), can convert narrow band laser light into multiple octave broad spectra. Average powers of several Watts and conversion efficiencies of more than 50% can be achieved. The properties of those spectra depend strongly on the input pulse duration. A particular feature of the femtosecond regime, which can be defined by an input pulse duration shorter than 200 fs, is a high spectral phase coherence. The main part part of this thesis describes the development of two diode-pumped sub-200 fs laser oscillators. On one hand, a very compact 20 MHz Yb:glass laser, on the other hand, a multi-Watt 44 MHz Yb:KGW oscillator. With the latter oscillator, we demonstrated to our knowledge the highest optical-to-optical efficiency of a diode-pumped multi-Watt laser with a pulse duration of 150-170 fs. These advances are based on our study of Yb:KGW slab-oscillators pumped by broad-area laser diodes. The thermal effects in the Yb:KGW crystal slabs pumped by high-power broad-area diodes were investigated in detail experimentally as well as numerically. These results constitute a considerable contribution to the development of diode-pumped Ytterbium tungstate lasers. In the second part of this work, we studied the properties of supercontinua generated in tapered fibers with Ytterbium lasers with typical wavelengths around 1 µm. While the intensity noise and the influence of the fiber geometry on the spectra were found to be similar to former results achieved with Ti:sapphire lasers, a different influence of a pulse pre-chirp on the spectra was observed for Ytterbium lasers.
Open Access
Complex 2D & 3D plasmonic nanostructures : Fano resonances, chirality, and nonlinearities
(2013) Hentschel, Mario; Giessen, Harald (Prof. Dr.)
This thesis covers two topics of the still emergent field of plasmonics. On the one hand we make use of the interaction of particle plasmon resonances to create 2D as well as 3D complex plasmonic structures which show radically different optical properties than the individual building blocks do. On the other hand we utilize the strongly enhanced local electric field associated with plasmonic nanostructures for nonlinear optical processes. In particular, we study the formation of Fano resonances in complex nanoparticles arrangements. So-called plasmonic oligomers, that are highly symmetric arrangements of metallic nanoparticles, are discussed in detail. These clusters support dark modes which lead to pronounced scattering minima in their otherwise broad dipolar scatting peaks. We demonstrate the amazing tunability of these clusters and the formation of higher order dark modes. Moreover, we discuss the plasmonic analogue of electromagnetically induced transparency (EIT) in 2D as well as 3D arrangements of metallic nano-bars. We show that such 3D particle groupings are capable of encoding their 3D arrangement in well pronounced and unique optical spectra. We thus envision that our structure can serve as a three-dimensional plasmon ruler enabling the optically determination of three-dimensional arrangements on the nanoscale. Taking the concept of plasmonic EIT one step further, we demonstrate that the destructive interference between normal plasmonic modes, which leads to plasmonic EIT and decreased absorbance in the structure, can be switch to constructive interference and thus enhanced absorbance. It can be argued that this phenomenon is the plasmonic analogue of electromagnetically induced absorbance (EIA). What is more, we discuss the formation of optical chirality in 3D arrangements of metallic nanoparticles which vastly outperform any naturally occurring chiral substances in the strength of their interaction with an external light field. We deduce the prerequisites for this strong response and demonstrate that only configurational chirality, that is a handed arrangement of equally sized particles, leads to a strong plasmonic chiral optical response. Compositional chirality, that is the use of different sized particles in an unhanded arrangement, is not favourable. This finding is in contrast to chemistry and molecular physics where a so-called chiral center, a carbon atom dressed with four different ligands, is the archetype chiral building block. Moreover, we show that it is possible to optically deduce the spatial arrangements of individual particles in these structures, as chirality is an inherently 3D property. Furthermore, we will demonstrate the formation of a strong and broadband chiral optical response upon the formation of charge transfer modes, that is, due to ohmic contact of the clusters constituents. Finally, we demonstrate the plasmonic analogue of diastereomers, structures possessing several chiral centers. We thus construct plasmonic composite structures consisting of two different handed sub-units. We show that the optical response, in striking contrast to their molecular counterparts, can be described in terms of fundamental building blocks. The chiral optical response of such complex structures can thus be traced back to the optical properties of the constituting elements. Finally, we investiagte nonlinear optical processes in plasmonic and plasmonic-dielectric-hybrid systems. In particular, we investigate third harmonic generation from dimer nanoantennas and show that the nonlinear optical response, in contrast to common belief, is not governed by gap nonlinearities but fully described by the linear optical properties of the antenna. A simple nonlinear harmonic oscillator model is shown to reproduce all experimental features. Moreover, we will discuss the selective filling of bowtie nanoantennas with the chi2 active material LiNbO3 and the nonlinear optical response of this hybrid system. As an outlook we discuss the role of symmetries in nonlinear optics and the perceived implications for nonlinear plasmon optics.
Open Access
Fabrication and optical characterization of metamaterials and nano-antennas
(2008) Guo, Hongcang; Giessen, Harald (Prof. Dr.)
In this thesis, we have investigated the optical properties of metamaterials and optical nano-antennas. The structures were fabricated with electron beam lithography and subsequent dry etching process. The linear transmission and reflection of these structures were measured with an Fourier transform infrared spectrometer. We also performed numerical simulations with a finite integration time domain algorithm. Split-ring resonators (SRRs) are promising building blocks for negative permeability metamaterials. By investigating the thickness dependence of the optical resonances of split-ring resonator metamaterials experimentally and numerically, we have demonstrated that the plasmonic model describes well the resonance behavior of SRRS in the optical regime. We have shown that the resonance of an SRR can be described by that of a cut-wire with an equivalent total length. We have also shown that combining with the hybridization concept, the plasmonic model offers a simple picture to understand the optical properties of coupled system such as double SRRs. Optical nano-antennas have attracted a great deal of attention due to their strong field enhancement and field confinement at resonance. In this thesis, we have investigated the optical properties of slot antennas using the bowtie slot antenna as a representative design. We demonstrated experimental results of bowtie slot antenna arrays in a thin metal film. Assisted by the numerical simulation tools, the resonance properties of bowtie slot antennas have been discussed in the framework of geometry dependence. We showed that two distinct types of resonances can be supported by bowtie slot antennas in the near infrared and visible spectral range, and they show a complementary behavior with respect to geometry dependence. We also investigated the effect of metal properties on the resonances of bowtie slot antennas.
Open Access
From near-field to far-field: plasmonic coupling in three-dimensional nanostructures
(2012) Taubert, Richard; Giessen, Harald (Prof. Dr.)
This thesis provides a comprehensive study of the coupling phenomena that occur in plasmonic nanostructures. Electromagnetic coupling between metallic nanoparticles leads to strong spectral modifications in the structures, which are determined using linear optical spectroscopy in the visible and infrared wavelength range. In contrast to previous investigations, the key aspect here are the properties of plasmonic far-field coupling in three-dimensionally arranged structures. These are fabricated by electron beam lithography in a multilayer process, which allows for a three-dimensional arrangement of plasmonic particles. We study coupling in a plasmonic dimer for different arrangements. In order to address the transition from the near-field to the far-field regime, we investigate a structure consisting of two nanowires stacked on top of each other for a wide range of interparticle spacings, and thus are able to examine near- as well as far-field coupling effects. In case of near-field coupling, only the quasistatic near fields of the plasmonic structures are important and the plasmon hybridization scheme gives an excellent qualitative description of all the observed phenomena. In contrast,the far-field regime is characterized by the occurrence of Fabry-Pérot modes due to the large vertical spacing between the particles. These couple to the particle plasmon resonances, forming new coupled modes which are extensively discussed. A situation of particular interest occurs whenever the interparticle distance fulfills the Bragg criterion, i.e., the vertical distance equals a multiple of half the particle plasmon resonance wavelength: then the coupled mode which spectrally approaches the single layer particle plasmon resonance becomes dark and a broad region of high reflectance forms. Increasing the number of oscillators stacked at this particular distance leads to the increase of the spectral width of the plasmonic response and the formation of a broad photonic band gap, which spans about one octave in the optical wavelength regime. In contrast to previous similar investigations which were carried out with semiconductor quantum well structures or atoms in optical lattices, the plasmonic particles exhibit an extraordinarily strong coupling to the light field. Therefore, we are able to explore the regime where the coupling between the oscillators is limited by their large radiative decay rate, rather than nonradiative decay channels. Finally, we address the intermediate coupling regime by investigating a structure which utilizes properties of both, near- and far-field coupling. A dipolar cut-wire is placed on top of a quadrupolar cut-wire pair. As long as the spacing between both, quadrupolar and dipolar oscillator is sufficiently small (near-field regime), the coupling leads to a transmittance peak in the spectra due to a destructive interference between the coupled modes. Hence, this effect is described as the plasmonic analog of electromagnetically induced transparency. On increasing distance, the relative phase of the oscillators changes and constructive instead of destructive interference is achieved. As a consequence, a sub-dipolar linewidth peak in the absorbance spectrum is observed. The phenomenon can thus be termed the plasmonic analog of electromagnetically induced absorption. In the pure near-field regime, the plasmonic fields exhibit no retardation phase due to their quasistatic nature. In the far-field regime, however, a coupling to the quadrupolar oscillator would not be possible. Hence, the occurrence of this effect relies on the intermediate regime, where properties of both, near- and far-field regime are present.
Open Access
Functional & active plasmonic systems and metasurfaces
(2022) Karst, Julian; Giessen, Harald (Prof. Dr.)
Funktionelle und aktive plasmonische Nanostrukturen und Metaoberflächen sind das Herzstück von einer Vielzahl an neuartigen optischen Technologien. Sie ermöglichen es Licht auf sehr kleinen Längenskalen zu bündeln und aktiv zu manipulieren. Insbesondere werden sie zur Miniaturisierung von elektro-optischen Bauteilen beitragen, welche für die Realisierung von zukunftsweisenden Technologien in verschiedensten Bereichen vonnöten sind. Hierzu zählen unter anderem Technologien aus den Bereichen virtuelle Realität (VR: engl: virtual reality) und erweiterte Realität (AR: engl: augmented reality), dynamische drei-dimensionale Holografie, miniaturisiertes LiDAR (engl.: Light Detection and Ranging), sowie nano-optische Sensoren für explosive und giftige Gase oder für (händige) chemische Substanzen. Die vorliegende Arbeit befasst sich mit mehreren Ansätzen für funktionelle und aktive plasmonische Systeme und Metaoberflächen. Hierbei analysieren wir insbesondere deren Funktionalitäten als auch die zugrundeliegenden Schaltmechanismen. Wir führen zunächst metallische Polymere für schaltbare plasmonische Systeme ein. Diese Polymere sind zwar seit den 1980er Jahren bekannt, wurden jedoch hauptsächlich für transparente und leitfähige Elektroden verwendet sowie optimiert. Wir zeigen, dass die Nanostrukturierung solcher Polymere es ermöglicht, Nanoantennen mit elektrisch schaltbaren optischen Eigenschaften herzustellen. Deren plasmonische Resonanzen lassen sich hierbei elektrisch ein- oder ausschalten. Unser Konzept basiert auf einem elektrisch-schaltbaren Metall-zu-Isolator-Übergang des Polymers, der durch CMOS-kompatible Spannungen von lediglich ±1 V kontrolliert werden kann. Schaltzeiten erreichen Displayfrequenzen von 33 Hz, was es unseren Nanoantennen ermöglicht, Anwendung in zukünftigen Display-Technologien zu finden. Zudem demonstrieren wir Langzeit-Stabilität über mehrere Hundert Zyklen. Durch gezielte periodische Anordnung unserer metallischen Polymer-Nanoantennen erschaffen wir ultra-dünne Metaoberflächen zur aktiven Laserstrahlablenkung sowie zur aktiv-steuerbaren Lichtfokussierung mit sehr hohem Kontrast. Wir realisieren zudem ein Metaobjektiv aus metallischem Polymer, welches neuartige Funktionalitäten, wie z.B. einen bifokalen Zustand, aufweist. Als Nächstes untersuchen wir aktive plasmonische Systeme auf Basis von wasserstoffsensitiven Metallen. Solche schaltbaren plasmonischen Antennen finden Anwendung in aktiven Metaoberflächen sowie in plasmonischen Wasserstoffsensoren. Häufig verwendete Materialien sind hierbei Palladium, Yttrium, oder Magnesium. Letzteres bietet hierbei den Vorteil, dass es ein exzellenter Wasserstoffspeicher ist. Zudem können die optischen Eigenschaften von plasmonischen Nanoantennen aus Magnesium mit Hilfe von Wasserstoff sehr stark beeinflusst werden. Jedoch sind die damit einhergehenden Schaltzeiten häufig sehr lang und die Degradierung von Magnesium während der Schaltvorgänge sehr hoch. Um die hierbei limitierenden Faktoren herauszufinden, vermessen wir Magnesium während der Hydrogenisierung mittels auf Streuung basierender optischer Rasternahfeldmikroskopie. Hiermit können wir die Wasserstoff-Diffusionsprozesse in Magnesium auf der Nanometer-Skala detailliert untersuchen. Es lässt sich dabei feststellen, dass diese Diffusionsprozesse stark von der polykristallinen Morphologie von Magnesium sowie durch dessen Volumenausdehnung beeinflusst werden. Darüber hinaus zeigen wir, dass nicht nur Wasserstoff in Form von Gas, sondern auch Alkohole in der Lage sind Metalle zu hydrogenisieren. Am Beispiel von Yttrium demonstrieren wir, dass sich die optischen Eigenschaften von plasmonischen Nanoantennen durch deren Eintauchen in Alkohol verändern lassen. Dabei fungieren die Nanoantennen als lokale nanooptische Indikatoren, um diesen flüssigen Hydrogenisierungsprozess optisch zu visualisieren. In verschiedensten Bereichen ist es zudem nötig die optische Aktivität/Chiralität bzw. Händigkeit einer chemischen Substanz zu ermitteln. So entscheidet, z.B., die Händigkeit einer Substanz darüber, ob sie als Medikament eingesetzt werden kann oder giftig für den menschlichen Körper ist. Diese Chiralität kann optisch detektiert und gemessen werden. Hierbei können plasmonische Nanoantennen die Sensitivität entscheidend verbessern. Deshalb analysieren wir zuletzt die spektrale Antwort einzelner chiraler Nanopartikel. Hierzu verwenden wir unsere automatisierte Messmethode, mit welcher sich die chiralen Streuspektren der Einzelpartikel detailliert untersuchen lassen. Dabei zeigen wir, dass die chiralen Einzelpartikelspektren stark von der jeweiligen Morphologie des Partikels abhängen. Unsere untersuchten Partikel weisen im Allgemeinen eine spiralförmige Struktur auf, welche zu einer enormen optischen Aktivität führt. Folglich können diese chiralen plasmonischen Nanopartikel in verschiedensten Sensorplattformen vielversprechende Anwendungen finden. Unter anderem könnte mit deren Verwendung die Sensitivität bzw. Empfindlichkeit von nanooptischen Sensoren für chirale Moleküle bzw. Flüssigkeiten enorm gesteigert werden. In Kombination mit schaltbaren plasmonischen Materialien rücken sogar aktiv steuerbare chirale Sensoren in greifbare Nähe.
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.
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.
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.
Open Access
Hybrid plasmonic structures for giant Faraday rotation
(2017) Flöß, Dominik; Giessen, Harald (Prof. Dr.)
Propagiert linear polarisiertes Licht durch ein magneto-optisches Medium, so bewirkt ein angelegtes statisches Magnetfeld eine Drehung der Polarisation der elektromagnetischen Welle. Dieses Phänomen wird als Faraday-Effekt bezeichnet. Was diesen Effekt besonders auszeichnet, ist die Tatsache, dass durch den Einfluss des angelegten Magnetfeldes sowohl die Zeitumkehrinvarianz, als auch die Lorentz-Reziprozität gebrochen werden. Aufgrund dieser Eigenschaft, werden Faraday-Rotatoren als Grundbaustein in einer Vielzahl von nichtreziproken optischen Systemen eingesetzt. Das wichtigste Beispiel sind optische Isolatoren, die eine Faraday-Rotation von 45° benötigen, um Licht in Vorwärtsrichtung zu transmittieren und in Rückwärtsrichtung vollständig zu blockieren. Sehr viele optische Komponenten, die Faraday-Rotatoren beeinhalten, unterliegen dem Trend hin zu immer stärkerer Miniaturisierung. Daraus folgt ebenfalls ein großer Bedarf an Faraday-Rotatoren mit kleinsten räumlichen Abmessungen. Allerdings ist die Realisierung solcher Systeme sehr anspruchsvoll, da die Faraday-Rotation proportional zur Dicke des verwendeten magneto-optischen Materials ist. Das bedeutet, dass mit einem kleineren magneto-optischen Kristall auch die maximal erreichbare Faraday-Rotation sinkt. Um diesem Verhalten entgegenzuwirken, werden in dieser Dissertation mehrere neuartige Methoden vorgestellt, die es erlauben den Faraday-Effekt eines Dünnfilms mithilfe von periodischen metallischen Nanostrukturen zu verstärken. Die verschiedenen Ansätze werden sowohl experimentell, als auch theoretisch untersucht. Weiterhin wird gezeigt, dass das magneto-optische Verstärkungsprinzip der Nanostrukturen mithilfe eines einfachen Oszillatormodells elegant beschrieben werden kann. Die in dieser Dissertation vorgestellten hybriden magnetoplasmonischen Systeme bestehen aus EuSe- und EuS-Dünnfilmen, sowie aus Gold-Nanogittern. Es wird gezeigt, dass diese weniger als 200 nm dicken Strukturen bei einer Temperatur von 20 K und einem statischen Magnetfeld von 5 T eine Faraday-Rotation von bis zu 14° erzeugen können. Weiterhin kann die Polarisation des transmittierten Lichts durch Umpolung und Variation des Magnetfeldes über einen 25° breiten Winkelbereich hinweg reguliert werden. Da die für optische Isolation benötigte Drehung von 45° nur einen Faktor drei größer ist als die von der Dünnfilmstruktur erreichte Faraday-Rotation, ist das hier präsentierte Konzept sehr vielversprechend und könnte wichtige Anwendungen im Bereich integrierter nichtreziproker photonischer Systeme finden. Besonders herauszustellen sind hier Anwendungen in den Bereichen optische Isolation, Lichtmodulation und Magnetfeldmessung.
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.
Open Access
Imaging microspectroscopy of functional nanoplasmonic systems
(2020) Sterl, Florian; Giessen, Harald (Prof. Dr.)
Open Access
Large-area low-cost fabrication of complex plasmonic nanostructures for sensing applications
(2015) Zhao, Jun; Giessen, Harald (Prof. Dr.)
In this thesis, we introduce hole-mask colloidal lithography and nanosphere lithography techniques for low-cost nanofabrication of large-area (about 1 cm^2) plasmonic nanostructures with different complex shapes. For the first one, we use thin film PMMA-gold hole-masks, which are first prepared with polystyrene colloids, combined with following tilted-angle-rotation evaporation to fabricate large-area randomly deposited plasmonic nanostructures. For the second one, we use hexagonal close-packed polystyrene nanosphere monolayers directly as evaporation masks to fabricate large-area periodic plasmonic nanostructures. We describe the fabrication process step by step, and manufacture a variety of different plasmonic nanostructures for different sensing applications. For example, we use split-ring-resonators for antenna-assisted surface-enhanced infrared absorption measurements to detect monolayer molecules with an up to 20000-fold enhancement factor. We also utilize asymmetric double split-ring- resonators for localized surface plasmon resonance sensing with experimental sensitivities of up to 520 nm/RIU and figures of merit up to 2.9. Furthermore, we investigate plasmonic oligomers consisting of touching triangular building blocks, which show fundamental modes, higher-order modes, as well as Fano resonances due to coupling between bright and dark modes within the same complex structures. Large-area low-cost direct contact Au-Pd hydrogen sensors are demonstrated, which show much improved spectral shifts as large as 30 nm upon hydrogen exposure. Additionally, we improve hole-mask colloidal lithography for three-dimensional and multishape fabrication. With multiple repetitions of hole-mask lithography, single-layer metasurfaces with complex, multi-shape plasmonic nanostructures can be created that exhibit desired optical functionalities. Large-area and low-cost fabrication of different samples with independently tunable resonances is demonstrated. These single-layer metasurfaces could find possible applications as bifunctional surface-enhanced infrared absorption and surface-enhanced Raman spectroscopy, multi-line, as well as broadband substrates. The fabrication method is particularly suited for the creation of large-area, single-layer C3-symmetric chiral metasurfaces, and this approach circumvents common problems with elliptical birefringence and can be utilized for interaction with chiral substances.
Open Access
Large-area plasmonics and sensors : fabrication of plasmonic nanostructures by laser interference lithography and femtosecond direct laser writing
(2017) Bagheri, Shahin; Giessen, Harald (Prof. Dr.)
The interaction of light with various types of metallic nanostructures reveals unique optical properties originating from the excitation of the localized surface plasmon resonances, which can be used for a wide range of spectroscopic and sensing applications. While, precisely defined and tailored nanostructures are essential building blocks for realizing such schemes, the commonly used electron-beam lithography is a cost-intensive and time-consuming method to create well-defined nanostructures over small areas. However, a low-cost and high-throughput fabricationmethod over large areas is crucial to advance plasmonic devices towards life science and technological applications. Building on these concepts, this thesis demonstrates the use of versatile large-area fabrication methods such Laser Interference Lithography (LIL) and Direct Laser Writing (DLW) for the fabrication of different plasmonic devices such as perfect absorber sensors and surface enhanced infrared absorption substrates. Both lithographic techniques allow for fast and homogeneous preparation of various nanoantenna geometries. Utilizing different plasmonic geometries, this work pursues laser interference lithography for large-area fabrication of plasmonic perfect absorber chemical sensors to reliably detect very small amounts of hydrogen. Furthermore, both laser interference lithography and direct laser writing allow for the preparation of large-area plasmonics nanoantenna arrays for surface-enhanced infrared spectroscopy, enabling the detection of small amounts of vibrationally active molecules. Additionally, both techniques combined with subsequent etching processes are employed for nanostructuring of so-called alternative plasmonic materials namely titanium nitride. Titanium nitride, known as a refractory plasmonic material, provides plasmonic properties comparable with gold and can sustain at high temperatures as investigated in the thesis.
Open Access
Linear & nonlinear plasmonic sensing : complex coupled plasmonic structures, functionalization, and nonlinear effects
(2016) Mesch, Martin; Giessen, Harald (Prof. Dr.)
Open Access
Microcavity plasmonics
(2011) Ameling, Ralf; Giessen, Harald (Prof. Dr.)
The understanding of light-matter interactions at the nanoscale lay the groundwork for many future technologies, applications and materials. The scope of this thesis is the investigation of coupled photonic-plasmonic systems consisting of a combination of photonic microcavities and metallic nanostructures. In such systems, it is possible to observe an exceptionally strong coupling between electromagnetic light modes of a resonator and collective electron oscillations (plasmons) in the metal. Near-field coupled nanowire pairs were placed between two metal layers that form a microcavity. Depending on the position of the nanowires in the microcavity, electric as well as magnetic modes of the light can be coupled to symmetric and antisymmetric plasmon modes exhibiting electric and magnetic dipoles. The measured coupling strengths both in the electric and magnetic case are extremely high. If the nanostructures are placed close to the mirrors of the cavity, both localized particle plasmons as well as propagating surface plasmons at the interfaces of the metal can be excited and strongly coupled to the photonic light modes. Furthermore, a first possible application of the structures as sensors was explored. The results have shown, that coupled photonic-plasmonic structures possess a considerably higher sensitivity to changes in their environment than conventional localized plasmon sensors due to a plasmon excitation phase shift that is depending on the environment. The thesis includes theoretical models and simulations of near- and far-field coupled plasmonic systems to explain the observed phenomena. Furthermore, the experimental techniques for the fabrication and characterization of multilayer nanostructures are presented. The results of the thesis contribute to a better understanding of light-metal interactions in three-dimensional near- and far-field coupled plasmonic nanostructures and disclose possibilities of their initial practical use.