Nonlinear optics in thin film interference coatings

Nonlinear optical effects play a crucial role in modern optical systems. They are applied in mode-locking for the generation of ultrashort optical pulses, in the generation of otherwise unavailable wavelengths, or for new approaches in measurement techniques. However, implementing the required nonlinear optical processes mainly relies on conventional optical systems comprising separate components and free-space constructions, which limits the possibilities for miniaturization and integration of functional groups. Contrary to this, optical interference coatings offer highly developed capabilities for combining optical functions into a single, monolithic stack of transparent materials. So far, the applications of optical coatings have generally been limited to the linear optical regime. If nonlinear effects were considered, it was mostly directed at their suppression to avoid undesired effects. This thesis, therefore, investigates the combination of selected nonlinear optical effects with specially designed optical coatings to create novel components as alternatives to established optical systems. Due to the amorphous nature of thin film coating materials, the two effects chosen for investigation are based on the third-order susceptibility χ(3). The first effect is the optical Kerr effect, which is utilized to achieve all-optical switching of incident light. In the second case, optical coatings are utilized to solve phase matching issues for the third harmonic generation in frequency tripling mirrors and significantly increase conversion efficiency. The manufacturing processes and material research are presented for both approaches, together with the experimental testing of the novel components’ function. It was found that while the frequency tripling mirrors provide a greatly enhanced efficiency compared to the third harmonic generated in more typical optical coatings, the total efficiency is currently limited by free-electron effects occurring during the conversion process. The optical switches show significant modulation of 20% in transmittance and 30% in reflectance with a repeatable process which can be clearly distinguished from laser-induced damage occurring at higher intensities. Therefore, the created optical switch can present a viable alternative to established switching concepts.Nichtlineare optische Effekte spielen eine essenzielle Rolle in modernen optischen Systemen. Sie finden Anwendung in der Modenkopplung zur Erzeugung ultrakurzer Laserpulse, bei der Generation von Laserwellenlängen, die auf andere Weise nicht verfügbar wären und bei neuartigen Messverfahren. Die Implementierung der benötigten nichtlinearen optischen Prozesse basiert dabei auf konventionellen optischen Aufbauten aus einzelnen Komponenten in Freistrahlkonfigurationen, was die Möglichkeiten für Miniaturisierung und Integration von Funktionsgruppen limitiert. Im Gegensatz dazu sind optische Interferenzbeschichtungen ein hochentwickeltes Verfahren für die Kombination von optischen Funktionen in einzelne, monolithische Schichtsysteme aus transparenten Materialien. Bisher waren die Anwendungen von optischen Beschichtungen dabei auf den linearen Bereich der optischen Wechselwirkung beschränkt. Wenn nichtlineare optische Effekt berücksichtigt wurden, dann üblicherweise nur, um diese als unerwünschte Effekte zu unterdrücken. Diese Arbeit untersucht daher die Kombination von ausgewählten nichtlinearen optischen Effekten mit speziell entworfenen optischen Beschichtungen. Ziel hierbei ist die Schaffung von neuartigen Komponenten, die als Alternative zu etablierten optischen Systemen fungieren können. Wegen der amorphen Struktur der Beschichtungsmaterialien, basieren die zwei betrachteten nichtlinearen Effekte auf der Suszeptibilität der dritten Ordnung χ(3). Der erste Effekt ist der optische Kerr-Effekt, welcher genutzt wird, um einfallendes Licht ausschließlich optisch zu schalten. Im zweiten Fall werden optische Beschichtungen genutzt, um das Problem der Phasenanpassung bei Erzeugung der dritten Harmonischen in frequenzverdreifachenden Spiegeln zu lösen und so die Konversionseffizienz deutlich zu erhöhen. Für beide dieser Anwendungen werden die Herstellungsverfahren, die Untersuchung der benötigten optischen Materialien, sowie die experimentelle Überprüfung der hergestellten Komponenten präsentiert. Die Untersuchungen haben gezeigt, dass die frequenzverdreifachenden Spiegel zwar die Konversionseffizienz deutlich steigern können, aktuell aber noch durch die Generation freier Elektronen sowie die daraus resultierenden Effekte limitiert werden. Die optischen Schalter zeigen eine deutliche Modulation ihrer optischen Eigenschaften mit Änderungen der Transmission und Reflexion um etwa 20% beziehungsweise 30%. Der Modulationsprozess ist dabei ein reproduzierbarer Prozess, der klar von der laserinduzierten Zerstörung bei höheren Intensitäten unterschieden werden kann. Der geschaffene optische Schalter kann daher eine funktionale Alternative zu etablierten Schaltkonzepten darstellen

Ultrafast switching with nonlinear optics in thin films

We demonstrate a novel, to the best of our knowledge, concept for an all-optical switch based on the optical Kerr effect in optical interference coatings. The utilization of the internal intensity enhancement in thin film coatings as well as the integration of highly nonlinear materials enable a novel approach for self-induced optical switching. The paper gives insight into the design of the layer stack, suitable materials, and the characterization of the switching behavior of the manufactured components. A modulation depth of 30% could be achieved, which prepares the way for later applications in mode locking

Third and fifth order nonlinear susceptibilities in thin HfO2 layers

Third harmonic generation (THG) from dielectric layers is investigated. By forming a thin gradient of HfO2 with continuously increasing thickness, we are able to study this process in detail. This technique allows us to elucidate the influence of the substrate and to quantify the layered materials third χ(3)(3ω: ω, ω, ω) and even fifth order χ(5)(3ω: ω, ω, ω, ω, − ω) nonlinear susceptibility at the fundamental wavelength of 1030 nm. This is to the best of our knowledge the first measurement of the fifth order nonlinear susceptibility in thin dielectric layers

Electrical and optical properties linked to laser damage behavior in conductive thin film materials

Epsilon-near-zero-materials (ENZ-materials) and their unique properties are key to the successful integration and miniaturization of optical components. Novel concepts, which promise significant progress in this field of research, such as optical switches and thin film electrooptical modulators, are possible when the electrical and optical properties of ENZ-materials are carefully exploited. To achieve a greater understanding of these properties, in this paper the electrical conductivity, optical transmittance, as well as absorption of thin indium tin oxide films, are investigated and linked to their laser-induced damage threshold in the ultra-short pulse regime. To the best of the authors’ knowledge, this is the first concise study linking the electrical properties of indium tin oxide to its properties regarding high-power laser applications. © 2020. All Rights Reserved

Correlation of structural and optical properties using virtual materials analysis

Thin film growth of TiO2 by physical vapor deposition processes is simulated in the Virtual Coater framework resulting in virtual thin films. The simulations are carried out for artificial, simplified deposition conditions as well as for conditions representing a real coating process. The study focuses on porous films which exhibit a significant anisotropy regarding the atomistic structure and consequently, to the index of refraction. A method how to determine the effective anisotropic index of refraction of virtual thin films by the effective medium theory is developed. The simulation applies both, classical molecular dynamics as well as kinetic Monte Carlo calculations, and finally the properties of the virtual films are compared to experimentally grown films especially analyzing the birefringence in the evaluation

Very thick mixture oxide ion beam sputtering films for investigation of nonlinear material properties

Currently, optical coating technology is facing a multitude of new challenges. Some of the new requirements are addressed to the spectral behavior of complex coatings, but in addition, the power handling capabilities gain in importance. Often, both demands are combined in the same component, for example in chirped mirrors for ultra-short pulse applications. The consequent demands on the accuracy of the layer thicknesses and the stability of the refractive indices require a deposition by sputtering processes. For high end components, Ion Beam Sputtering (IBS) is often the method of choice. Utilizing the Co-sputtering technique, IBS additionally allows a higher flexibility in the possible coating materials by mixing two pure oxides into one ternary composite material. These composite materials are also advantageous for researching third order nonlinear effects, which can limit the functionality of optics at high powers. The layer thicknesses required for this fundamental research often exceed 100 µm, which therefore makes low stress and absorption in the layer materials mandatory. A reduction of these decisive properties can be achieved by a thermal treatment of the sample. Usually, this is performed by a post-deposition annealing. Alternatively, the coating temperature can be increased. This is rarely done for IBS processes, but it can be assumed, that the effect is comparable to that of ex-situ annealing. In this work, different ternary mixtures of Al2O3/SiO2, HfO2/Al2O3 as well as Nb2O5/Al2O3 were investigated for their layer stress and absorption, applying both, in-situ temperature treatment as well as post manufacturing annealing. It is observed that suitable thermal treatment as well as material composition can significantly reduce layer stress and absorption in the deposited layer. This enabled the manufacturing of layers with thicknesses of over 180 µm as well as the measurement of nonlinear properties of the deposited materials

Femtosecond laser-induced modifications of frequency tripling mirrors

It is well known that dielectric materials change their properties during irradiation by (high-power) lasers. While in some cases this is desirable and used for laser conditioning, in many cases these material modifications need to be avoided or minimized to maintain the function of optical elements, like mirrors and beam splitters. Recently dielectric stacks have been proposed as nonlinear optical elements for frequency conversion and switches. These dielectric multi-layer coatings are exposed to large incident intensities, which may lead to permanent refractive index changes. We characterize material modifications of a frequency tripling mirror to assess their impact on conversion efficiency and to suggest mitigation measures