title: Ultrafast Nonlinear Spectroscopy of Nanostructures
Creators: name: Obermeier, Julian
Publisher: name: University of Bayreuth
Year: 2022
Subjects: subject: Nonlinear Plasmonics
subject: Nanostructures
subject: Ultrafast Spectroscopy
subject: Higher Harmonics Generation
subject: 530

description: Nanostructures like subwavelength metal nanoparticles or quantum dots build the bridge between the atomic length scale and the macroscopic one. Although they consist of thousands of atoms, they show properties only explainable by quantum mechanical approaches. Nowadays, those systems are widely used in commercially available technologies but are still object of scientific research, even down to the clarification of fundamental physical questions. The formation of strongly coupled systems, formed by two or more nanostructures, sources for entangled photons and the nonlinear response of nanosystems are topics of worldwide research in the field of nanooptics. Especially the later offers many open questions and often it is not clear, if restrictions like, e.g., symmetry restrictions from the macroscopic scale still hold in the nanoscopic universe. We address some of those open questions in this thesis experimentally as well as theoretically. The use of ultrafast pump-probe techniques allow us to reveal temporal dynamics on the femto second timescale, while spectroscopic measurements complete the picture. For theoretical modeling, we combine intuitive analytical models with numerical approaches like optimization algorithms and the Finite Element Method. Our research ranges from isolated gold nanoparticles over complex waveguide-like nanocircuits to layered semiconductor structures with embedded quantum dots. The first two chapters lay a basis of physical framework for the later investigated topics. The field of plasmonics is embedded in the theory of electromagnetism and we give an insight in the field of nonlinear optics. Moreover, we introduce computational electrodynamics with a focus on the Finite Element Method and nanostructures that are of interest in the subsequent work. The following two chapters focus on second-harmonic generation on the nanoscale. We demonstrate theoretically and experimentally, that the current understanding has to be expanded. In contrast to the past understanding, subwavelength particles made of a symmetric material do not necessarily have to possess a geometrical asymmetry to emit second-harmonic light. In fact, it is already sufficient if an optical or plasmonic mode of appropriate symmetry is present. We prove our hypothesis in a complex plasmonic nanocircuit, carefully exclude influences that could weaken our statement and, moreover, explore the nonlinear operation of the nanocircuit to its full extend. The later is described by an intuitive analytical model. The next chapter marks the transition towards time resolved experimental techniques. We investigate the temporal dynamics of a low density, quantum dot based semiconductor saturable absorber mirror. We reveal time constants of the recovery process after saturation and determine characteristic constants of the saturation process itself, offering enough information for a future application as mode locking device in an ultrafast red emitting vertical-external-cavity surface emitting laser. Moreover, we also conduct experiments with varying wavelength, that reveal the structure's behavior in spectral proximity to the desired operation wavelength. The last chapter of the thesis focuses again on plasmonic nanostructures but lays focus on the temporal dynamics of quasi free, hot electrons and their role in nonlinear optical processes. Up to now, this relation, as well as the connection to the bandstructure of the material itself, remains rather unclear. We present our experimental approach, involving a multi-color pump-probe setup. We create hot electrons in the conduction band with a 400nm pump pulse, probe the structure with a delayed infrared probe pulse and investigate the impact on the nonlinear signals. Namely, second-harmonic generation, third-harmonic generation and multi-photon luminescence. We observe partly differing changes in all three of them regarding the sign of the modulation as well as recovery and decay times. In the subsequent measurements, we sacrifice the spectral information for a better signal-to-noise ratio by a Lock-In detection scheme with a single photon counting device. Therefor, we focused on the dominating signal, given by changes in the photo luminescence. We investigate excitation power dependent changes in the pump-probe peak and present first steps towards a theoretical model involving a two-temperature model for the thermalization of the generated hot electrons. Moreover, we present a technique for ultrafast multi-color pulse characterization. We use difference-frequency generation in order to obtain cross-correlation frequency resolved optical gating traces from nonlinear micro crystals and also gold nanorods themselves in the sample plane of the experiment.