Generation and control of solid-state high harmonics at the nanoscale
Keywords: Physics.

High-harmonic generation (HHG) lies at the heart of attosecond science. Since the first experimental observation of HHG in noble gases, extensive theoretical and experimental studies have been performed on HHG from gas phase medium including both noble gases as well as small molecues for the past 30 years, making HHG a valuable tool to probe attosecond dynamics in various physical and chemical systems. A recent new progress in this field is the observation of high harmonics in a crystalline bulk solid reported in the year of 2011. Fundamentally, high-harmonic generation from solids is a spectroscopy technique and understanding the mechanism will allow us to study the fundamental strong light-matter interaction processes happen at femto- and attosecond time scale in the condensed matter phase. In terms of application, observation of high-harmonics in solids such as semiconductors makes it possible to engineer and control the ultrafast strong light-matter interaction at nanoscales by patterning the solids target with subwavelength nanostructures, which could eventually lead to novel compact ultrafast photonic devices operating at extremely short wavelengths. In this thesis, we report a few experiments that reveal the fundamental mechanism responsible for solid-state high-harmonic generation as well as the control of the process at subwavelength scale by applying nanofabrication technology. The first experiment is an optical pump-probe study on HHG from a ZnO bulk crystal. The behavior of the high-harmonics signal generated from a strong probe pulse under a direct band gap linear excitation suggests that a Rabi-type of interband transition plays an important role in high-harmonic generation in solids under the experiment condition. A second experiment is the first demonstration of high-harmonic generation in a two-dimensional material (in this case, monolayer MoS$_{2}$). Based on the measured high-harmonic spectra we suggest that the Berry curvature of the material has important effects when electrons are strongly driven by the excitation field. We further find that the generation process is more efficient from monolayer compared to an equivalent layer in bulk, revealing hints of the correlation effects on the high-harmonic generation process. A third experiment covered in this thesis is to enhance and control high-harmonic generation from an all-dielectric metasurface. The enhancement of high harmonics is a direct result of the enhanced pump field in the metasurface when the device is resonantly excited. The overall harmonic yield from field-enhancing nanostructures is ultimately limited by the laser induced damage of the sample, and this motivates the last work in this thesis. In this work, we demonstrate the guiding of above-gap high harmonics in a slotted waveguide geometry by allowing high harmonics propagating in the vacuum channel with a greatly reduced absorption. Coherent propagation effects are observed, which is otherwise absent in bulk due to strong absorption. With the reduced absorption, we further demonstrate the enhanced high harmonics at high excitation intensities up to the damage threshold of the bulk.