Enhancing Nonlinear Optics in Thin Materials

Abstract

This thesis examines various nonlinear optical processes to either control the frequency or switch the amplitude of light all-optically. This is achieved in subwavelength thin samples designed to overcome issues such as phase-matching. In order to achieve a significant nonlinear optical response, we utilise plasmonic resonances and nano-antennas. First, we enhance the high-harmonic generation in graphene. A heterostructure of gold nano-ribbons on graphene with a thin insulator in-between enhances the incoming electric field and enormously enhances the high-harmonic generation. A thousandfold enhancement of the fifth-harmonic generations is measured. Next, we study the all-optical switching of an epsilon-near-zero plasmon in indium tin oxide (ITO). The resonance provides near-perfect absorption and occurs for layers of just 60 nm thickness. Utilising the Kretschmann-geometry in a pump-probe scheme enables phase-matching to the resonance and studying the nonlinear changes of the reflection. Significant absolute changes of 45 % are measured, with an initial reflection of ~1 % thanks to the near-perfect absorption resonance. A novel two-beam coupling contribution is identified and will be essential to take into account in other studies. The addition of cross-shaped nano-antennas allows for improvement on various critical issues. Optical switching is now possible for lower intensities, normal incidence and better control of polarisation. The symmetric cross-shape enables a nonlinear dichroic response by which only the probe polarisation parallel to the pumped cross-bar is undergoing a significant nonlinear shift. We analyse the complete polarisation ellipses and identify two wavelength regimes in which the amplitude or phase can be modulated independently. Finally, we study the angle and frequency-dependent phase-modulation in ITO films of various thicknesses. We present good agreement with the temporal refraction for low angle of incidence. However, we find an additional dependence for higher angles. Close to the epsilon-near-zero case, we even report an opposing frequency shifting contribution. We show that the additional phase contribution origins from the temporal changes to the spatial refractive index boundary. Hence, we refer to the process as spatiotemporal refraction. This contribution can tailor the frequency shift and allow for better designability of simultaneous ultrafast changes in amplitude and phase. In summary, both graphene and ITO have proven themselves as useful nonlinear active media. The enhanced response makes them promising materials for controlling frequency and amplitude in applications such as optical communication. Beyond that, the work on ITO even unveils novel effects such as spatiotemporal refraction.