Linear and Nonlinear Plasmonics with Monocrystalline Gold Flakes

Modern plasmonics strives to meet the demands of next-generation quantum technologies while opening new research frontiers in mesoscopic solid-state physics. However, the success of these advancements largely depends on the reduction of electromagnetic losses in metallic materials, which constitutes the most ubiquitous problem of current nanoplasmonic devices.

This thesis presents experimental investigations of the plasmonic properties of (quasi-) monocrystalline gold flakes, which emerged recently as a material platform to supersede the traditionally-used polycrystalline gold films.
First, the optical response in the linear regime, including nonlocal effects, is discussed in detail, and prospective functionalities for advanced plasmonic devices are experimentally demonstrated. Second, the nonlinear response arising from the interaction of crystalline gold with intense ultrashort light pulses is considered, with experiments revealing that monocrystalline flakes produce a strong anisotropic second-order nonlinear response which is markedly absent in polycrystalline films. In addition, two-photon luminescence microscopy is used to study the nonlinear absorption dynamics in gold flakes that are few tens of nanometers in thickness, exploiting their strong intrinsic third-order susceptibility. Preliminary results indicate that hot carrier excitation and relaxation dynamics is significantly altered when the gold thickness approaches mesoscopic dimensions.

The results presented in this thesis confirm that monocrystalline gold flakes are among the best candidates for the experimental exploration of nonlocal and nonlinear plasmonic phenomena, and can be used for substantial improvement of existing plasmonic devices.