Hybrid phase-changing nanostructures: from reconfigurable plasmonic devices to ultrafast dynamics

Abstract

Ultrafast photoinduced phase transitions in quantum materials could revolutionize data-storage and telecommunications technologies by modulating transport in integrated nanocircuits at terahertz speeds. In phase-changing materials (PCMs), microscopic charge, orbital and lattice degrees of freedom interact cooperatively to modify macroscopic electrical and optical properties. Although these interactions are well documented for bulk single crystals and thin films, little is known when such PCMs are nanostructured and implemented in nanoscale switching configurations. This dissertation presents a generalizable concept of incorporating a quantum material – vanadium dioxide (VO₂) – to create functionality in plasmonics, a new device technology that interfaces electronic and photonic components in a single chip. By designing, simulating and fabricating hybrid plasmonic/PC nanostructures, we demonstrate at the single nanostructure level how signal modulation can be achieved when the VO₂ component undergoes its characteristic insulator-to-metal transition. Furthermore, a subwavelength hybrid nanomodulator is demonstrated that is both thermodynamically and wavelength tunable. Reconfigurability is enabled by spatially confining electromagnetic fields to nanoscale volumes using a metallic nanostructure while simultaneously tailoring its near-field environment with a PC nanostructure. By providing the first ultrafast optical studies of a hybrid nanomaterial, this dissertation also reports a novel all-optical technique to trigger VO₂ PT on timescale faster than its single phonon cycle, accompanied by a decrease in switching threshold. The mechanism is based on ballistic hot electrons created by ultrafast optical excitation of gold nanoparticles, which are injected through the gold/VO₂-nanostructure interface. Density functional calculations show that the injected electrons cause the catastrophic collapse of the 6 THz optical phonon mode, associated with the structural phase transition of VO₂. Most importantly, the hybrid nanostructures discussed in this dissertation combine generic plasmonic (gold) and PC (VO₂) components. Therefore, this work aims to be generalizable, serving as a platform for designing other hybrid nanostructures operating at nanometer length scale and on femtosecond timescale for the next generation all-optical nanophotonic devices. Key scientific issues regarding the viability of such hybrid nanomodulators are also addressed, such as interfacial effects, intrinsic size-dependent switching of VO₂ and the potential for coherent control of the structural dynamics in VO₂.