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Hybrid Plasmonic Phase-Changing Nanostructures: Active Reconfigurable Devices to Ultrafast Dynamics

CNM Nanoscience Seminar
Kannatassen Appavoo, Vanderbilt University
January 9, 2013 11:00AM to 12:00PM
Building 440, Room A105-106
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 andlattice 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 talk 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 isenabled 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, we also report 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 here 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 arealso addressed, such as interfacial effects, intrinsic size-dependent switching of VO2 and the potential for coherent control of the structural dynamics inVO2.