We present an optical modulator based on a silicon ring resonator coated with vanadium-dioxide (VO 2 ) motivated by the need for compact silicon-compatible optical switches operating at THz speeds. VO 2 is a functional oxide undergoing metal-insulator transition (MIT) near 67 o C, with huge changes in electrical resistivity and near-infrared transmission. The MIT can be induced thermally, optically (by ultra-fast laser excitation in less than 100 fs), and possibly with electric field. VO 2 is

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... sily deposited on silicon and its ultrafast switching properties in the near-infrared can be used to tune the effective index of ring resonators in the telecommunication frequencies instead of depending on the weak electro-optic properties of silicon. The VO 2 -silicon hybrid ring resonator is expected to operate at speeds up to 10 THz at low Qfactor and with shorter cavity lifetimes, thus enabling compact, faster, more robust devices. We have made ring resonator structures on SOI substrates with rings varying in diameter from 3-10 µm coupled to 5 mm-long nanotapered waveguides at separations of 200 nm. Rings were coated with 80 nm of VO 2 by pulsed laser deposition. As proof-ofconcept, by switching the VO 2 top layer thermally, we were able to modulate the resonance frequency of the ring to match with the predictions from our FDTD simulations.