On-chip signal modulation, processing and routing are now largely carried out in hard-wired circuits occupying most of the area of CPU chips, creating speed bottlenecks and increasing thermal loading. We have demonstrated on-chip, all-optical modulation in hybrid silicon-vanadium dioxide (VO2) ring resonators, in which a nanoscale patch of VO2 acts as the switch, driven by the enormous change in index of refraction in the IMT (Δn~1.3). In principle, the femtosecond IMT transition time of VO2 enables modulation speeds up to 500 GHz. However, the IMT is accompanied by a structural phase transition (SPT) from a monoclinic (M) to a rutile (R) phase, and nanosecond return to the M phase would reduce modulation frequencies below 1 GHz. Moreover, VO2 is lossy, so that optimizing switching contrast and power consumption simultaneously requires that the VO2 modulator should have lateral dimensions of order 100 nm, about the size of a single grain. This talk highlights advances in fabrication and performance of ultrafast switching of the nanoscale VO2 thin-film modulator. Studies of ultrafast excitation and relaxation of VO2 confirm the existence of an excited monoclinic phase (mM) with a fast recovery time compatible with Tbps switching. We show how nanoscale modulator design, and particularly optimizing the resonant ring, can achieve power requirements compatible with systems specifications for all-optical modulators, and elaborate on the effects of dopants. Finally, we show how optical switching in the hybrid ring resonator can be achieved using near band-edge pumping of the VO2 nanopatch at wavelengths in the telecommunications band.
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