  Low driving voltage (Vpp), high-speed silicon microring modulator plays a
critical role in energy-efficient optical interconnect and optical computing
systems owing to its ultra-compact footprint and capability for on-chip
wavelength-division multiplexing. However, existing silicon microring
modulators usually require more than 2 V of Vpp, which is limited by the
relatively weak plasma dispersion effect of silicon and the small capacitance
density of the reversed PN-junction. Here we present a highly efficient
metal-oxide semiconductor capacitor (MOSCAP) microring modulator through
heterogeneous integration between silicon photonics and titanium-doped indium
oxide, which is a high-mobility transparent conductive oxide (TCO) material
with a strong plasma dispersion effect. The device is co-fabricated by Intel's
photonics fab and TCO patterning processes at Oregon State University, which
exhibits a high electro-optic modulation efficiency of 117 pm/V with a low VpiL
of 0.12 Vcm, and consequently can be driven by an extremely low Vpp of 0.8 V.
At a 11 GHz modulation bandwidth where the modulator is limited by the high
parasitic capacitance, we obtained 25 Gb/s clear eye diagrams with energy
efficiency of 53 fJ/bit and demonstrated 35 Gb/s open eyes with a higher
driving voltage. Further optimization of the device is expected to increase the
modulation bandwidth up to 52 GHz that can encode data at 100 Gb/s for
next-generation, energy-efficient optical communication and computation with
sub-volt driving voltage without using any high voltage swing amplifier.
