  A high-modulation-efficiency optical modulator integrated on silicon (Si) is
a key enabler for low-power and high-capacity optical interconnects. However,
Si-based optical modulators suffer from low phase modulation efficiency owing
to the weak plasma dispersion effect in Si. Therefore, it is essential to find
a novel modulation scheme that is compatible with a Si photonics platform.
Here, we demonstrate an InGaAsP/Si hybrid metal-oxide-semiconductor (MOS)
optical modulator with a Mach-Zehnder interferometer (MZI) formed by direct
wafer bonding with an Al2O3 bonding interface. Electron accumulation at the
InGaAsP MOS interface enables the extraction of the electron-induced refractive
index change in InGaAsP, which is significantly greater than that in Si. The
presented modulator exhibits a phase modulation efficiency of 0.047 Vcm, which
is approximately 5 times higher than that of Si MOS optical modulators. This
approach provides a new efficient scheme of phase modulation on a Si photonics
platform for low-power, high-speed, and high-density optical links.
