  Despite fundamentally challenging in integrated (nano)photonics, achieving
chip-based light nonreciprocity becomes increasingly urgent in signal
processing and optical communications. Because of material incompatibilities in
conventional approaches based on Faraday effects, alternative solutions have
resorted to nonlinear processes to obtain one-way transmission. However,
revealed dynamic reciprocity in a recent theoretical analysis has pinned down
the functionalities of these nonlinear isolators. To overcome this dynamic
reciprocity, we here report the first demonstration of a nonlinear optical
isolator on a silicon chip enforced by phase-matched parametric amplification.
Using a high-Q microtoroid resonator, we realize highly nonreciprocal transport
at the 1,550 nm wavelength when waves are simultaneously launched in both
forward and backward directions. Our design, compatible with current CMOS
technique, yields convincing isolation performance with sufficiently low
insertion loss for a wide range of input power levels. Moreover, our work
evidences the possibility of designing chip-based real nonlinear isolators for
information processing and laser protection.
