Photonic Crystal Cavity IQ Modulators in Thin-Film Lithium Niobate

Thin-Film Lithium Niobate is an emerging integrated photonic platform showing great promise due to its large second-order nonlinearity at microwave and optical frequencies, cryogenic compatibility, large piezoelectric response, and low optical loss at visible and near-infrared wavelengths. These properties enabled Mach–Zehnder interferometer-based devices to demonstrate amplitude- and in-phase/quadrature (IQ) modulation at voltage levels compatible with complementary metal-oxide-semiconductor (CMOS) electronics. Maintaining low-voltage operation requires centimeter-scale device lengths, making it challenging to realize the large-scale circuits required by ever-increasing demands for information density in communications as well as classical- and quantum information processing. Reduced device sizes reaching the 10 μm scale are possible with photonic crystal cavities. So far, their operation has been limited to amplitude modulation and required circulators or lacked cascadability. Here, we demonstrate a compact IQ modulator using two photonic crystal cavities operating as phase shifters in a Fabry–Perot-enhanced Michelson interferometer configuration. It supports cascadable amplitude and phase modulation at GHz bandwidths with CMOS-compatible voltages.