Monolithic Barium Titanate Nanophotonics and Electro-optics

Barium titanate-on-insulator (BTOI) is a compelling material for high-speed integrated photonic modulators due to its large Pockels coefficient (r42 > 1200 pm/V in bulk), which allows for the miniaturization of modulators while maintaining strong electro-optic performance. Sub-wavelength nanostructures monolithically etched into BTOI are particularly exciting, as they offer a path toward subwavelength light-matter interaction and reduced modulator energy consumption. Here, we design, fabricate, and characterize monolithic one-dimensional nanophotonic crystals (PhCs) and high-Q (230k) photonic-crystal Fabry-Perot (FP) cavities in BTOI. We develop and optimize a nanofabrication process that yields anisotropic (75-degree sidewalls) and deep etching that features low optical loss, with racetrack resonators achieving intrinsic quality factors near 1 million and propagation losses of about 0.5 dB/cm. Our photonic crystals exhibit bandgap contrasts greater than 40 dB, and FP cavities reach loaded quality factors up to 230k. We verify ferroelectric domain alignment via second-harmonic generation microscopy and extract an effective Pockels coefficient of 154 pm/V. By probing the microwave response at the PhC band edge, where modulation bandwidth is set by the material's electro-optic response rather than cavity photon lifetime, we measure a 3-dB electro-optic bandwidth of 11 GHz and a 6-dB bandwidth of 21 GHz, consistent with the frequency-dependent roll-off of BTO's r42 coefficient near 10 GHz. Finally, we show a variety of modulation effects in resonators and at photonic crystal band edges, including sideband-resolved modulation, resonant bandwidth-limited modulation, and photonic-crystal based single sideband modulation and frequency comb generation.