Strong-confinement low-index-rib-loaded waveguide structure for etchless thin-film integrated photonics

Novel thin films consisting of optical materials such as lithium niobate and barium titanate enable various high-performance integrated photonic devices. However, the nanofabrication of these devices requires high-quality etching of these thin films, necessitating the long-term development of the fabrication recipe and specialized equipment. Here we present a strong-confinement low-index-rib-loaded waveguide structure as the building block of various passive and active integrated photonic devices based on novel thin films. By optimizing this low-index-rib-loaded waveguide structure without etching the novel thin film, we can simultaneously realize strong optical power confinement in the thin film, low optical propagation loss, and strong electro-optic coupling for the fundamental transverse electric mode. Based on our low-index-rib-loaded waveguide structure, we designed and fabricated a thin film lithium niobate (TFLN) modulator, featuring a 3-dB modulation bandwidth over 110 GHz and a voltage-length product as low as 2.26 V·cm, which is comparable to those of the state-of-the-art etched TFLN modulators. By alleviating the etching of novel thin films, the proposed structure opens up new ways of fast proof-of-concept demonstration and even mass production of high-performance integrated electro-optic and nonlinear devices based on novel thin films.