Highly Efficient Intramodal and Intermodal Acousto‐Optic Modulation on an Etchless Lithium Niobate Integrated Platform

Abstract Harnessing light–sound interaction in photonic integrated circuits can enable many classical and quantum applications on a chip. With excellent piezoelectricity and broad transparency window, lithium niobate on insulator (LNOI) is an ideal platform for integrated acousto‐optic devices. Here, by utilizing bound states in the continuum for ultralow‐loss waveguiding, highly efficient intramodal and intermodal acousto‐optic modulation is demonstrated on an etchless lithium niobate integrated platform in the telecommunication band. The maximal intramodal acousto‐optic modulation efficiency is −13.17 dB, measured at microwave power of 14 dBm and drive frequency of 0.76 GHz, corresponding to a V π L of 1.133 V cm. The intermodal acousto‐optic modulation is realized with TM 00 –TM 10 mode conversion, achieving a single‐sideband contrast of >20 dB over a broad optical bandwidth at drive frequency of ≈0.663 GHz, corresponding to a V π L of 1.993 V cm measured at ≈1551.6 nm. These devices establish a direct link between optical and microwave photons on LNOI platform, paving the way for many applications such as acoustic sensing, magnetic‐free isolation, on‐chip heterodyne detection, and programmable photonic routing.