On‐Chip Brillouin Amplifier in Suspended Lithium Niobate Nanowaveguides

Thin film lithium niobate (TFLN) has emerged as a leading material platform for integrated nonlinear photonics, enabling transformative applications such as broadband Kerr soliton microcomb and high‐speed electro‐optic modulation. While stimulated Brillouin scattering has been numerically proposed in TFLN, achieving sufficient gain remains challenging due to the requirement for the simultaneous low optical and mechanical losses of the device. In this work, the angle‐dependence of Brillouin gain coefficients in x‐cut membrane‐suspended TFLN nanowaveguides is systematically characterized, taking into account the anisotropy of the photoelastic coefficients in lithium niobate. A Brillouin gain coefficient of 129.5 m−1W−1${\rm m}^{-1}{\rm W}^{-1}$ is reported and the Brillouin frequency tuning is further demonstrated through variations in either pump frequency or chip operating temperature. Based on the suspended TFLN nanowaveguide, by optimizing the confinement of both photonic and phononic modes, a Brillouin amplifier with a record‐high gain of 8.5 dB is achieved. This result not only validates the feasibility of strong guided Brillouin interaction using suspended TFLN nanowaveguides, but also paves the way for novel on‐chip sensing and signal processing applications.