Nonlinear Transverse Mode Conversion Based on Ferroelectric Domain Engineering in Thin Film Lithium Niobate

Abstract The transverse mode of a waveguide is an emerging degree of freedom of photons, and the nonlinear frequency conversion of which is essential to construct multi‐dimensional multiplexed photonic integrated circuits for high‐capacity optical communications. The challenges of nonlinear conversion of waveguide modes are to simultaneously fulfill the phase‐matching conditions and maximize the nonlinear overlap integral. In this work, a scheme is proposed to achieve waveguide mode conversion based on 2D ferroelectric domain engineering in the emerging integrated photonic material platform of thin‐film lithium niobate (TFLN), where the longitudinal periodic distribution of the second‐order nonlinearity ensures the quasi‐phase‐matched frequency conversion between the guided‐modes; while the transverse distribution can maximize the nonlinear overlap integral. As a proof‐of‐principle experiment, a nonlinear waveguide is designed and fabricated with 2D ferroelectric domain structure on a Z‐cut TFLN, and demonstrates the conversion from TM 00 mode of the fundamental wave to TM 10 mode of the second harmonic wave. Efficient nonlinear conversions from TM 00 mode to high‐order modes are numerically verified. The proposed nonlinear guided‐mode conversion scheme has the advantages of flexible design, high conversion efficiency, and scalability, and will lay the foundation for the development of classical and quantum photonic chips with multiple degrees of freedom.