Low-loss plasmonic waveguide with high optical confinement on the thin-film lithium niobate-on-insulator (LNOI) platform

Thin-film lithium niobate-on-insulator (LNOI) is a promising platform for integrated photonics, offering a large second-order nonlinearity for electro-optic modulation and nonlinear applications. Moving to thinner films can reduce the separation of electrodes, enabling a possible approach to improve modulation efficiency since larger electric fields can be produced across the thin film for a given drive voltage. However, the reduction in optical confinement factor for thinner films would act against the improvement in modulation efficiency. In this work, we avoid this trade-off by developing a thin film LN waveguide which can maintain high optical confinement. We use the hybrid plasmonic structure for the first time on thin-film LNOI platform. By careful design, we can reduce the propagation losses to 0.011 dB/μm and 0.046 dB/μm at 1550 nm in simulation and experiment respectively. The plasmonic waveguide has an effective mode area of only 0.064 μm², which is one order of magnitude smaller than typical thin-film LNOI waveguides. The high optical confinement enables efficient light-matter interactions and can improve the modulation efficiency of electro-optic modulators in ultra-compact devices implemented on the thin-film LNOI platform.