Nonlinear LightBoosting of Anisotropic Lithium Niobateby Anapole States in Plasmonic Nanocavities

Nonlinear frequency conversion has garnered extensive attention in advancing the functionality of integrable nanophotonics. In recent decades, besides tailoring phase-matching conditions, boosting second-harmonic (SH) generation in nonlinear optics by precisely manipulating the light–matter interactions at the nanoscale was crucial for driving diverse applications that span nonlinear imaging, sensing, and quantum optics on a chip. In this work, we propose an effective strategy for boosting the local SH generation of lithium niobate (LN) and steering its spatial far-field radiation by utilizing the excitation of magnetic anapole states in a gap-plasmon nanocavity. For the first-order magnetic anapole state of the gap-plasmon cavity, when the polarization component along the gap direction aligns with the second-order nonlinear susceptibility (χ_<i>eee</i>⁽²⁾) of anisotropic LN, a dramatically enhanced SH generation with a conversion efficiency of up to ∼0.022 W^–1 is achieved, which is about 6 orders of magnitude higher than other nanostructured counterparts. Additionally, when the optical axis of LN is perpendicular to the gap direction, the far-field SH radiation exhibits pronounced polarization-dependent anisotropy. By adjusting the structural parameters, we present the first- or higher-order magnetic anapole states across a range of wavelengths, thereby allowing for the precise control of SH radiation spectrally. Our findings might pave the way for the development of LN-based photonic devices such as on-chip high-efficiency nonlinear light sources.