Strong Polarization‐Tuned Optical Nonlinearity Via Femtosecond‐Laser Plasmonic Nanolithography in Lithium Niobate

Abstract Despite the advantages of lithium niobate (LN)‐based photonic integration platforms in various applications, the inherently weak third‐order nonlinear optical response of conventional materials limits the miniaturization and energy efficiency of nonlinear optical devices in compact optical systems. Localized surface plasmons (LSPs) provide a promising solution to this miniaturization challenge by confining and enhancing light fields at deep subwavelength scales. However, due to nanofabrication limitations, strongly coupled single‐crystal LN‐LSP structures have yet to be realized. Here, it is demonstrated Au nanoparticle‐LN hybrid plasmonic structures are assembled via plasmonic nanolithography. By leveraging plasmon‐mediated energy deposition and photon momentum transfer under femtosecond laser irradiation, the nanoparticles within the single‐crystalline region are formed from implanted elements. Nanoparticles are transformed into nanorods elongated along the polarization of the laser. With plasmons excited in distinct axial directions, the resulting nanorod‐LN hybrid plasmonic material exhibits polarization‐dependent nonlinearity, with the nonlinear absorption coefficient for long‐axis polarized light augmented by five orders of magnitude compared to the pure LN. Utilizing this feature, a Q‐switched laser is developed exhibiting pronounced polarization‐dependent behavior. This research offers a viable approach for directly incorporating plasmonic nanostructures into crystals, enhancing and tuning optical properties, and thereby broadening the optical and photonic applications of LN‐based platforms.