Non-reciprocal laser nano-structuring modulation via surface plasmon-polariton backscattering and spontaneous polarization synergy

The non-reciprocal (directional-dependent) nanostructuring phenomenon in birefringent crystals during ultrafast laser processing has long been considered an inherent limitation. Through systematic investigation of lithium niobate (LiNbO₃) surface nanostructuring, the synergistic interplay between surface plasmon-polariton (SPP) backscattering dynamics and the crystal’s intrinsic spontaneous polarization field for the non-reciprocity of femtosecond laser nanostructuring in LiNbO₃ is demonstrated in this study. By strategically modulating laser polarization and scanning direction, the propagation of SPP can be well controlled, thereby enhancing the electric field via the interaction between SPP backscattering and spontaneous polarization, enabling amplification, suppression, and the complete removal of the non-reciprocal behavior of laser-induced periodic surface structures (LIPSS). Abundant phase modulations are achieved via only single laser polarized femtosecond laser irradiation on the LiNbO₃ by the modulation of non-reciprocal LIPSS, which successfully realizes different vortex beam distributions and depth-dependent pseudo-color nano-patterings. It provides a novel direct nanostructuring approach for LiNbO₃ with high controllability, efficiency, and uniformity via opposite laser writing and polarization adjustments, which is expected to enable advanced nano-optics devices, including optical data storage, shaping, and geometric phase optical elements.