Dual‐Wavelength Focusing through Fresnel Zone Plate Fabricated in Lithium Niobate Crystal by Femtosecond Laser Micromachining

Abstract

Herein, a simple method is proposed that realizes the dual‐wavelength focusing of second‐harmonic (SH) and fundamental frequency (FF) through amplitude‐type nonlinear Fresnel zone plate (FZP). The FZP is fabricated by femtosecond laser micromachining with 0 or +1 binary modulation of quadratic susceptibility in lithium niobate crystal. The evolution of the SH beam focusing intensity patterns with 2 cm focal length at 532 nm wavelength is theoretically and experimentally studied. Based on the birefringence phase matching, the measured normalized conversion efficiency for peak power of 1.9 × 10−5% W−1 cm−2 is achieved in the SH focal spot. The amplitude‐type nonlinear FZP realizes the focusing of the FF beam and SH beam simultaneously, which opens wide possibilities in the integration of laser beam shaping, nonlinear frequency conversion, and laser focusing for ultra‐compact on‐chip device requirement.