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.

中文翻译：

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飞秒激光微加工通过铌酸锂晶体制成的菲涅耳环带进行双波长聚焦

本文提出了一种简单的方法，即通过振幅型非线性菲涅耳波带片（FZP）实现二次谐波（SH）和基频（FF）的双波长聚焦。FZP是通过飞秒激光微加工制造的，铌酸锂晶体中二次磁化率的0或+1二进制调制。理论和实验研究了在532 nm波长下焦距为2 cm的SH光束聚焦强度模式的演变。基于双折射相位匹配，测得的峰值功率为1.9×10 ^-5％％W ^-1  cm ^-2的归一化转换效率是在SH焦点实现的。振幅型非线性FZP同时实现了FF光束和SH光束的聚焦，这为集成激光束整形，非线性频率转换和满足超小型片上设备要求的激光聚焦提供了广阔的可能性。