Fresnel zone plate (FZP) is a kind of diffractive optical element that produces amplitude and phase modulation of light. The FZP is also known as the diffractive lens and has a wide range of applications with the wavelength ranging from visible to X-ray. Nowadays, many methods have been proposed to produce FZP for beam focusing, including photolithography combined with etching technique, spatial light modulator (SLM) method, and laser lithography technique. However, these methods achieved beam focusing with monochromatic light in the linear optics and had some limitations when applied to the nonlinear field.

In this article, we demonstrated the realization of the dual-wavelength focusing of second-harmonic (SH) and fundamental frequency (FF) via a diffractive lens in lithium niobate (LiNbO3) crystal. The diffractive lens was fabricated by femtosecond laser micromachining and had a geometric FZP pattern with 0 or +1 binary modulation of quadratic susceptibility. We named this SH diffractive lens in LiNbO3 as amplitude-type nonlinear FZP (A-NFZP). The modulated A-NFZP was cut for a specific angle between the optical axis and the z-axis of the crystal, where the phase-matching condition is fulfilled through the birefringence phase matching (BPM) in the longitudinal direction. We have fabricated the A-NFZP with a 1 cm focal length at the wavelength of 1064 nm. The evolution of the SH beam focusing intensity patterns in the direction of beam propagation has been theoretically and experimentally studied. 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.

Figure: The schematic illustration of the dual-wavelength focusing experiment through amplitude-type Fresnel zone plate and the process of the femtosecond laser micromachining.

This research was published in“Bing Zhu, Xiangmin Liu, Haigang Liu, Yi’an Liu, Xiongshuo Yan, Yuping Chen*, and Xianfeng Chen, Dual-Wavelength Focusing through Fresnel Zone Plate Fabricated in Lithium Niobate Crystal by Femtosecond Laser Micromachining, Phys. Status Solidi RRL. (2020). doi: 10.1002/pssr.202000213.”

Link：https://doi.org/10.1002/pssr.202000213