![]() These include surface FZPs fabricated on polymethyl methacrylate, polycarbonate, fused silica, and the end facet of multimode fibers by using laser ablation volumetric FZPs fabricated in borosilicate glass, fused silica, and sapphire by using laser-induced localized refractive index change or void generation and nonlinear FZPs fabricated in lithium niobate crystal by using laser-induced quadratic susceptibility erasion. Various FZPs based on different material systems have been fabricated through femtosecond laser direct writing. The small heat-affected zone increases the processing accuracy of femtosecond lasers compared with that of other laser beams, which makes femtosecond laser processing a flexible and high-precision tool for fabricating micro–nano structures. Because of the ultrashort pulse duration, the laser pulse ends before the energy is transferred from the excited electrons to the lattice, which minimizes the thermal effect. However, these methods require a multistep manufacturing process, the processing of masks, or rigorous environmental conditions, which makes processing time-consuming and costly, thus limiting the practical application of FZPs and their integration with other optical devices.įemtosecond lasers have attracted broad attention in micro–nano fabrication because of their unique ultrashort pulse duration, extremely high peak intensity, and material universality as well as their inherently flexible and maskless laser processing. Similar to other diffractive optical devices, FZPs are commonly fabricated using high-accuracy processing methods, such as photolithography, electron-beam lithography, or focused ion beam etching. The light beams transmitted or reflected by FZPs interfere constructively at the designed points, and hence, they are generally used in optical systems as the refractive lens for focusing and have been extensively applied in hybrid optics, microsensors, optical tweezers, and interconnections. The Fresnel zone plate (FZP) is a typical diffractive optical device that consists of alternate concentric rings of various thicknesses (phase type) or optical transmittances (amplitude type). Planar diffractive optical devices, however, have attracted broad attention because they are ultrathin, light weight, and easy to integrate. Refractive optics such as lens, grating, axicon, and microlens arrays are widely used in optical systems and daily life, but their bulky size and poor installation flexibility render them incapable of meeting the growing demand for miniaturization in integrated optical systems. Furthermore, the fabrication of other functional ultrathin lenses, such as axial multifocal zone plates, petal-like zone plates, and FZP arrays, is described, revealing the wide potential for the flexible and scalable fabrication method in on-chip integrated optical systems. The fabricated FZPs possess an excellent broadband focusing and imaging ability in the visible spectrum. FZPs with an area varying across three orders of magnitude are presented to demonstrate the capability of cross-scale fabrication. The fabricated FZPs are spliced by the printed element structures with no FZP size limitation in theory. The FZPs are split into a series of element patterns that are printed in order by using corresponding modulated femtosecond pulses. This paper details a high efficiency method for fabricating ultrathin FZPs of different scales on metal films by using holographic femtosecond lasers. ![]() However, challenges remain in fabricating FZPs with high efficiency and satisfying the requirement for cross-scale fabrication. To meet the growing demand for photonic integration and device miniaturization, planar diffractive Fresnel zone plates (FZPs) are widely applied in integrated optical systems.
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