Extended-depth-of-focus Fresnel zone plate via genetic algorithm and photothermal lithography

Fresnel zone plates (FZPs) are widely used diffractive optical elements (DOEs); however, their inherently limited depth of focus (DOF) restricts performance in applications requiring stable axial intensity over extended propagation distances. In this work, we propose and experimentally demonstrate an extended-depth-of-focus Fresnel zone plate (EFZP) that achieves a significantly elongated focal region without the need for additional optical components. The EFZP design is optimized using a genetic algorithm (GA) to simultaneously enhance axial depth and maintain controlled transverse focusing performance. When operated at a wavelength of 0.532 µm, the optimized EFZP exhibits an extended DOF of 67.62 µm, representing an improvement of nearly 60× compared with a traditional FZP (TFZP) of identical aperture size and focal length. The EFZP is fabricated using a simplified two-step process based on direct laser writing, inducing phase transitions in phase change material thin films, followed by selective wet etching. Structural characterization confirmed high fabrication accuracy with dimensional deviations below 5%. Experimental verification using a wide-field microscopy system shows strong agreement with numerical simulations and confirms stable focusing performance across the extended focal range. This work provides a robust and efficient framework for developing high-performance, compact diffractive optics with tailored axial intensity profiles, while extending the use case of traditional direct laser writing systems.