Computationally Optimized Fresnel Zone Plates for Extended Depth of Focus

Date of Award

5-9-2026

Degree Name

M.S. in Electro-Optics

Department

Department of Electro-Optics and Photonics

Advisor/Chair

Imad Agha

Abstract

Fresnel zone plates (FZPs) provide a compact and flexible approach for wavefront shaping and focusing, but conventional FZPs usually suffer from a limited depth of focus imposed by diffraction. This limitation restricts their performance in applications that require stable focusing over an extended axial range. To address this issue, this thesis investigates the design, fabrication, and experimental characterization of extended depth-of-focus Fresnel zone plates (EFZPs) for generating elongated focal regions with quasi-non-diffracting behavior. The theoretical part of this work begins with the optical transmission theory of traditional FZPs and then introduces a linear multi-focal EFZP design based on the normalized angular spectrum compression concept. Rayleigh–Sommerfeld diffraction simulation is employed to evaluate the propagated optical field and to analyze the axial and transverse focusing characteristics. To improve the optical performance, a genetic-algorithm-based optimization framework is developed. The fitness function jointly considers the effective depth of focus, the continuity of the above-threshold focal region, axial ripple suppression, and transverse spot-size confinement. In addition, by reducing the original multi-parameter EFZP formulation to a compact two-parameter representation, a dimensionless ratio governing the normalized axial intensity profile is identified. Numerical evidence suggests that EFZP designs sharing the same value of this ratio exhibit similar axial intensity distributions up to an affine rescaling, providing a useful guideline for parameter selection in the design process. To experimentally realize the optimized design, EFZP structures are fabricated on GST phase-change thin films by laser direct writing lithography and subsequent selective wet etching. Optical microscopy and scanning electron microscopy confirm that the fabricated concentric-ring structures agree well with the theoretical design. An optical characterization system based on 532 nm laser illumination, microscopic imaging, and axial scanning is then established to measure the focusing performance of the fabricated EFZP. The experimental results demonstrate that the fabricated EFZP produces an elongated focal region and maintains a compact transverse focal spot over an extended axial range. The measured axial intensity evolution agrees well with the simulation trend, although additional oscillations and background interference are observed in the practical system. Compared with a traditional FZP of similar specifications, the EFZP achieves a depth-of-focus enhancement of nearly 60 times. These results verify the feasibility of combining theoretical modeling, numerical optimization, GST-based fabrication, and optical characterization to realize high-performance EFZPs, and they provide a useful basis for further studies on ripple suppression, dual-wavelength design, and parameter-scaling laws in diffractive optical devices.

Keywords

Engineering, Nanotechnology, Optics, Physics

Comments

OCLC No. 1591818577

Rights Statement

Copyright 2026, author.

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