Abstract
Flexible, planar diffractive optical focusing elements are of interest due to their relative design simplicity, large-area scalability, and low cost of manufacturing compared to refractive optics, metamaterial lenses, and microlens arrays. Fresnel zone plates have been considered for this application. However, the relatively large side-bands produced in the zone plate focal plane result in a reduced signal contrast, which is not ideal for many applications. In order to eliminate this issue, photon sieves have been investigated. Developed in 2001, photon sieves are essentially zone plates with the Fresnel rings segmented into a large number of circular pinholes. Replacing the continuous zones with pinholes allows one to modulate the number and position of pinholes in a photon sieve, which allows for relatively simple manipulation of the incident wavefront, tighter focusing, and near-zero focal plane side-lobes. These properties have led to photon sieves being evaluated for applications in remote sensing, beam shaping, optical multiplexing, photolithography, lenses in telescopes and imaging, among other things. However, the reported focusing efficiency of photon sieves has been significantly lower (<7%) than their corresponding zone plate (>70%), which provides the motivation for this work.
In this work, photon sieves were fabricated on lightweight, flexible polyimide substrates via a novel pulsed laser ablation method in order to demonstrate a low-cost large area fabrication process for their feasibility in space-based and other applications. Flexible, single-step phase-type devices with efficiencies of ~11% were demonstrated and performance agreed well with finite-difference time domain (FDTD) simulations. In order to further increase efficiencies, a multi-step photon sieve concept was investigated, modeled, fabricated, and tested. Testing of a fabricated four-step photon sieve showed efficiencies of ~50%, nearly five times the highest reported value for a photon sieve in the scientific literature. Higher step numbers were simulated via FDTD, and a fundamental efficiency limit was found to approach 70%. We envision that these drastically increased efficiencies will allow photon sieves to be adopted for widespread applications, where previously low efficiency hindered their use.
Additionally, several fundamental limitations of photon sieves were investigated as they related to the current state of the art. Key metrics such as operating bandwidth, focusing efficiency, and fabrication tolerances were analyzed both numerically and experimentally, and new limitations on photon sieve performance were demonstrated. For example, it was found that an inherent trade-off exists between photon sieve bandwidth and focusing efficiency in the current design paradigm, and that in order to improve this metric, a new paradigm must be adopted.
Lastly, as a result of the increased efficiencies developed in this work, photon sieves were demonstrated as focusing- optics in a laser ablation system. The fundamental differences in ablation processes and focused beam characteristics between photon sieve and traditional lens were investigated. It was found that photon sieve beams have a less intense heat affected zone (HAZ) compared to traditional lenses, which results in more consistent marking at various laser beam energy densities. This reduced HAZ and consistency has important implications in many laser-processing applications, especially semiconductor processing and optical surgery, where high precision and localization is required.
