Considering Grayscale Process and Material Properties for Robust Multilevel Diffractive Flat Optics

Arbitrarily designed flat optics directly manipulate the light wavefront to reproduce complex optical functions, enabling more compact optical assemblies and microsystem integration. Phase-shifting micro-optical devices rely on locally tailoring the optical path length of the wavefront through binary or multilevel surface relief micro- and nanostructures. Considering the resolution and tolerances of the production processes and the optical properties of the substrate and coating materials is crucial for designing robust multilevel diffractive flat optics. In this work, we evaluate the impact of the grayscale laser lithography resolution and geometry constraints on the efficiency of surface-relief diffractive lenses, and we analyze the process parameter space that limit lens performance. We introduce a spectral bandwidth metric to help evaluate the broad-spectrum performance of different materials. We simulate and experimentally observe that the diffractive focusing is dominated by the periodic wavelength-dependent phase discontinuities arising in the profile transitions of multilevel diffractive lenses.