Designing high-efficiency extended depth-of-focus metalens via topology-shape optimization

Abstract

Longitudinal optical field modulation is of critical importance in a wide range of applications, including optical imaging, spectroscopy, and optical manipulation. However, it remains a considerable challenge to realize a uniformly distributed light field with extended depth-of-focus. Here, a high-efficiency extended depth-of-focus metalens is proposed by adjoint-based topology-shape optimization approach, wherein the theoretical electric field intensity corresponding to a variable focal-length phase is utilized as the figure of merit. Using a dozen of metalens with random structure parameters as initial structures, the average focal depth of topology-shape optimized metalens is greatly improved up to 18.80 μm (about 29.7λ), which is 1.54 times higher than the diffraction-limited focal depth. Moreover, all the topology-shape optimized metalens exhibit high diffraction efficiency exceeding 0.7 over the whole focal depth range, which is approximately three times greater than that of the forward design. Our results offer a new insight into the design of extended depth-of-focus metalens and may find potential applications in imaging, holography, and optical fabrication.

Electrically switchable 2N-channel wave-front control with N cascaded polarization-dependent metasurfaces

“…These non-ideal factors can be further reduced with improved design approach of metasurfaces, e.g. inverse design [66][67][68][69][70] or adjoint optimization 71,72 , thus the accumulated stray light, cross-talk and loss with increased cascading layers can be within control. These methods go beyond the present work, but we have been engaged in related researches.…”

Multiscale Optical Field Manipulation via Planar Digital Optics

“…In contrast with the forward design approach, the inverse design could optimize the desired complex functionality by reshaping the regular meta-atoms into arbitrarily irregular topology. − One of the typical inverse design approaches is adjoint-based topology optimization, which is a local gradient descent algorithm requiring only two steps of full-wave electromagnetic simulation for each iteration process . Recently, it has demonstrated great success in realizing complex functionalities of metasurfaces and meta-optics, such as meta-gratings, − metalens, − disordered metasurfaces, , and other freeform metasurfaces. , For example, as shown in Figure a, Sell et al demonstrated a topology-optimized large-angle deflection meta-grating with 75% absolute deflection efficiency at 1050 nm . As shown in Figure b, Sang et al comprehensively investigated the efficiency gap between the conventional high numerical aperture (NA) metalens and the theoretical limitation of vector diffraction theory .…”

Inverse design of sub-diffraction focusing metalens by adjoint-based topology optimization

“…Topology optimization is one of the widely used gradient-based approaches that can be combined with other algorithms, such as the adjoint method, boundary optimization algorithm, and machine learning. The gradient optimization methods based on the design of adjoint source can be applied in the optimization of other metasurface functional devices, such as beam deflectors [40,41], ultrahigh numerical aperture metalens [39], and high-efficiency extended depth-of-focus metalens [42]. In large-scale electromagnetic simulations, the adjoint method can significantly reduce the number of simulations for calculating the influence of structural change on merit function [43].…”