Optical metasurfaces are two-dimensional arrays of meta-atoms that modify different characteristics of light such as phase, amplitude, and polarization. One intriguing feature that distinguishes them from conventional optical components is their multifunctional capability. However, multifunctional metasurfaces with efficiencies approaching those of their single-functional counterparts require more degrees of freedom. Here we show that 2.5D metastructures, which are stacked layers of interacting metasurface layers, provide sufficient degrees of freedom to implement efficient multifunctional devices. The large number of design parameters and their intricate intercoupling make the design of multifunctional 2.5D metastructures a complex task, and unit-cell approaches to metasurface design produce suboptimal devices. We address this issue by designing 2.5D metastructures using the adjoint optimization technique. Instead of designing unit cells individually, our technique considers the structure as a whole, accurately accounting for interpost and inter-layer coupling. As proof of concept, we experimentally demonstrate a double-wavelength metastructure, designed using adjoint optimization, that has significantly higher efficiencies than a similar device designed with a simplified approach conventionally used in metasurface design. The 2.5D metastructure architecture empowered by the optimization-based design technique is a general platform for realizing high-performance multifunctional components and systems.

We report a one-step additive manufacturing process to fabricate metalenses for visible wavelengths. Nanostructures with aspect ratios larger than eight and critical dimensions smaller than 60 nm were produced using nanoimprint lithography and a titanium dioxide nanocrystal-based imprint material, resulting in inorganic structures exhibiting a refractive index of n = 1.9. As a demonstration, we fabricate metalenses with numerical apertures (NAs) of 0.2 and focusing efficiencies over 50%. Manufacturability was assessed by performing 15 manual imprints in 30 min (2 min of process time per imprint) with a single stamp. All imprinted lenses exhibit comparable performance, paving the way for high-throughput and low-cost manufacturing of flat optical devices. Metalenses with a diameter of 4 mm were also fabricated to investigate the success of large area replication using this process, showing efficiencies of 43%, indicating good macroscopic imprinting.

Optical metasurfaces are planar arrangements of subwavelength meta-atoms that implement a wide range of transformations on incident light. The design of efficient metasurfaces requires that the responses of and interactions among meta-atoms are accurately modeled. Conventionally, each meta-atom’s response is approximated by that of a meta-atom located in a periodic array. Although this approximation is accurate for metastructures with slowly varying meta-atoms, it does not accurately model the complex interactions among meta-atoms in more rapidly varying metasurfaces. Optimization-based design techniques that rely on full-wave simulations mitigate this problem but thus far have been mostly applied to topology optimization of small metasurfaces. Here, we describe an adjoint-optimization-based design technique that uses parametrized meta-atoms. Our technique has a lower computational cost than topology optimization approaches, enabling the design of large-scale metasurfaces that can be readily fabricated. As proof of concept, we present the design and experimental demonstration of high numerical aperture metalenses with significantly higher efficiencies than their conventionally designed counterparts.

One of the important advantages of optical metasurfaces over conventional diffractive optical elements is their capability to efficiently deflect light by large angles. However, metasurfaces are conventionally designed using approaches that are optimal for small deflection angles and their performance for designing high numerical aperture devices is not well quantified. Here we introduce and apply a technique for the estimation of the efficiency of high numerical aperture metasurfaces. The technique is based on a particular coherent averaging of diffraction coefficients of periodic blazed gratings and can be used to compare the performance of different metasurface designs in implementing high numerical aperture devices. Unlike optimization-based methods that rely on full-wave simulations and are only practicable in designing small metasurfaces, the gradient averaging technique allows for the design of arbitrarily large metasurfaces. Using this technique, we identify an unconventional metasurface design and experimentally demonstrate a metalens with a numerical aperture of 0.78 and a measured focusing efficiency of 77%. The grating averaging is a versatile technique applicable to many types of gradient metasurfaces, thus enabling highly efficient metasurface components and systems.