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Production of flat optics incorporating subwavelength features, particularly at visible frequencies, remains a significant challenge. Here, we establish a framework for the design of effective medium metasurfaces (EMM), relying on nanoimprinting of mesoporous silicon to realize a patterned refractive indexn(x,y) corresponding to an arbitrary transmitted phase profileϕ(x,y). The method is used to design the stamp profile required to produce a Fresnel lens and the theoretical performance of the metalens is examined using the finite-difference time-domain method. Additionally, we demonstrate neural network aided Monte Carlo analysis as a method to model the effects of metasurface fabrications errors on EMM performance and process yield. 

Abstract Dynamic and responsive structural color devices present promising opportunities for sensing and display technologies, with applications including point‐of‐care diagnostics, portable/wearable sensors, and low‐power full‐color displays. However, it is often difficult to generate a large and quantitatively meaningful colorimetric response especially toward weak stimuli. Here, dual‐band hyperchromatic structural color (HSC) is presented as an approach to overcome these challenges. Within this framework, a dual‐band mesoporous silicon rugate filter metamaterial co‐designed is experimentally realized for use with a dichromatic (red/green) illuminant. This is shown to enable an amplified red‐to‐green color transition with a substantially smaller wavelength shift than conventional structural color devices, Δλ ≪ |λ_G–λ_R|, as well as a direct quantitative mapping between the observed chromaticity and the input stimulus. This approach is experimentally demonstrated for the spatiotemporally resolved sensing of refractometric stimuli including small‐molecules and volatile organic compounds (VOCs) with a ≈0.08 nm equivalent spectral resolution. This approach is entirely scanning‐free, enabled by simple color imaging, and does not require advanced spectroscopic (hyperspectral) imaging or interferometric imaging to obtain meaningful quantitative and spatiotemporally resolved information pertaining to the sensor's attributes. These results demonstrate dual‐band HSC as a promising approach for realizing low‐cost and high performance dynamic structural color devices and sensors. 

The wavefronts emerging from phase gradient metasurfaces are typically sensitive to incident beam properties such as angle, wavelength, or polarization. While this sensitivity can result in undesired wavefront aberrations, it can also be exploited to construct multifunctional devices which dynamically vary their behavior in response to tuning a specified degree of freedom. Here, we show how incident beam tilt in a one dimensional metalens naturally offers a means for changing functionality between diffraction limited focusing and the generation of non-paraxial accelerating light beams. This attractively offers enhanced control over accelerating beam characteristics in a simple and compact form factor. 

Abstract We report the realization of digital and gradient index flat‐optics and planar waveguides using the ‘nanoimprinting refractive index’ (NIRI) technique applied to mesoporous silicon. This technique combines the distinct optical and mechanical metamaterial qualities of mesoporous silicon, including its widely tunable effective refractive index and ability to undergo plastic deformation with a near zero Poisson ratio. Nanoimprinting with premastered and reusable stamps containing analog or digital features enables the continuous or discontinuous patterning of refractive index with high contrast Δn ≥ 0.8 and subwavelength resolution. Using NIRI we experimentally demonstrate a wavefront shaping flat microlens array operating in the visible (405–635 nm) and mesoporous silicon and silica waveguides operating near 1310 nm. This study demonstrates the viability of patterning arbitrary refractive index distributions,n(x,y), on the surface of a chip while circumventing the challenges and limitations of top‐down lithographic techniques – thus opening a low‐cost and scalable approach for the realization of advanced planar optical technologies.