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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. 

We report the fabrication of gradient index flat optics and waveguides using the ‘nanoimprinting refractive index’ (NIRI) technique applied to mesoporous silicon substrates. Optical wavefront shaping and waveguiding are demonstrated in the visible and near-infrared respectively. 

We implement 1D moiré patterns in silicon photonic nanowires to demonstrate a wide range of effects such as tunable photon transport and localization, high-Q cavities and coupled resonator optical waveguide behavior by modulation of lattice mismatch and crystal length. © 2022 The Author(s) 

Analogous to twistronic and twistoptic systems derived from 2D moiré potentials wherein the twist angle is a key parameter, we modulate the 1D moiré pattern in a silicon nanowire and observe lattice mismatch and phase to be critical parameters for controlling photon transport or localization. 

We report a novel colorimetric sensing paradigm using multi-chromatic light from an RGB laser combined with a structural color sensor for fast, ultra-sensitive, and spatio-temporally resolved detection of surface biomolecules by human eye or smartphone. 

We demonstrate how a simple 1D flat lens can be utilized to not only focus light but to generate non-paraxial accelerating beams. We further report how illumination angle and wavelength degrees of freedom allow dynamic transition between these two functionalities. 

Colorimetric sensors offer the prospect for on-demand sensing diagnostics in simple and low-cost form factors, enabling rapid spatiotemporal inspection by digital cameras or the naked eye. However, realizing strong dynamic color variations in response to small changes in sample properties has remained a considerable challenge, which is often pursued through the use of highly responsive materials under broadband illumination. In this work, we demonstrate a general colorimetric sensing technique that overcomes the performance limitations of existing chromatic and luminance-based sensing techniques. Our approach combines structural color optical filters as sensing elements alongside a multichromatic laser illuminant. We experimentally demonstrate our approach in the context of label-free biosensing and achieve ultrasensitive and perceptually enhanced chromatic color changes in response to refractive index changes and small molecule surface attachment. Using structurally enabled chromaticity variations, the human eye is able to resolve ∼0.1-nm spectral shifts with low-quality factor (e.g., Q ∼ 15) structural filters. This enables spatially resolved biosensing in large area (approximately centimeters squared) lithography-free sensing films with a naked eye limit of detection of ∼3 pg/mm 2 , lower than industry standard sensors based on surface plasmon resonance that require spectral or angular interrogation. This work highlights the key roles played by both the choice of illuminant and design of structural color filter, and it offers a promising pathway for colorimetric devices to meet the strong demand for high-performance, rapid, and portable (or point-of-care) diagnostic sensors in applications spanning from biomedicine to environmental/structural monitoring. 

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.