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Flat optics consisting of nanostructures of high-refractive index materials produce lenses with thin form factors that tend to operate only at specific wavelengths. Recent attempts to achieve achromatic lenses uncover a tradeoff between the numerical aperture (NA) and bandwidth, which limits performance. Here, we propose a new approach to design high-NA, broadband, and polarization-insensitive multilayer achromatic metalenses (MAMs). We combine topology optimization and full-wave simulations to inversely design MAMs and fabricate the structures in low-refractive index materials by two-photon polymerization lithography. MAMs measuring 20 µm in diameter operating in the visible range of 400 to 800 nm with 0.5 and 0.7 NAwere achieved with efficiencies of up to 42%. We demonstrate broadband imaging performance of the fabricated MAM under white light and RGB narrowband illuminations. These results highlight the potential of the 3D-printed multilayer structures for realizing broadband and multifunctional meta-devices with inverse design. [ABSTRACT FROM AUTHOR]

A printable hybrid hydrogel is fabricated by embedding poly(N-isopropylacrylamide) (PNIPAm) microparticles within a water-rich silica-alumina (Si/Al)-based gel matrix. The hybrid gel holds water content of up to 70 wt%, due to its unique Si/Al matrix. The hybrid hydrogel can respond to both heat and electrical stimuli, and can be directly printed layer-by-layer using a commercial 3-dimensional printer, without requiring any curing. The hybrid ink is printed onto a transparent, flexible conductive electrode composed of silver nanoparticles and sustains bending angles of up to 180°, which enables patterning of various flexible devices such as smart windows and a 3D optical waveguide valve. [ABSTRACT FROM AUTHOR]

We report a robust methodology forsynthesizing monodisperse copper-tin-selenide(CTSe) nanocrystals (NCs) of tunable size, shape, and compositionincluding single-crystalline nanosheets, nanoplates, spheres, andtetrahedra. Both the identity and concentration of the selenium precursorplay important roles in determining the morphology and crystal structureof the CTSe NCs. In contrast to previous studies, we demonstratedbroad tunability of the Cu to Sn ratio. The size of CTSe NCs continuouslydecreased with increasing Sn incorporation. Moreover, the near-infrared(NIR) localized surface plasmon resonance (LSPR) in CTSe alloy NCswas tuned over a broad range by varying the Cu:Sn ratio. The LSPRred-shifted and decreased in intensity with increasing Sn content.This indicates that the free charge carrier concentration can be manipulatedby varying the cation ratio. The cation deficiency responsible forself-doping in these NCs decreases with increasing Sn content. Theresulting CTSe NCs and related materials with tunable size, shape,band gap, and doping level provide new opportunities in solution-processedoptoelectronic devices. [ABSTRACT FROM AUTHOR]