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The optical properties of phase‐change materials (PCMs) can be tuned to multiple levels by controlling the transition between their amorphous and crystalline phases. In multi‐material PCM structures, the number of discrete reflectance levels can be increased according to the number of PCM layers. However, the effect of increasing the number of layers on quenching and reversibility has not been thoroughly studied. In this work, the phase‐change physics and thermal conditions required for reversible switching of single and multi‐material PCM switches are discussed based on thermo‐optical phase‐change models and laser switching experiments. By using nanosecond laser pulses, 16 different reflectance levels in Ge2Sb2Te5 are demonstrated via amorphization. Furthermore, a multi‐material switch based on Ge2Sb2Te5 and GeTe with four discrete reflectance levels is experimentally proven with a reversible multi‐level response. The results and design principles presented herein will impact active photonics applications that rely on dynamic multi‐level operation, such as optical computing, beam steering, and next‐generation display technologies. [ABSTRACT FROM AUTHOR]

The colorful wings of butterflies result from the interaction between light and the intricate chitinous nanostructures on butterflies' scales. This study demonstrates that just by reproducing the chitinous ridges present in butterfly scales (i.e., without any other secondary structure), the entire color palette is achieved. This result is achieved using a new methodology based on the controlled reproduction of parts of the biological structure of complex chitinous systems using their native chemistry, enabling the isolation of different features' contributions. Here the contribution of the ridges and their variations as producing and modulating color hue is isolated. The results suggest that complicated butterfly scales may be non‐ideal solutions for producing color when multifunctionality is not considered. [ABSTRACT FROM AUTHOR]

Metasurfaces are planarized and miniaturized versions of conventional optical elements. Subwavelength‐thick single‐layer metalenses have diffraction limited resolution for on‐axis imaging but relatively low resolution for off‐axis imaging due to off‐axial aberrations. The aberrations of planar single‐layer metalenses have been corrected by patterning two metasurfaces on both sides of a substrate to form metalens doublets with a thickness of hundreds of micrometers to millimeters. The multilevel diffractive lenses are demonstrated to achieve wide angle imaging with a thickness of several micrometers, however, the off‐axial aberrations are not compensated. Here, an epsilon‐greedy algorithm‐based scheme for achieving a planar wavelength‐thick single‐layer aberration‐compensated (SLAC) flat lens consisting of dielectric nanoring structures fabricated by 3D printing are proposed. The scheme is experimentally validated via a SLAC flat lens. This SLAC flat lens has a thickness of 1 µm, a numerical aperture of 0.45, a focal length of 1 mm, a full field of view (FOV) of 32° that enables aberration‐compensated imaging along the focal plane and monochromatic microscopic imaging with resolution better than 2.2 µm at a wavelength of 633 nm. This scheme can lead to ultrathin wide‐FOV flat lens designs as well as low‐cost mass production, which has various applications in miniscopes, mobile camera modules, and machine vision. [ABSTRACT FROM AUTHOR]

The dielectric metasurface hologram promises higher efficiencies due to lower absorption than its plasmonic counterpart. However, it has only been used, up to now, for controlling linear-polarization photons to form single-plane holographic images in the near-infrared region. Here, we report a transmission-type metahologram achieving images in three colors, free from high-order diffraction and twin-image issues, with 8-level modulation of geometric phase by controlling photon spin via precisely patterned Si nanostructures with varying orientations. The resulting real and virtual holographic images with spin dependence of incident photons natively enable the spin degeneracy removal of light, leading to a metahologram-enabled spin Hall effect of light. Low-absorption dielectrics also enable us to create holograms for short-wavelength light down to 480 nm, thus spanning the three primary colors. It possesses the potential for compact color-display chips using mature semiconductor processes, and holds significant advantages over previous metaholograms operating at longer wavelengths. [ABSTRACT FROM AUTHOR]

A compact and responsive thermoelectric photodetector is introduced for the mid‐infrared. By resonantly coupling mid‐infrared light to a Sb2Te3‐Bi2Te3 thermoelectric junction, a thermocouple is formed that is directly heated by narrow‐band mid‐infrared radiation. Near‐perfect absorption is achieved at this hot junction through the resonantly enhanced coupling of light to free‐electrons in the Bi2Te3 and Sb2Te3 materials. The fabricated devices operate at 3.6 µm and demonstrate a responsivity of 10.2 V W−1, a specific detectivity of 4.6  × 106 cm Hz1/2 W−1, and a bandwidth in the order of 1 kHz. The optimal detection wavelength can be spectrally tuned by changing the resonant cavity dimensions. This work shows a path toward miniaturized mid‐infrared detectors and spectrometers with high sensitivity, responsivity, and bandwidth. Importantly, the device presented here is ideal for industrial production, which it is hoped will provide wider access to mid‐infrared technologies for chemical sensing, medicine, and security. [ABSTRACT FROM AUTHOR]