ABSTRACT: Among various flat optical devices, metasurfaces have presented their great ability in efficient manipulation of light fields and have been proposed for variety of devices with specific functionalities. However, due to the high phase dispersion of their building blocks, metasurfaces significantly suffer from large chromatic aberration. Here we propose a design principle to realize achromatic metasurface devices which successfully eliminate the chromatic aberration over a continuous wavelength region from 1200 to 1680?nm for circularly-polarized incidences in a reflection scheme. For this proof-of-concept, we demonstrate broadband achromatic metalenses (with the efficiency on the order of ?12%) which are capable of focusing light with arbitrary wavelength at the same focal plane. A broadband achromatic gradient metasurface is also implemented, which is able to deflect wide-band light by the same angle. Through this approach, various flat achromatic devices that were previously impossible can be realized, which will allow innovation in full-color detection and imaging.Metasurfaces suffer from large chromatic aberration due to the high phase dispersion of their building blocks, limiting their applications. Here, Wang et al. design achromatic metasurface devices which eliminate the chromatic aberration over a continuous region from 1200 to 1680?nm in a reflection schleme.
Project description:Planar structured interfaces, also known as metasurfaces, are continuously attracting interest owing to their ability to manipulate fundamental attributes of light, including angular momentum, phase, or polarization. However, chromatic aberration, limiting broadband operation, has remained a challenge for metasurfaces-based optical components and imagers. The limitation stems from the intrinsic dispersion of existing materials and design principles. Here we report and experimentally demonstrate polarization-independent fishnet-achromatic-metalenses with measured average efficiencies over 70% in the continuous band from the visible (640?nm) to the infrared (1200?nm). Results of the scalable platform are enabling for applications requiring broad bandwidth and high efficiency including energy harvesting, virtual reality and information processing devices, or medical imaging.
Project description:Metasurfaces offer a unique platform to precisely control optical wavefronts and enable the realization of flat lenses, or metalenses, which have the potential to substantially reduce the size and complexity of imaging systems and to realize new imaging modalities. However, it is a major challenge to create achromatic metalenses that produce a single focal length over a broad wavelength range because of the difficulty in simultaneously engineering phase profiles at distinct wavelengths on a single metasurface. For practical applications, there is a further challenge to create broadband achromatic metalenses that work in the transmission mode for incident light waves with any arbitrary polarization state. We developed a design methodology and created libraries of meta-units-building blocks of metasurfaces-with complex cross-sectional geometries to provide diverse phase dispersions (phase as a function of wavelength), which is crucial for creating broadband achromatic metalenses. We elucidated the fundamental limitations of achromatic metalens performance by deriving mathematical equations that govern the tradeoffs between phase dispersion and achievable lens parameters, including the lens diameter, numerical aperture (NA), and bandwidth of achromatic operation. We experimentally demonstrated several dielectric achromatic metalenses reaching the fundamental limitations. These metalenses work in the transmission mode with polarization-independent focusing efficiencies up to 50% and continuously provide a near-constant focal length over ??=?1200-1650?nm. These unprecedented properties represent a major advance compared to the state of the art and a major step toward practical implementations of metalenses.
Project description:Metasurfaces provide a compact, flexible, and efficient platform to manipulate the electromagnetic waves. However, chromatic aberration imposes severe restrictions on their applications in broadband polarization control. Here, we propose a broadband achromatic methodology to implement polarization-controlled multifunctional metadevices in mid-wavelength infrared with birefringent meta-atoms. We demonstrate the generation of polarization-controlled and achromatically on-axis focused optical vortex beams with diffraction-limited focal spots and switchable topological charge (L ? = 0 and L ? = 2). Besides, we further implement broadband achromatic polarization beamsplitter with high polarization isolation (extinction ratio up to 21). The adoption of all-silicon configuration not only facilitates the integration with CMOS technology but also endows the polarization multiplexing meta-atoms with broad phase dispersion coverage, ensuring the large size and high performance of the metadevices. Compared with the state-of-the-art chromatic aberration-restricted polarization-controlled metadevices, our work represents a substantial advance and a step toward practical applications.
Project description:Conventional imaging systems comprise large and expensive optical components that successively mitigate aberrations. Metasurface optics offers a route to miniaturize imaging systems by replacing bulky components with flat and compact implementations. The diffractive nature of these devices, however, induces severe chromatic aberrations, and current multiwavelength and narrowband achromatic metasurfaces cannot support full visible spectrum imaging (400 to 700 nm). We combine principles of both computational imaging and metasurface optics to build a system with a single metalens of numerical aperture ~0.45, which generates in-focus images under white light illumination. Our metalens exhibits a spectrally invariant point spread function that enables computational reconstruction of captured images with a single digital filter. This work connects computational imaging and metasurface optics and demonstrates the capabilities of combining these disciplines by simultaneously reducing aberrations and downsizing imaging systems using simpler optics.
Project description:Metasurfaces offer exciting opportunities that enable precise control of light propagation, optical intensity, phase and polarization. Plasmonic metasurface based quarter-wave plates have been recently studied to realize the conversion between linear polarization and circular polarization. However, it is still quite challenging to directly measure the birefringent phase retardation introduced by metasurface wave plates with a reliable technique. Here, we report a high-performance broadband metasurface quarter-wave plate made of anisotropic T-shaped plasmonic antennas in near-infrared wavelength range, where the achromatic nearly 90° transmitted phase retardation through the metasurface is precisely characterized with an optical vortex based interferometric approach. Based on the measured transmission amplitude and phase of two orthogonal linear polarization components, nearly unit degree of linear polarization is extracted from the Stokes parameters, indicating excellent broadband polarization conversion between linearly and circularly polarized light through the metasurface. Our results will be an important step forward in the advancement of integrated metasurface devices for polarization conversion and beam manipulation, structured light control, as well as new spectroscopic and interferometric techniques for metasurface characterization.
Project description:Chromatic dispersion spatially separates white light into colours, producing rainbows and similar effects. Detrimental to imaging but essential to spectroscopy, chromatic dispersion is the result of material properties in refractive optics and is considered an inherent characteristic of diffractive devices such as gratings and flat lenses. Here, we present a fundamental relation connecting an optical system's dispersion to the trajectories light takes through it and show that arbitrary control over dispersion may be achieved by prescribing specific trajectories, even in diffractive systems. Using cascaded metasurfaces (2D arrays of sub-micron scatterers) to direct light along predetermined trajectories, we present an achromatic twisted metalens and experimentally demonstrate beam deflectors with arbitrary dispersion. This new insight and design approach usher in a new class of optical systems with wide-ranging applications.
Project description:Dispersion engineering is essential to the performance of most modern optical systems including fiber-optic devices. Even though the chromatic dispersion of a meter-scale single-mode fiber used for endoscopic applications is negligible, optical lenses located on the fiber end face for optical focusing and imaging suffer from strong chromatic aberration. Here we present the design and nanoprinting of a 3D achromatic diffractive metalens on the end face of a single-mode fiber, capable of performing achromatic and polarization-insensitive focusing across the entire near-infrared telecommunication wavelength band ranging from 1.25 to 1.65 µm. This represents the whole single-mode domain of commercially used fibers. The unlocked height degree of freedom in a 3D nanopillar meta-atom largely increases the upper bound of the time-bandwidth product of an achromatic metalens up to 21.34, leading to a wide group delay modulation range spanning from -8 to 14 fs. Furthermore, we demonstrate the use of our compact and flexible achromatic metafiber for fiber-optic confocal imaging, capable of creating in-focus sharp images under broadband light illumination. These results may unleash the full potential of fiber meta-optics for widespread applications including hyperspectral endoscopic imaging, femtosecond laser-assisted treatment, deep tissue imaging, wavelength-multiplexing fiber-optic communications, fiber sensing, and fiber lasers.
Project description:We exploit the inherent dispersion in diffractive optics to demonstrate planar chromatic-aberration-corrected lenses. Specifically, we designed, fabricated and characterized cylindrical diffractive lenses that efficiently focus the entire visible band (450?nm to 700?nm) onto a single line. These devices are essentially pixelated, multi-level microstructures. Experiments confirm an average optical efficiency of 25% for a three-wavelength apochromatic lens whose chromatic focus shift is only 1.3??m and 25??m in the lateral and axial directions, respectively. Super-achromatic performance over the continuous visible band is also demonstrated with averaged lateral and axial focus shifts of only 1.65??m and 73.6??m, respectively. These lenses are easy to fabricate using single-step grayscale lithography and can be inexpensively replicated. Furthermore, these devices are thin (<3??m), error tolerant, has low aspect ratio (<1:1) and offer polarization-insensitive focusing, all significant advantages compared to alternatives that rely on metasurfaces. Our design methodology offers high design flexibility in numerical aperture and focal length, and is readily extended to 2D.
Project description:Underwater acoustic metasurfaces have broad application prospects for the stealth of underwater objects. However, problems such as a narrow operating frequency band, poor operating performance, and considerable thickness at low frequencies remain. In this study a reverse design method for ultra-thin underwater acoustic metasurfaces for low-frequency broadband is proposed using a tandem fully connected deep neural network. The tandem neural network consists of a pre-trained forward neural network and a reverse neural network, based on which a set of elements with flat phase variation and an almost equal phase shift interval in the range of 700–1150 Hz is designed. A diffuse underwater acoustic metasurface with 60 elements was designed, showing that the energy loss of the metasurface in the echo direction was greater than 10 dB. Our work opens a novel pathway for realising low-frequency wideband underwater acoustic devices, which will enable various applications in the future.
Project description:Nanostructured metasurfaces offer unique capabilities for subwavelength control of optical waves. Based on this potential, a large number of metasurfaces have been proposed recently as alternatives to standard optical elements. In most cases, however, these elements suffer from large chromatic aberrations, thus limiting their usefulness for multiwavelength or broadband applications. Here, in order to alleviate the chromatic aberrations of individual diffractive elements, we introduce dense vertical stacking of independent metasurfaces, where each layer is made from a different material, and is optimally designed for a different spectral band. Using this approach, we demonstrate a triply red, green and blue achromatic metalens in the visible range. We further demonstrate functional beam shaping by a self-aligned integrated element for stimulated emission depletion microscopy and a lens that provides anomalous dispersive focusing. These demonstrations lead the way to the realization of ultra-thin superachromatic optical elements showing multiple functionalities-all in a single nanostructured ultra-thin element.