Control over the dispersion of the refractive index is essential to the performance of most modern optical systems. These range from laboratory microscopes to optical fibres and even consumer products, such as photography cameras. Conventional methods of engineering optical dispersion are based on altering material composition, but this process is time-consuming and difficult, and the resulting optical performance is often limited to a certain bandwidth. Recent advances in nanofabrication have led to high-quality metasurfaces with the potential to perform at a level comparable to their state-of-the-art refractive counterparts. In this Review, we introduce the underlying physical principles of metasurface optical elements (with a focus on metalenses) and, drawing on various works in the literature, discuss how their constituent nanostructures can be designed with a highly customizable effective index of refraction that incorporates both phase and dispersion engineering. These metasurfaces can serve as an essential component for achromatic optics with unprecedented levels of performance across a broad bandwidth or provide highly customized, engineered chromatic behaviour in instruments such as miniature aberration-corrected spectrometers. We identify some key areas in which these achromatic or dispersion-engineered metasurface optical elements could be useful and highlight some future challenges, as well as promising ways to overcome them. Flat metasurface optics provides an emerging platform for combining semiconductor foundry methods of manufacturing and assembling with nanophotonics to produce high-end and multifunctional optical elements. This Review highlights the design of metasurfaces, recent advances in the field and initial promising applications.

Flat optics with dispersion-engineered metasurfaces

Image processing has become a critical technology in a variety of science and engineering disciplines. Although most image processing is performed digitally, optical analog processing has the advantages of being low-power and high-speed, but it requires a large volume. Here, we demonstrate flat optics for direct image differentiation, allowing us to significantly shrink the required optical system size. We first demonstrate how the differentiator can be combined with traditional imaging systems such as a commercial optical microscope and camera sensor for edge detection with a numerical aperture up to 0.32. We next demonstrate how the entire processing system can be realized as a monolithic compound flat optic by integrating the differentiator with a metalens. The compound nanophotonic system manifests the advantage of thin form factor as well as the ability to implement complex transfer functions, and could open new opportunities in applications such as biological imaging and computer vision. Vertical integration of a metalens to realize compound nanophotonic systems for optical analog image processing is realized, significantly reducing the size and complexity of conventional optical systems.

Flat optics for image differentiation

Flat optics has emerged as a key player in the area of structured light and its applications, owing to its subwavelength resolution, ease of integration, and compact footprint. Although its first generation has revolutionized conventional lenses and enabled anomalous refraction, new classes of meta-optics can now shape light and dark features of an optical field with an unprecedented level of complexity and multifunctionality. Here, we review these efforts with a focus on metasurfaces that use different properties of input light—angle of incidence and direction, polarization, phase distribution, wavelength, and nonlinear behavior—as optical knobs for tuning the output response. We discuss ongoing advances in this area as well as future challenges and prospects. These recent developments indicate that optically tunable flat optics is poised to advance adaptive camera systems, microscopes, holograms, and portable and wearable devices and may suggest new possibilities in optical communications and sensing. Description Actively structuring light The development of metasurfaces has provided a route to replacing bulk optical components with thin layers of engineered materials. In a review, Dorrah and Capasso highlight some of the recent advances in wavefront shaping using multifunctional meta-optics. They focus on the ability to tune the response of the metasurface by simply tuning one or more degrees of freedom of incident light, for example, by varying its angle of incidence, polarization, wavelength, or phase. The key feature of these metasurfaces is that although they are static, they can produce a tunable response without the need for complex switching. These developments enable multifunctional and lightweight components for technologies such as augmented and virtual reality displays, drone-based sensing, and endoscopy. —ISO A review discusses methods to control the functionality of optical metasurfaces by the incident light. BACKGROUND Structuring the degrees of freedom of light—including its phase, amplitude, and polarization—has opened new frontiers in science and technology alike. Adaptive cameras, microscopes, portable and wearable devices, optical communications, and laser machining are only a few of the domains that have evolved over the past decade owing to the advances in wavefront-shaping platforms. Flat optics composed of subwavelength-spaced optical scatterers—also known as metasurfaces or meta-optics—are key enabling tools for structured light not only for their compact footprint and complementary metal-oxide semiconductor (CMOS) compatibility but also because of their versatility and custom design. Although flat optics, or at least in its first generation, has led to the development of effects like anomalous refraction and diffraction-limited focusing, new classes of metasurfaces can now mold the flow of light in much more complex ways. Dispersion engineering and polarization optics are two prominent areas in which the metasurface’s ability to spatially manipulate each wavelength and/or polarization state, independently, cannot be paralleled using bulk optical components. At the heart of these developments is a carefully engineered light-matter interaction at the level of the meta-atom, which allows a passive metasurface to produce this complex response. ADVANCES The ability to manipulate light in different ways, depending on its properties, is intriguing because it allows a passive device to produce many functions without the need for active switching—that is, light itself can be used as an optical control knob. For example, the same flat optic may behave as a lens or a mirror depending on the incidence angle of light. Likewise, by changing light’s polarization, a metasurface can switch between different holograms or modify its focal length. In this spirit, a new generation of meta-optics can now perform parallel processing of the polarization of input light in the transverse plane or in 3D, reducing the function of many polarizers and waveplates into a single optical component that can be integrated in polarimeters and cellphone cameras. The spatial phase distribution of incoming light is another degree of freedom that can also be used as a switch, allowing a static flat optic to project different holograms by varying the helicity of the incident wavefront or its phase profile in general. Moreover, the ability to impart different phase and/or amplitude profiles on different wavelengths, independently, has enabled a wide class of versatile metalenses and compact pulse-shaping tools. Harnessing the nonlinear interaction of light with meta-atoms has also enabled multiwavelength holography on high harmonic-generated signals in addition to an asymmetric response. This versatility has made flat optics an ideal platform for the generation of structured light and has inspired many applications. The figure depicts the use of a metasurface as a multipurpose device (akin to a Swiss knife) that can mix and match output light by tuning the five abovementioned control knobs, without the need for any complex circuitry to drive the meta-atoms. Tunable behavior of this kind relies on an intricate light-matter interaction at the nanoscale, which is often difficult to replicate with other wavefront-shaping tools. With such tunability, many existing technologies can be dramatically miniaturized, enabling compact spectrometers, polarization-sensitive cameras, lightweight augmented reality and virtual reality headsets, and biomedical devices. OUTLOOK As the area of structured light matures, the quest for more sophisticated metasurfaces is also on the rise, aided by advanced nanofabrication, powerful computation capabilities for revealing new meta-atom libraries, and recent developments in actively tunable materials for time-varying control. Structured light with tunable metasurfaces is poised to reveal new functionalities and to replace conventional optical systems with on-chip photonic components. This includes integrating metasurfaces in laser cavities, Fabry-Perot resonators, fiber-based devices, and active wavefront-shaping tools. With the emerging trends in inverse design and topology optimization, new standardized protocols for large-scale multilayer metasurface fabrication and innovative material platforms will push the limits of multifunctional meta-optics and structured light from 2D to 3D and from static to animate, thus tackling the open challenges in this wide field of research and unlocking many new paths. Tunable structured light with static meta-optics. Different properties of input light may act as control knobs for tuning the optical response of the metasurface. These degrees of freedom include the angle of incidence (A), polarization state (B), orbital angular momentum (or spatial structure in general) (C), wavelength (D), and intensity level (manifested in a nonlinear interaction) (E). By changing one or more of these properties at the input of the metasurface, one can obtain a different light pattern at the output (depicted by the different puzzle pieces of Harvard University’s logo). This tunability relies on an intricate light-matter interaction at the level of the meta-atom, which often cannot be replicated by other conventional wavefront shaping platforms.

Tunable structured light with flat optics

Metasurfaces enable the design of optical elements by engineering the wavefront of light at the subwavelength scale. Due to their ultrathin and compact characteristics, metasurfaces possess great potential to integrate multiple functions in optoelectronic systems for optical device miniaturisation. However, current research based on multiplexing in the 2D plane has not fully utilised the capabilities of metasurfaces for multi-tasking applications. Here, we demonstrate a 3D-integrated metasurface device by stacking a hologram metasurface on a monolithic Fabry-Perot cavity-based colour filter microarray to simultaneously achieve low-crosstalk, polarisation-independent, high-efficiency, full-colour holography, and microprint. The dual functions of the device outline a novel scheme for data recording, security encryption, colour displays, and information processing. Our 3D integration concept can be extended to achieve multi-tasking flat optical systems by including a variety of functional metasurface layers, such as polarizers, metalenses, and others.

3D-Integrated metasurfaces for full-colour holography

Quantitative phase imaging (QPI) of transparent samples plays an essential role in multiple biomedical applications, and miniaturizing these systems will enable their adoption into point-of-care and in vivo applications. Here, we propose a compact quantitative phase gradient microscope (QGPM) based on two dielectric metasurface layers, inspired by a classical differential interference contrast (DIC) microscope. Owing to the multifunctionality and compactness of the dielectric metasurfaces, the QPGM simultaneously captures three DIC images to generate a quantitative phase gradient image in a single shot. The volume of the metasurface optical system is on the order of 1 mm3. Imaging experiments with various phase resolution samples verify the capability to capture quantitative phase gradient data, with phase gradient sensitivity better than 92.3 mrad μm−1 and single-cell resolution. The results showcase the potential of metasurfaces for developing miniaturized QPI systems for label-free cellular imaging and point-of-care devices. Using two dielectric metasurface layers, a compact quantitative phase gradient microscope that can capture quantitative phase gradient images in a single shot is reported with phase gradient sensitivity better than 92.3 mrad μm−1 and single-cell resolution.

Single-shot quantitative phase gradient microscopy using a system of multifunctional metasurfaces

Metasurfaces provide a versatile platform for manipulating the wavefront of light using planar nanostructured surfaces. Transmissive metasurfaces, with full 2π phase control, are a particularly att...

Multilayer Noninteracting Dielectric Metasurfaces for Multiwavelength Metaoptics

Optical metasurfaces have become versatile platforms for manipulating the phase, amplitude, and polarization of light. A platform for achieving independent control over each of these properties, however, remains elusive due to the limited engineering space available when using a single-layer metasurface. For instance, multiwavelength metasurfaces suffer from performance limitations due to space filling constraints, while control over phase and amplitude can be achieved, but only for a single polarization. Here, we explore bilayer dielectric metasurfaces to expand the design space for metaoptics. The ability to independently control the geometry and function of each layer enables the development of multifunctional metaoptics in which two or more optical properties are independently designed. As a proof of concept, we demonstrate multiwavelength holograms, multiwavelength waveplates, and polarization-insensitive 3D holograms based on phase and amplitude masks. The proposed architecture opens a new avenue for designing complex flat optics with a wide variety of functionalities.

/pdf/multifunctional-metaoptics-based-on-bilayer-metasurfaces-45dw8x3xxl.pdf

Multifunctional metaoptics based on bilayer metasurfaces.

We report a multilayer dielectric metasurface platform that increases the design flexibility of metasurfaces. We numerically and experimentally demonstrate multiwavelength metaoptics using this platform. © 2019 The Author(s)

Noninteracting Multilayer Dielectric Metasurfaces for Multiwavelength Metaoptics

Solid-state narrow-band light emitters are on-demand for quantum optoelectronics. Current approaches based on defect engineering in low-dimensional materials usually introduce a broad range of emission centers. Here, we report narrow-band light emission from covalent heterostructures fused to the edges of graphene nanoribbons (GNRs) by controllable on-surface reactions from molecular precursors. Two types of heterojunction (HJ) states are realized by sequentially synthesizing GNRs and graphene nanodots (GNDs) and then coupling them together. HJs between armchair GNDs and armchair edges of the GNR are coherent and give rise to narrow-band photoluminescence. In contrast, HJs between the armchair GNDs and the zigzag ends of GNRs are defective and give rise to nonradiative states near the Fermi level. At low temperatures, sharp photoluminescence emissions with peak energy range from 2.03 to 2.08 eV and line widths of 2-5 meV are observed. The radiative HJ states are uniform, and the optical transition energy is controlled by the band gaps of GNRs and GNDs. As these HJs can be synthesized in a large quantity with atomic precision, this finding highlights a route to programmable and deterministic creation of quantum light emitters.

Engineering Edge States of Graphene Nanoribbons for Narrow-Band Photoluminescence.

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Multilayer Noninteracting Dielectric Metasurfaces for Multiwavelength Metaoptics

Multifunctional metaoptics based on bilayer metasurfaces.

Noninteracting Multilayer Dielectric Metasurfaces for Multiwavelength Metaoptics

Engineering Edge States of Graphene Nanoribbons for Narrow-Band Photoluminescence.