Tunable Infrared Metasurface on a Soft Polymer Scaffold

Magneto-Mechanical Bilayer Metamaterial with Global Area-Preserving Density Tunability for Acoustic Wave Regulation

2D metamaterials have immense potential in acoustics, optics, and electromagnetic applications due to their unique properties and ability to conform to curved substrates. Active metamaterials have attracted significant research attention because of their on-demand tunable properties and performances through shape reconfigurations. 2D active metamaterials often achieve active properties through internal structural deformations, which lead to changes in overall dimensions. This demands corresponding alterations of the conforming substrate, or the metamaterial fails to provide complete area coverage, which can be a significant limitation for their practical applications. To date, achieving area-preserving active 2D metamaterials with distinct shape reconfigurations remains a prominent challenge. In this paper, magneto-mechanical bilayer metamaterials are presented that demonstrate area density tunability with area-preserving capability. The bilayer metamaterials consist of two arrays of magnetic soft materials with distinct magnetization distributions. Under a magnetic field, each layer behaves differently, which allows the metamaterial to reconfigure its shape into multiple modes and to significantly tune its area density without changing its overall dimensions. The area-preserving multimodal shape reconfigurations are further exploited as active acoustic wave regulators to tune bandgaps and wave propagations. The bilayer approach thus provides a new concept for the design of area-preserving active metamaterials for broader applications.

Ultracompact Nanophotonics: Light Emission and Manipulation with Metasurfaces

Internet of Things (IoT) technology is prosperous for the betterment of human well-being. With the expeditious needs of miniature functional devices and systems for adaptive optics and light manipulation at will, relevant sensing techniques are thus in the urgent stage of development. Extensive developments in ultrathin artificial structures, namely metasurfaces, are paving the way for the next-generation devices. A bunch of tunable and reconfigurable metasurfaces with diversified catalogs of mechanisms have been developed recently, enabling dynamic light modulation on demand. On the other hand, monolithic integration of metasurfaces and light-emitting sources form ultracompact meta-devices as well as exhibiting desired functionalities. Photon-matter interaction provides revolution in more compact meta-devices, manipulating light directly at the source. This study presents an outlook on this merging paradigm for ultracompact nanophotonics with metasurfaces, also known as metaphotonics. Recent advances in the field hold great promise for the novel photonic devices with light emission and manipulation in simplicity.

Nanophotonics for light detection and ranging technology

Light detection and ranging (LiDAR) technology, a laser-based imaging technique for accurate distance measurement, is considered one of the most crucial sensor technologies for autonomous vehicles, artificially intelligent robots and unmanned aerial vehicle reconnaissance. Until recently, LiDAR has relied on light sources and detectors mounted on multiple mechanically rotating optical transmitters and receivers to cover an entire scene. Such an architecture gives rise to limitations in terms of the imaging frame rate and resolution. In this Review, we examine how novel nanophotonic platforms could overcome the hardware restrictions of existing LiDAR technologies. After briefly introducing the basic principles of LiDAR, we present the device specifications required by the industrial sector. We then review a variety of LiDAR-relevant nanophotonic approaches such as integrated photonic circuits, optical phased antenna arrays and flat optical devices based on metasurfaces. The latter have already demonstrated exceptional functional beam manipulation properties, such as active beam deflection, point-cloud generation and device integration using scalable manufacturing methods, and are expected to disrupt modern optical technologies. In the outlook, we address the upcoming physics and engineering challenges that must be overcome from the viewpoint of incorporating nanophotonic technologies into commercially viable, fast, ultrathin and lightweight LiDAR systems.

Scalable spectrally selective mid-infrared meta-absorbers for advanced radiative thermal engineering

Metamaterials with spectrally selective absorptance operating in the mid-infrared range have attracted much interest in numerous applications. However, it remains a challenge to economically fabricate scalable meta-absorbers with tailorable absorptance bands. This work demonstrates a conceptually simple and low-cost yet effective design strategy to achieve spectrally selective absorption with tailorable band positions at MIR by colloidal lithography. The strategy ingeniously uses residual diameter fluctuations of circular resonators etched through monodisperse colloidal particles for achieving superposition of multiple magnetic resonances and thereby a more than doubled absorption band, which is neglected in previous works. The proposed meta-absorber features densely packed thick aluminum resonators with a rather narrow diameter distribution and enhanced capacitive coupling among them. Moreover, the tailorability of the absorption band can be achieved by a parameterized variation in the fabrication process. As a proof of concept, infrared stealth and radiative cooling are demonstrated based on our meta-absorbers. The design and fabrication strategy create versatile metamaterials for advanced radiative thermal engineering.

Metasurface Color Filters Using Aluminum and Lithium Niobate Configurations

Two designs of metasurface color filters (MCFs) using aluminum and lithium niobate (LN) configurations are proposed and numerically studied. They are denoted as tunable aluminum metasurface (TAM) and tunable LN metasurface (TLNM), respectively. The configurations of MCFs are composed of suspended metasurfaces above aluminum mirror layers to form a Fabry-Perot (F-P) resonator. The resonances of TAM and TLNM are red-shifted with tuning ranges of 100 nm and 111 nm, respectively, by changing the gap between the bottom mirror layer and top metasurface. Furthermore, the proposed devices exhibit perfect absorption with ultra-narrow bandwidth spanning the whole visible spectral range by composing the corresponding geometrical parameters. To increase the flexibility and applicability of proposed devices, TAM exhibits high sensitivity of 481.5 nm/RIU and TLNM exhibits high figure-of-merit (FOM) of 97.5 when the devices are exposed in surrounding environment with different refraction indexes. The adoption of LN-based metasurface can enhance FWHM and FOM values as 10-fold and 7-fold compared to those of Al-based metasurface, which greatly improves the optical performance and exhibits great potential in sensing applications. These proposed designs provide an effective approach for tunable high-efficiency color filters and sensors by using LN-based metamaterial.

Recent Advances in MEMS Metasurfaces and Their Applications on Tunable Lens

The electromagnetic (EM) properties of metasurfaces depend on both structural design and material properties. microelectromechanical systems (MEMS) technology offers an approach for tuning metasurface EM properties by structural reconfiguration. In the past 10 years, vast applications have been demonstrated based on MEMS metasurfaces, which proved to have merits including, large tunability, fast speed, small size, light weight, capability of dense integration, and compatibility of cost-effective fabrication process. Here, recent advances in MEMS metasurface applications are reviewed and categorized based on the tuning mechanisms, operation band and tuning speed. As an example, the pros and cons of MEMS metasurfaces for tunable lens applications are discussed and compared with traditional tunable lens technologies followed by the summary and outlook.

Fabrication of multi-material 3D structures by the integration of direct laser writing and MEMS stencil patterning

The construction of a complex, 3D optical metamaterial challenges conventional nanofabrication techniques. These metamaterials require patterning of both a deformable mechanical substrate and an optically-active structure with ∼200 nm resolution and precision. The soft nature of the deformable mechanical materials often precludes the use of resist-based techniques for patterning. Furthermore, FIB deposition approaches produce metallic structures with considerable disorder and impurities, impairing their optical response. In this paper we discuss a novel solution to this nanofabrication challenge - the integration of direct laser writing and MEMS stencil patterning. We demonstrate a variety of methods that enable this integration and then show how one can produce optically-active, 3D metamaterials. We present optical characterization data on one of these metamaterials to demonstrate the viability of our nanofabrication approach.

Integrating microsystems with metamaterials towards metadevices

Electromagnetic metamaterials, which are a major type of artificially engineered materials, have boosted the development of optical and photonic devices due to their unprecedented and controllable effective properties, including electric permittivity and magnetic permeability. Metamaterials consist of arrays of subwavelength unit cells, which are also known as meta-atoms. Importantly, the effective properties of metamaterials are mainly determined by the geometry of the constituting subwavelength unit cells rather than their chemical composition, enabling versatile designs of their electromagnetic properties. Recent research has mainly focused on reconfigurable, tunable, and nonlinear metamaterials towards the development of metamaterial devices, namely, metadevices, via integrating actuation mechanisms and quantum materials with meta-atoms. Microelectromechanical systems (MEMS), or microsystems, provide powerful platforms for the manipulation of the effective properties of metamaterials and the integration of abundant functions with metamaterials. In this review, we will introduce the fundamentals of metamaterials, approaches to integrate MEMS with metamaterials, functional metadevices from the synergy, and outlooks for metamaterial-enabled photonic devices.

Tunable/Reconfigurable Metasurfaces: Physics and Applications

Metasurfaces, ultrathin metamaterials constructed by planar meta-atoms with tailored electromagnetic (EM) responses, have attracted tremendous attention due to their exotic abilities to freely control EM waves. With active elements incorporated into metasurface designs, one can realize tunable and/or reconfigurable metadevices with functionalities controlled by external stimuli, opening a new platform to dynamically manipulate EM waves. In this article, we briefly review recent progress on tunable/reconfigurable metasurfaces, focusing on their working mechanisms and practical applications. We first describe available approaches, categorized into different classes based on external stimuli applied, to realize homogeneous tunable/reconfigurable metasurfaces, which can offer uniform manipulations on EM waves. We next summarize recent achievements on inhomogeneous tunable/reconfigurable metasurfaces with constitutional meta-atoms locally tuned by external knobs, which can dynamically control the wave-fronts of EM waves. We conclude this review by presenting our own perspectives on possible future directions and existing challenges in this fast developing field.