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The manipulation of light has become the focus of various modern optical technologies. The emergence of bound states in the continuum (BICs) offers an alternative platform for controlling light, including the confinement and manipulation of polarization, amplitude, and phase. Currently, research on photonic BICs is maturing, with extensive exploration of methods for achieving BICs and their various applications, including lasing, sensing, and enhanced light‐matter interaction. In this review, an overview of photonic BICs is provided. Specifically, the unique properties of BICs are first presented, followed by their state‐of‐the‐art applications in passive systems, ranging from sensing to waveguiding, beam shaping, and chirality. The paradigm‐shifting developments in active systems resulting from the hybridization of BICs with active and novel materials are then highlighted. Finally, some of the challenges facing photonic BICs are discussed, along with future directions in terms of physics, design, fabrication, engineering, and tunability. [ABSTRACT FROM AUTHOR]

Molding the wavefront of acoustic waves into the desired shape is of paramount significance in acoustics, which however are usually constrained by the acoustical response of naturally available materials. The emergence of acoustic metamaterials built by assembling artificial subwavelength elements provides distinct response to acoustic waves unattainable in nature. More recently, acoustic metasurfaces, a class of metamaterials with a reduced dimensionality, empower new physics and lead to extended functionalities different from their three-dimensional counterparts, enabling controlling, transmitted or reflected acoustic waves in ways that were not possible before. In this review paper, we present a comprehensive view of this rapidly growing research field by introducing the basic concepts of acoustic metasurfaces and the recent developments that have occurred over the past few years. We review the interesting properties of acoustic metasurfaces and their important functionalities of wavefront manipulation, followed by an outlook for promising future directions and potential practical applications.

Beam steering plays a vital role in light detection and ranging (LiDAR). However, the scanning speed is below megahertz (MHz) in current LiDAR systems. Frequency‐gradient metasurface has been reported to achieve gigahertz (GHz) beam steering with 25° scanning angle in two dimensions. However, the fabrication of millions of form‐varying nanoscale meta‐atoms poses challenges in operation efficiency, massive production, and overall cost‐effectiveness when the size goes beyond millimeter scale. Here, a dynamic beam steering comprising periodic grating nanostructures fabricated through nanoimprint lithography with an ultrafast scanning rate up to GHz (≈81 ps) and ultra‐wide three‐dimensional (3D) fields of view up to 53° × 23° is reported, which shows the powerful acquisition of 3D space data. The detection distance of object reaches to ≈230 m. This work demonstrates a wide‐angle and high‐speed beam steering, and paves a robust way to realize cost‐efficient, high‐volume manufacturing, and powerful 3D all‐solid‐state LiDAR for next‐generation intelligent vision and self‐driving technologies. [ABSTRACT FROM AUTHOR]

Geometric metasurfaces primarily follow the physical mechanism of Pancharatnam–Berry (PB) phases, empowering wavefront control of cross‐polarized reflective/transmissive light components. However, inherently accompanying the cross‐polarized components, the copolarized output components have not been attempted in parallel in existing works. Here, a general method is proposed to construct phase‐modulated metasurfaces for implementing functionalities separately in co‐ and cross‐polarized output fields under circularly polarized (CP) incidence, which is impossible to achieve with solely a geometric phase. By introducing a propagation phase as an additional degree of freedom, the electromagnetic (EM) energy carried by co‐ and cross‐polarized transmitted fields can be fully phase‐modulated with independent wavefronts. Under one CP incidence, a metasurface for separate functionalities with controllable energy repartition is verified by simulations and proof‐of‐principle microwave experiments. A variety of applications can be readily expected in spin‐selective optics, spin‐Hall metasurfaces, and multitasked metasurfaces operating in both reflective and transmissive modes. [ABSTRACT FROM AUTHOR]

Structured interfaces with subwavelength features enable modulations of phase, amplitude, and polarization on demand, leading to a plethora of flat-profile devices and metasurfaces. Plasmonic and dielectric metasurfaces have been intensively explored, building up the frameworks of flat optics for ultrathin and integrated nanophotonics. The in situ controllability and tunability of aforementioned family of metasurfaces, however, has been a grand challenge, due to the intrinsic limitations of the materials. Semiconductors with diversified catalogs of material candidates thus demonstrate promising potentials, owing to the mature and versatile technologies developed nowadays. The fuse of semiconductors and nanostructured metasurfaces has been witnessed more recently, paving a distinct avenue toward active, tunable, reconfigurable light manipulation for next-generation optical nanodevices. Judicious selection of the active materials for metasurfaces empowers the active functionality of the designer applications. This paper presents a review of this merging semiconductor paradigm for active metasurfaces across a wide range of spectrum and shows unprecedented potentials in the future interface-based optoelectronics, quantum optics, nano-optics, and surface engineering with full compatibility of semiconductor foundry. [ABSTRACT FROM AUTHOR]

Abstract: The emerging invisibility schemes mainly adopt transformation optics, scattering cancellation, light diffusion, and metasurface‐based phase restoration techniques. However, those aforementioned invisibility achievements natively depend on the predefined curvature, polarization, frequency, and/or angle of the incident wave. Here, an invisibility strategy of using an ultrathin parabolic‐phase metasurface and its applications to achieve diffusive invisibility for dual‐polarization channels and multifrequency channels is reported. Such strategy can intrinsically split the wavenumber of the scattering wave and therein is termed as wavenumber‐splitting metasurface. For verification, two proof‐of‐concept examples are experimentally characterized. The first prototype manifests dual‐polarized near‐isotropic diffusive scattering immune from wide‐angle incidences. The second demonstration exhibits bifunctionality of combined diffusive invisibility and vortex scattering in dual‐polarization channels. In both cases, theoretical, numerical and experimental results agree well, illustrating a well‐separated triple‐band versatile scattering behavior. This approach addresses the fundamental issue of real invisibility under bistatic detection without complex optimization, thanks to the physical essence of numerous splitting wave vectors. Such strategy opens an upstream way to realize invisibility as well as holding the potentials for downstream applications such as stealth and camouflaging devices. [ABSTRACT FROM AUTHOR]

Metasurfaces with interfacial phase discontinuities provide a unique platform for manipulating light propagation both in free space and along a surface. Three‐dimensional focusing of visible light is experimentally exhibited as a bi‐functional phenomenon by controlling the radial orientation of identical plasmonic dipoles, generating a desired phase profile along the interface. With this technique, the in‐plane and out‐of‐plane refractions are manipulated by an ultrathin flat lens such that a beam can be focused into a 3D spot either in a real or virtual focal plane, which can be reversed via manipulation of the circularly polarized status of the incident light. Both the inverted real image and the upright virtual image of an arbitrary object are experimentally demonstrated using the same flat lens in the visible range, which paves the way towards robust application of phase discontinuity devices. [ABSTRACT FROM AUTHOR]

The spectral resolution and range of conventional spectrometers are typically limited by optical aberrations of their focusing elements, mainly due to chromatically induced astigmatism and an intrinsically curved focal plane. Traditional approaches to overcome this challenge require additional optical components which introduce significant bulk and design complexity to the system and prevent easy integration with portable devices. Here a single planar off‐axis focusing metalens consisting of subwavelength TiO2 nanofins whose focal spots lie along a plane and undergo minimal focal spot broadening for almost 200 nm across the visible spectrum is demonstrated. This allows us to achieve a miniature aberration‐corrected spectrometer with nanometer spectral resolution, while having a beam propagation distance of only 4 cm to the camera plane. This is achieved by dispersion engineering: tailoring the phase, group delay (GD) and GD dispersion of the metalens. This approach is general and can also be used to introduce customized functionalities to the metalens such as a linear dispersion in the frequency domain with minimal additional overhead. [ABSTRACT FROM AUTHOR]

A single metasurface is demonstrated with powerful manipulating capability to arbitrarily and independently tune the phase profiles of orthogonal polarization states using subwavelength tunable cross‐shaped element (TCE) arrays. Here, the arbitrary control of the phase profiles must be distinguished from the semiarbitrary phase control using multiple predesigned metasurfaces with one‐to‐one functionality. The decoupled tuning of phase profiles for orthogonal polarization states bestows the metasurface with additional degree of freedom to manipulate the electromagnetic wave for multifunctionality. As a proof of concept, a tunable polarization beam splitter, a dynamic beam steering component, and a quarter waveplate are demonstrated using one single metasurface. This metasurface has promising applications on versatile and function‐switchable photonic components, such as polarization optical devices, compact functional devices, communication systems, and wave controlling devices, to name just a few. Arbitrary and independent polarization control is realized using a single metasurface constituted by tunable cross‐shaped element (TCE), of which the orthogonal resonance modes can be independently tuned. Here, tunable polarization beam splitter, dynamic beam steerer, and quarter waveplate are demonstrated using this single metasurface. [ABSTRACT FROM AUTHOR]