Optical metasurfaces, composed of planar arrays of sub-wavelength dielectric or metallic structures that collectively mimic the operation of conventional bulk optical elements, have revolutionized the field of optics by their potential in constructing high-efficiency and multi-functional optoelectronic systems with a compact form factor. By engineering the geometry, placement, and alignment of its constituent elements, an optical metasurface arbitrarily controls the magnitude, polarization, phase, angular momentum, or dispersion of incident light. The study of metasurface now spans various multidisciplinary fields in both fundamental research on light-matter interaction [1–3], and emerging applications from solid-state LiDAR [4,5] to compact imaging, spectroscopy, and quantum optical devices [6–11]. High-performance metasurface devices have been experimentally demonstrated over the entire optical spectrum from the deep ultraviolet to the terahertz (THz) [12–16], and have been employed to manipulate optical waves in both spatial and temporal domains [17–21].This special issue on “Recent Advances in Optical Metasurfaces” includes five review articles and five research articles, covering various topics ranging from metasurface design to practical applications. Qiu et al. [22] provide a comprehensive review of the fundamentals and applications of spin-decoupled Pancharatnam – Berry (PB) metasurfaces. Different from traditional PB-phase-based metasurfaces which impinge phase modulations with opposite signs onto the left-handed and right-handed circularly polarized light, the spin-decoupled PB metasurfaces release the above spin-locked limitation and allow independent and arbitrary control over orthogonal circular polarizations. The recent development of bianisotropic metasurfaces has allowed versatile control over the state of polarization and propagation direction of light. Xiong et al. [23] discuss the electromagnetic properties of photonic bianisotropic structures using the finite element method. The authors show that the vector wave equation with the presence of bianisotropy is self-adjoint under the scalar inner product and propose a balanced formulation of weak form in the practical implementation that outperforms the standard formulation in finite element modeling. Realizing active devices with adjustable functionalities is of great interest to the metasurface research community. Bi et al. [24] review the physical mechanisms and device applications of magnetically controllable metasurfaces. Magnetic field manipulation has advantages of ultra-fast response, non-contact and continuous adjustment, thus paving the way toward realizing multi-functional and dynamic metasurface-based devices and systems.Several typical as well as emerging applications of the metasurface technology, are covered by this special issue. Fu et al. [25] give a comprehensive review of metalenses, tiny planar imaging devices enabled by metasurface technology. The article covers the basic phase modulation techniques, design principles, characterization methods, and functional applications of metalenses. Although a metalens might not fully compete with a conventional lens in terms of imaging quality at the current stage, it possesses unique advantages in terms of multi-dimensional and multi-degree-of-freedom control over an incident light, thus facilitating novel functionalities that are extremely difficult or even impossible to implement using conventional technology. The electromagnetic absorber is another typical application of metasurface technology. Gandhi et al. [26] propose a polarization-insensitive metasurface absorber operating in the THz regime. The device consists of metal-dielectric-metal resonators and exhibits absorption greater than 90% over the 2.54 to 5.54 THz range. In recent years, edge detection using metasurfaces has raised a significant interest and could find promising applications in all-optical computing and artificial intelligence. Wan et al. [27] review the development of dielectric metasurfaces for spatial differentiation and edge detection. The article focuses on the underlying principles of dielectric metasurfaces as first- or second-order spatial differentiators and their applications in biological imaging and machine vision.Non-diffractive beams are highly desired for a number of applications, including biomedical imaging, particle manipulation, and material processing. Liu et al. [28] investigate dual non-diffractive THz beam generation using dielectric metasurfaces. The authors design and experimentally implement Bessel beams and abruptly autofocusing beams, two representative kinds of non-diffractive beams with dramatically opposite focusing properties. With its compact footprint and multiple functionalities, metasurface offers new possibilities in constructing high-performance optical sensors. Ye et al. [29] introduce an ultra-sensitive optical sensing platform based on the parity-time-reciprocal scaling (PTX)-symmetric non-Hermitian metasurfaces. Such devices leverage exotic singularities, such as the exceptional point and the coherent perfect absorber-laser point, to significantly enhance the sensitivity and detectability of photonic sensors. Ren et al. [30] propose a U-shaped THz metamaterial with polarization-sensitive and actively-controllable electromagnetically induced transparency, which could find useful applications in tunable integrated devices such as biosensors, filters, and THz modulators. Realizing large-scale and low-cost fabrication of metasurface could greatly facilitate the technology’s practical applications. Oh et al. [31] review the development of nanoimprint lithography for high-throughput fabrication of optical metasurfaces. The authors elaborate various imprint methods for scalable fabrication of metasurfaces and share their perspectives on the technology’s future development.We hope that this special issue on “Recent Advances in Optical Metasurfaces” could provide useful information for metasurface researchers and inspire new ideas for their future exploration. We thank all authors for their contribution to this special issue, and reviewers for their valuable comments. In the end, we would like to express sincere gratitude to the editors of Frontiers of Optoelectronics for providing us such an excellent opportunity to put together this special issue and their invaluable assistance along the way.

Given a constitutive relation of the bianisotropic medium, it is not trivial to study how light interacts with the photonic bianisotropic structure due to the limited available means of studying electromagnetic properties in bianisotropic media. In this paper, we study the electromagnetic properties of photonic bianisotropic structures using the finite element method. We prove that the vector wave equation with the presence of bianisotropic is selfadjoint under scalar inner product. we propose a balanced formulation of weak form in the practical implementation, which outperforms the standard formulation in finite element modeling. Furthermore, we benchmark our numerical results obtained from finite element simulation in three different scenarios. These are bianisotropydependent reflection and transmission of plane waves incident onto a bianisotropic slab, band structure of bianisotropic photonic crystals with valley-dependent phenomena, and the modal properties of bianisotropic ring resonators. The first two simulated results obtained from our modified weak form yield excellent agreements either with theoretical predictions or available data from the literature, and the modal properties in the last example, i.e., bianisotropic ring resonators as a polarizationdependent optical insulator, are also consistent with the theoretical analyses.

The applications of terahertz (THz) technology can be greatly extended using non-diffractive beams with unique field distributions and non-diffractive transmission characteristics. Here, we design and experimentally demonstrate a set of dual non-diffractive THz beam generators based on an all-dielectric metasurface. Two kinds of non-diffractive beams with dramatically opposite focusing properties, Bessel beam and abruptly autofocusing (AAF) beam, are considered. A Bessel beam with longdistance non-diffractive characteristics and an AAF beam with low energy during transmission and abruptly increased energy near the focus are generated for x- and y-polarized incident waves, respectively. These two kinds of beams are characterized and the results agree well with simulations. In addition, we show numerically that these two kinds of beams can also carry orbital angular momentum by further imposing proper angular phases in the design. We believe that these metasurface-based beam generators have great potential use in THz imaging, communications, non-destructive evaluation, and many other fields.

In this paper, we introduce an ultra-sensitive optical sensing platform based on the parity-time-reciprocal scaling (PTX)-symmetric non-Hermitian metasurfaces, which leverage exotic singularities, such as the exceptional point (EP) and the coherent perfect absorber-laser (CPAL) point, to significantly enhance the sensitivity and detectability of photonic sensors. We theoretically studied scattering properties and physical limitations of the PTXsymmetric metasurface sensing systems with an asymmetric, unbalanced gain-loss profile. The PTX-symmetric metasurfaces can exhibit similar scattering properties as their PT-symmetric counterparts at singular points, while achieving a higher sensitivity and a larger modulation depth, possible with the reciprocal-scaling factor (i.e., X transformation). Specifically, with the optimal reciprocalscaling factor or near-zero phase offset, the proposed PTXsymmetric metasurface sensors operating around the EP or CPAL point may achieve an over 100 dB modulation depth, thus paving a promising route toward the detection of small-scale perturbations caused by, for example, molecular, gaseous, and biochemical surface adsorbates.

Electromagnetically induced transparency (EIT) phenomenon is observed in simple metamaterial which consists of concentric double U-shaped resonators (USRs). The numerical and theoretical analysis reveals that EIT arises from the bright-bright mode coupling. The transmission spectra at different polarization angle of incident light shows that EIT transparency window is polarization sensitive. More interestingly, Fano resonance appears in the transmission spectrum at certain polarization angles. The sharp and asymmetric Fano lineshape is high valuable for sensing. The performance of sensor is investigated and the sensitivity is high up to 327 GHz/RIU. Furthermore, active control of EIT window is realized by incorporating photosensitive silicon. The proposed USR structure is simple and compact, which may find significant applications in tunable integrated devices such as biosensor, filters, and THz modulators.

Halide perovskites have attracted tremendous attention as semiconducting materials for various optoelectronic applications. The functional metal-halide octahedral units and their spatial arrangements play a key role in the optoelectronic properties of these materials. At present, most of the efforts for material exploration focus on substituting the constituent elements of functional octahedral units, whereas designing the spatial arrangement of the functional units has received relatively little consideration. In this work, via a global structure search based on density functional theory (DFT), we discovered a metastable threedimensional honeycomb-like perovskite structure with the functional octahedral units arranged through mixed edgeand corner-sharing. We experimentally confirmed that the honeycomb-like perovskite structure can be stabilized by divalent molecular cations with suitable size and shape, such as 2,2′-bisimidazole (BIM). DFT calculations and experimental characterizations revealed that the honeycomb- like perovskite with the formula of BIMPb2I6, synthesized through a solution process, exhibits high electronic dimensionality, a direct allowed bandgap of 2.1 eV, small effective masses for both electrons and holes, and high optical absorption coefficients, which indicates a significant potential for optoelectronic applications. The employed combination of DFT and experimental study provides an exemplary approach to explore prospective optoelectronic semiconductors via spatially arranging functional units.

Manipulating circularly polarized (CP) electromagnetic (EM) waves at will is significantly important for a wide range of applications ranging from chiralmolecule manipulations to optical communication. However, conventional EM devices based on natural materials suffer from limited functionalities, bulky configurations, and low efficiencies. Recently, Pancharatnam–Berry (PB) phase metasurfaces have shown excellent capabilities in controlling CP waves in different frequency domains, thereby allowing for multi-functional PB meta-devices that integrate distinct functionalities into single and flat devices. Nevertheless, the PB phase has intrinsically opposite signs for two spins, resulting in locked and mirrored functionalities for right CP and left CP beams. Here we review the fundamentals and applications of spindecoupled metasurfaces that release the spin-locked limitation of PB metasurfaces by combining the orientation- dependent PB phase and the dimension-dependent propagation phase. This provides a general and practical guideline toward realizing spin-decoupled functionalities with a single metasurface for orthogonal circular polarizations. Finally, we conclude this review with a short conclusion and personal outlook on the future directions of this rapidly growing research area, hoping to stimulate new research outputs that can be useful in future applications.

The dynamic control of the metasurface opens up a vital technological approach for the development of multifunctional integrated optical devices. The magnetic field manipulation has the advantages of sub-nanosecond ultra-fast response, non-contact, and continuous adjustment. Thus, the magnetically controllable metasurface has attracted significant attention in recent years. This study introduces the basic principles of the Faraday and Kerr effect of magneto-optical (MO) materials. It classifies the typical MO materials according to their properties. It also summarizes the physical mechanism of different MO metasurfaces that combine the MO effect with plasmonic or dielectric resonance. Besides, their applications in the nonreciprocal device and MO sensing are demonstrated. The future perspectives and challenges of the research on MO metasurfaces are discussed.

Lens is a basic optical element that is widely used in daily life, such as in cameras, glasses, and microscopes. Conventional lenses are designed based on the classical refractive optics, which results in inevitable imaging aberrations, such as chromatic aberration, spherical aberration and coma. To solve these problems, conventional imaging systems impose multiple curved lenses with different thicknesses and materials to eliminate these aberrations. As a unique photonic technology, metasurfaces can accurately manipulate the wavefront of light to produce fascinating and peculiar optical phenomena, which has stimulated researchers’ extensive interests in the field of planar optics. Starting from the introduction of phase modulation methods, this review summarizes the design principles and characteristics of metalenses. Although the imaging quality of existing metalenses is not necessarily better than that of conventional lenses, the multi-dimensional and multi-degree-of-freedom control of metasurfaces provides metalenses with novel functions that are extremely challenging or impossible to achieve with conventional lenses.

Dielectric metasurfaces-based planar optical spatial differentiator and edge detection have recently been proposed to play an important role in the parallel and fast image processing technology. With the development of dielectric metasurfaces of different geometries and resonance mechanisms, diverse on-chip spatial differentiators have been proposed by tailoring the dispersion characteristics of subwavelength structures. This review focuses on the basic principles and characteristic parameters of dielectric metasurfaces as first- and second-order spatial differentiators realized via the Green’s function approach. The spatial bandwidth and polarization dependence are emphasized as key properties by comparing the optical transfer functions of metasurfaces for different incident wavevectors and polarizations. To present the operational capabilities of a two-dimensional spatial differentiator in image information acquisition, edge detection is described to illustrate the practicability of the device. As an application example, experimental demonstrations of edge detection for different biological cells and a flower mold are discussed, in which a spatial differentiator and objective lens or camera are integrated in three optical pathway configurations. The realization of spatial differentiators and edge detection with dielectric metasurfaces provides new opportunities for ultrafast information identification in biological imaging and machine vision.

Metasurfaces are composed of periodic subwavelength nanostructures and exhibit optical properties that are not found in nature. They have been widely investigated for optical applications such as holograms, wavefront shaping, and structural color printing, however, electron-beam lithography is not suitable to produce largearea metasurfaces because of the high fabrication cost and low productivity. Although alternative optical technologies, such as holographic lithography and plasmonic lithography, can overcome these drawbacks, such methods are still constrained by the optical diffraction limit. To break through this fundamental problem, mechanical nanopatterning processes have been actively studied in many fields, with nanoimprint lithography (NIL) coming to the forefront. Since NIL replicates the nanopattern of the mold regardless of the diffraction limit, NIL can achieve sufficiently high productivity and patterning resolution, giving rise to an explosive development in the fabrication of metasurfaces. In this review, we focus on various NIL technologies for the manufacturing of metasurfaces. First, we briefly describe conventional NIL and then present various NIL methods for the scalable fabrication of metasurfaces. We also discuss recent applications of NIL in the realization of metasurfaces. Finally, we conclude with an outlook on each method and suggest perspectives for future research on the high-throughput fabrication of active metasurfaces.