We design and numerically analyze a high-quality (Q)-factor, high modulation depth, multiple Fano resonance device based on periodical asymmetric paired bars in the near-infrared regime. There are four sharp Fano peaks arising from the interference between subradiant modes and the magnetic dipole resonance mode that can be easily tailored by adjusting different geometric parameters. The maximal Q-factor can exceed 10 in magnitude, and the modulation depths ΔT can reach nearly 100%. Combining the narrow resonance line-widths with strong near-field confinement, we demonstrate an optical refractive index sensor with a sensitivity of 370 nm/RIU and a figure of merit of 2846. This study may provide a further step in sensing, lasing, and nonlinear optics.

elements. Thus, realizing phase manipulation of EM waves at the nanoscale has become a key pursuit for the development of modern optics and nanophotonics.Metamaterials are 3D artificial nanostructures composed of periodic subwavelength unit cells that resonantly couple to the incident EM waves, exhibiting effective electric and magnetic responses not found in nature. [1][2][3] However, these promising potential applications are hindered in their applications due to the challenges of fabricating the required complex 3D nanostructures and the inherent metallic losses and strong dispersion of plasmonic elements at optical frequencies. Planar metamaterials, or so-called metasurfaces, can be fabricated using existing technologies, such as the lithography method and have attracted increasing attention due to their feasibility, low loss, and ease of fabrication. [4,5] The most prominent advantage of metasurfaces is that they can generate spatial phase discontinuities over the full 2π range with an optically thin interface; moreover, the resolution is less than one wavelength. Thus, wavefronts can be shaped with a distance of much less than the wavelength. With the increasing development of metasurfaces, the aforementioned limitations can be solved using various ultrathin optical devices, which have properties superior to their conventional counterparts. [6][7][8][9][10][11][12] Here, we concentrate on the new capabilities of metasurfaces in recent years in manipulating the phase and propagation behaviors of EM waves. In Section 2, we briefly introduce the underlying mechanisms of three types of phase discontinuities. In Section 3, we review the basic applications of phase modulation using metasurfaces. In Section 4, we review more complex and advanced information photonics that have emerged from metasurfaces. In the last section, we provide concluding remarks and an outlook on future development directions. Three Basic Types of Phase Discontinuities Generated by Metasurfaces Resonance PhaseThe pioneering approach to achieve phase discontinuities was to use the dispersion of various metallic nanoantennas, as shown in the left panel of Figure 1a. The optical energy is coupled to surface EM waves propagating back and forth along the antenna surface. Due to the localized surface plasmon resonance, these waves are accompanied by oscillating free electrons Relative to conventional phase-modulation optical elements, metasurfaces (i.e., 2D versions of metamaterials) have shown novel optical phenomena and promising functionalities with more compact platforms and more straightforward fabrication processes. With the ability to generate a spatial phase variation, optical wavefronts can be manipulated into arbitrary shapes at will, enabling new phenomena and integrated ultrathin optical devices to be explored. This review is focused on recent developments regarding phase manipulation of electromagnetic waves with metasurfaces. Starting from their underlying physics for realizing full 2π phase manipulation, an overview of the applica...

Artificial microstructures, which allow us to control and change the properties of wave fields through changing the geometrical parameters and the arrangements of microstructures, have attracted plenty of attentions in the past few decades. Some artificial microstructure based research areas, such as metamaterials, metasurfaces and phononic topological insulators, have seen numerous novel applications and phenomena. The manipulation of different dimensions (phase, amplitude, frequency or polarization) of wave fields, particularly, can be easily achieved at subwavelength scales by metasurfaces. In this review, we focus on the recent developments of wave field manipulations based on artificial microstructures and classify some important applications from the viewpoint of different dimensional manipulations of wave fields. The development tendency of wave field manipulation from single-dimension to multidimensions provides a useful guide for researchers to realize miniaturized and integrated optical and acoustic devices.

A novel approach for the design of a spin‐selective directional coupler and spin‐selective wavelength demultiplexing device by integrating an all‐dielectric metasurface into a silicon nitride waveguide is proposed and demonstrated. Taking advantage of the intrinsic chirality of the electromagnetic field inside the waveguide, the propagation direction of the excited waveguide mode can be easily controlled by the spin and wavelength of the incident light. The coupling efficiency of the spin‐selective directional coupler toward one side can reach ≈51.6% at 1836 nm. In addition, the working wavelengths, coupling efficiency, peak width, and mode type can be easily tailored by adjusting the geometric parameters of the waveguide and the antenna arrays. This study may provide a further step in the development of photonic integrated circuits, integrated quantum optics, chiral optics, and high‐bit‐rate telecommunication applications.

applications have been realized based on the metamaterials, such as negative index materials, [3] invisibility cloaks, [4] and zeroindex materials. [5] Nevertheless, metamaterials are usually bulky, difficult to be fabricated, and suffer from high energy losses, which hinder their practical applications in modern photonic systems. In recent years, planar metasurfaces, the 2D equivalents of metamaterials, have attracted plenty of attentions due to their extraordinary abilities in controlling the polarization, amplitude, phase, and dispersion of electromagnetic waves. [6][7][8] Compared with bulk metamaterials, the metasurfaces have many advantages, such as ultrathin thicknesses, low losses, ease of fabrication and integration. Over the past years, single-layer metasurfaces have been widely applied in realizing polarization conversion, [9] beam deflectors, [10,11] metalenses, [12,13] holograms, [14] coding, [15] structural colors, [16,17] nonlinear metasurfaces, [18] and some other applications. Nevertheless, the interactions between lights and ultrathin single-layer metasurfaces are usually limited, resulting in low efficiency and limited controllability in some applications. [19] Moreover, the degrees of freedom for light manipulation provided by a single-layer metasurface are usually not enough in realizing multifunctional devices and some other sophisticated photonic systems.Few-layer metasurface that contains more than one functional layer provides an effective method to overcome the drawbacks of both bulk metamaterials and single-layer metasurfaces. Cheng et al. employed the concept of few-layer metasurfaces to discuss the advantages and emergent functionalities of them in ref. [20]. Few-layer metasurfaces retain the advantages of single-layer metasurfaces and can provide more degrees of freedom to manipulate electromagnetic waves. More importantly, the abundant layer effects, such as the multiple wave interference between layers and the near-field coupling effects, can enhance the interactions between lights and structures and improve the efficiency of few-layer systems. In addition, the combination of different functional layers can produce novel functions that single-layer metasurfaces can hardly realize. For example, by breaking the mirror symmetry along the propagation direction, few-layer metasurfaces can realize asymmetric transmission of linearly polarized lights. [21] By vertically integrating different metasurfaces on one substrate, optical systems with different functions can be miniaturization and integration. Recently, the few-layer metasurfaces have also been extended in the acoustic fields to realize some novel applications, such Metamaterials are 3D artificial structures proposed to surpass conventional natural materials and realize novel functions beyond traditional optical elements. Nevertheless, they are usually bulky and difficult to be fabricated. As 2D equivalents of metamaterials, metasurfaces have been proposed to overcome the drawbacks of metamaterials and fully control the polarizati...

Optical vortices (OVs) carrying orbital angular momentum (OAM) have attracted considerable interest in the field of optics and photonics owing to their peculiar optical features and extra degree of freedom for carrying information. Although there have been significant efforts to realize OVs using conventional optics, it is limited by large volume, high cost, and lack of design flexibility. Optical metasurfaces have recently attracted tremendous interest due to their unprecedented capability in the manipulation of the amplitude, phase, polarization, and frequency of light at a subwavelength scale. Optical metasurfaces have revolutionized design concepts in photonics, providing a new platform to develop ultrathin optical devices for the realization of OVs at subwavelength resolution. In this article, we will review the recent progress in optical metasurface-based OVs. We provide a comprehensive discussion on the optical manipulation of OVs, including OAM superposition, OAM sorting, OAM multiplexing, OAM holography, and nonlinear metasurfaces for OAM generation and manipulation. The rapid development of metasurface for OVs generation and manipulation will play an important role in many relevant research fields. We expect that metasurface will fuel the continuous progress of wearable and portable consumer electronics and optics where low-cost and miniaturized OAM related systems are in high demand.

We report the design, characterization, and experimental demonstration of a broadband nearperfect absorber in the visible regime based on strong interference in ultrathin molybdenum disulfide films obtained by the spin-coating method. The absorber is polarization-insensitive, and the absorption peak maintains a high value for large angles of incidence, which provides more efficient absorption for nonpolarized or oblique incident beams. The experimental results show that the absorption can reach more than 87% between 400 nm and 640 nm, which is in reasonable agreement with the simulated results. This work may offer a further step in the development of solar absorption-based nano-optoelectronic devices. Published by AIP Publishing.

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