Plasmonic and Dielectric Metasurfaces: Design, Fabrication and Applications

Abstract

Two-dimensional metasurfaces are widely focused on for their ability for flexible light manipulation (phase, amplitude, polarization) over sub-wavelength propagation distances. Most of the metasurfaces can be divided into two categories by the material type of unit structure, i.e., plasmonic metasurfaces and dielectric metasurfaces. For plasmonic metasurfaces, they are made on the basis of metallic meta-atoms whose optical responses are driven by the plasmon resonances supported by metallic particles. For dielectric metasurfaces, the unit structure is constructed with high refractive index dielectric resonators, such as silicon, germanium or tellurium, which can support electric and magnetic dipole responses based on Mie resonances. The responses of plasmonic and dielectric metasurfaces are all relevant to the characteristics of unit structure, such as dimensions and materials. One can manipulate the electromagnetic field of light wave scattered by the metasurfaces through designing the dimension parameters of each unit structure in the metasurfaces. In this review article, we give a brief overview of our recent progress in plasmonic and dielectric metasurface-assisted nanophotonic devices and their design, fabrication and applications, including the metasurface-based broadband and the selective generation of orbital angular momentum (OAM) carrying vector beams, N-fold OAM multicasting using a V-shaped antenna array, a metasurface on conventional optical fiber facet for linearly-polarized mode (LP₁₁) generation, graphene split-ring metasurface-assisted terahertz coherent perfect absorption, OAM beam generation using a nanophotonic dielectric metasurface array, as well as Bessel beam generation and OAM multicasting using a dielectric metasurface array. It is believed that metasurface-based nanophotonic devices are one of the devices with the most potential applied in various fields, such as beam steering, spatial light modulator, nanoscale-resolution imaging, sensing, quantum optics devices and even optical communication networks.

1. Introduction

A “metamaterial” is a man-made material whose properties can be attained by designing the unit structure, having been widely focused on for a dozen years since it was firstly reported by Smith et al. in 2000 [1]. The unit structure in metamaterial, called meta-atoms or meta-molecules, must be considerably smaller than the operating wavelength, and the distance between neighboring meta-atoms also has a subwavelength scale [2]. This subwavelength scale inhomogeneity makes the whole metamaterial uniform on the macro of performance, and this phenomenon makes this artificial structure essentially a “material” rather than a device. Therefore, one can arbitrarily arrange the so-called meta-atoms into periodic arrays to build one-dimensional (1D) materials (chains), two-dimensional (2D) materials (metasurfaces) and three-dimensional (3D) materials (metamaterials), as shown in Figure 1 [3]. For the metamaterial, one can even manipulate its macroscopical properties, such as permittivity ε and permeability μ. The research of metamaterials is essentially related to the exploitation of the electromagnetic parameter space for better control of electromagnetic waves, as depicted in Figure 1. Utilizing metamaterials, one can enter regions of the electromagnetic parameter space that are not observed in conventional material, but that are not forbidden by Maxwell’s equations, such as the third quadrant of the parameter space (negative-index materials), zero-index materials and the space far away from the non-magnetic line [4,5,6,7]. Taking advantage of these attractive features, as shown in Figure 1, metamaterials have been widely applied in optical negative index materials [8,9,10,11,12], nonlinear optics [13,14,15,16], optical magnetism [17,18,19], super resolution [20,21,22,23,24,25], giant artificial chirality [26,27], electromagnetic cloaks [28,29,30,31,32], etc.

Besides 3D materials, 2D metasurfaces are also widely focused on for their ability for flexible light manipulation (phase, amplitude, polarization) over subwavelength propagation distances [33,34,35,36,37]. Most of the metasurfaces can be divided into two categories by the material type of unit structure: plasmonic and dielectric, as displayed in Figure 2. For plasmonic metasurfaces, they are made on the basis of metallic meta-atoms whose optical responses are driven by the plasmon resonances supported by metallic particles. When a metallic particle is placed in an electric field, the conduction electrons escape from their equilibrium positions with respect to the core ions, resulting in a polarization of the particle and generating a depolarizing field. In a time-varying external field, this collective motion in the metallic particle can be seen as a Lorentzian oscillator whose characteristic peak in the displacement amplitude is around the resonance frequency and accompanied by a phase shift of π over the spectral width of the resonance [38,39,40,41,42,43,44,45,46]. For dielectric metasurfaces, the unit structure is constructed with high refractive index dielectric resonators, such as silicon, germanium or tellurium, which can support electric and magnetic dipole responses based on Mie resonances [47]. When a dielectric particle is illuminated by a light wave whose frequency is below or near the bandgap frequency of the material of particle, both the magnetic dipole (first Mie resonance) and electric dipole resonances (second Mie resonance) are excited. The magnetic and electric Mie resonance can then enhance the magnetic and electric field at the particle’s center at optical frequencies, respectively, and this enhancement is related to the intrinsic properties of dielectric particles [48,49,50,51,52,53,54,55,56,57,58]. Therefore, the responses of plasmonic and dielectric metasurfaces are all relevant to the characteristics of the unit structure, such as dimensions and materials. One can manipulate the electromagnetic field of the light wave scattered by the metasurfaces through designing the dimension parameters of each unit structure in the metasurfaces [59].

To produce the designed metasurfaces, many attractive previous works have reported the fabrication techniques, such as electron-beam lithography [60,61,62,63,64,65], focused-ion beam [66,67], interference lithography [68,69,70,71,72] and nanoimprint lithography [73,74,75,76,77]. Electron-beam lithography is a standard method that can fabricate the metasurface with high nanoscale resolution. Focused-ion beam is a more appropriate method for rapid prototyping. Interference lithography can provide a solution for realizing large-scaled metasurface fabrication. Nanoimprint lithography is a more promising technique for combining many advantages of the former, such as high-resolution, large-scaled production and low processing cost [73]. The unique properties and the development of fabrication techniques have allowed metasurfaces to be applied in various fields, such as flat optics [33,34,35,78,79,80,81,82,83,84,85,86,87,88,89,90,91,92,93,94,95,96,97,98,99,100], nonlinear effects [101,102,103,104,105,106,107], photonic Hall effects [108,109,110], electromagnetically-induced transparency [111], cloaking [112,113,114,115,116,117], etc., as shown in Figure 2. For instance, the applications of metasurfaces in the field of flat optics mainly include anomalous reflection/refraction [34,81,82], wave plates [79,80], flats lens/axicons [83,84,85], mirrors [88], hologram [90,91,92,93,94,95], filters [96], optical vortex generation [34,85,98,99], polarization beam splitter [100], etc. The applications of metasurfaces in the field of nonlinear effects mainly include second/third harmonic generation [102,103,104,105] and enhanced absorbing [106,107].

In this review article, we go over our recent progress in plasmonic and dielectric metasurface-assisted nanophotonic devices and their design, fabrication and applications, including the metasurface-based broadband and selective generation of orbital angular momentum (OAM) carrying vector beams [118,119], N-fold OAM multicasting using a V-shaped antenna array [120], metasurfaces on a conventional optical fiber facet for linearly-polarized mode (LP₁₁) generation [121], graphene split-ring metasurface-assisted terahertz coherent perfect absorption [122], OAM beam generation using a nanophotonic dielectric metasurface array [123] and Bessel beam generation and OAM multicasting using dielectric metasurface array [124].

2. Plasmonic Metasurfaces

Plasmonic metasurfaces are composed of metal nanoparticles based on the plasmon resonances. Several typical types of plasmonic metasurfaces and previous works are displayed in

Table 1. Several typical types of plasmonic metasurfaces.

	Metal V-shaped nano-antennas to generate optical beams carrying orbital angular momentum (OAM). Reproduced with permission from [34], Copyright The American Association for the Advancement of Science, 2011.
	Au patch antennas separated from a metal back plane by a MgF2 spacer to realize anomalous reflections. Reproduced with permission from [81], Copyright American Chemical Society, 2012.
	H-shaped microwave nano-antenna arrays to realize high efficiency conversion from propagating waves to surface waves. Reproduced with permission from [82], Copyright Nature Publishing Group, 2012.
	Metal nanorods to generate a hologram. Reproduced with permission from [95], Copyright Nature Publishing Group, 2013.
	Freestanding nanofabricated fishnet metasurface to function as a broadband band pass filter. Reproduced with permission from [96], Copyright Nature Publishing Group, 2013.
	Rectangular apertures arranged in an array with rotational symmetry in metal film to generate OAM beams. Reproduced with permission from [98], Copyright The Optical Society, 2012.
	Metal nanorods to generate broadband OAM beams. Reproduced with permission from [99], Copyright American Chemical Society, 2012.
	Split-ring resonators to generate the second harmonic. Reproduced with permission from [102], Copyright Nature Publishing Group, 2015.

: (1) metal V-shaped nano-antennas are designed and fabricated on silicon substrate using the electron-beam lithography method to manipulate the phase of scattering light and to generate OAM beams [34]; (2) Au patch antennas separated from a metal back plane by a MgF₂ spacer are designed and fabricated using the electron-beam evaporation electron-beam lithography technique to realize anomalous reflections [81]; (3) metal H-shaped nano-antenna arrays are designed and fabricated using printed circuit boards to realize high efficiency conversion from propagating waves to surface waves in the microwave regime [82]; (4) metal nanorods are deposited on a glass substrate to generate a hologram [95]; (5) freestanding nanofabricated fishnet metasurfaces are designed and fabricated using the electron-beam lithography method to realize a broadband band pass filter [96]; (6) metal rectangular apertures are arranged in an array with rotational symmetry in metal film to generate OAM beams [98]; (7) metal nanorods are designed and fabricated on glass substrate with a standard electron-beam lithography and lift-off process to generate broadband OAM beams [99]; (8) metal split-ring resonators are designed and fabricated on glass substrate with a standard electron-beam lithography to enhance second harmonic generation [102]. All of these works are based on manipulating the plasmon responses of nano-resonators by altering the dimensions, directions and even arrangement of unit structures. Recently, we have also made some progresses in plasmonic metasurfaces on the basis of previous works.

2.1. Metasurfaces-Based Broadband and Selective Generation of Orbital Angular Momentum Carrying Vector Beams [118,119]

Design: We propose and design compact metal-assisted metasurfaces to enable broadband generation of OAM-carrying vector beams. Figure 3a,b depicts the structure and geometric parameters of metal-assisted metasurfaces. We design two concentric rings in a gold film with a thickness of h = 200 nm. Each ring is composed of 42 rectangular apertures with gradually-varied orientation. The rectangular aperture array in the gold film can enhance the transmission of linearly-polarized light (perpendicular to the aperture direction), which might be explained as follows: (i) the localized waveguide resonance (each air aperture can be regarded as a truncated rectangular waveguide with four metal walls and two sides open to air); (ii) the property of the surface plasmon resonance due to the aperture array. Hence, each rectangular aperture can be regarded as a localized linear polarizer. By controlling the orientation angle of rectangular apertures, we can construct the desired spatially-variant polarizers to generate OAM-carrying vector beams with a right or left circularly-polarized input light beam. Figure 3c shows an example of the phase front and spatial polarization after passing though the metasurfaces, which indicates the generation of the OAM-carrying vector beam from the circularly-polarized beam.

Figure 4 depicts the spatial distributions of phase, power and polarization of output beams under the excitation of input left circularly-polarized light (E_in = [1 i] ^T). We set the wavelength at 1550 nm to characterize the properties of the generated OAM-carrying vector beams. The employed metasurfaces have an orientation angle of α ($\mathsf{\varphi }$) = l$\mathsf{\varphi }$ + α₀, where l varies from +3 to −3 and α₀ = 0. We use E₁ and E₂ to represent the electric field components along the directions of e₁ ($\mathsf{\varphi }$) and e₂ ($\mathsf{\varphi }$) in Figure 3b, respectively. The first row of Figure 4 shows the spatial phase distribution of E₁, indicating that output beams carry OAM with a charge number of l from +3 to −3. The second and third rows of Figure 4 show spatial power distributions (P₁ ∝ |E₁|², P₂ ∝ |E₂|²) along the directions of e₁ ($\mathsf{\varphi }$) and e₂ ($\mathsf{\varphi }$), respectively. It is found that the power component P₁ is much larger than P₂. The extinction ratio (ER), defined by 10 × log₁₀(P₁/P₂), exceeds 20 dB. Hence, the electric field component E₂ along the direction of e₂ ($\mathsf{\varphi }$) can be ignored. The fourth and fifth rows of Figure 4 show spatial power distributions (P_x, P_y) along x and y axes, respectively. The alternative bright and dark power distribution implies that the polarization state rotates with the azimuthal angle $\mathsf{\varphi }$. The sixth row of Figure 4 shows the calculated spatial polarization distribution, implying the generation of vector beams with a polarization order of l from +3 to −3. The obtained results shown in Figure 4 confirm the successful generation of OAM-carrying vector beams using metal-assisted metasurfaces.

We also study the operation bandwidth. Metasurfaces with an orientation angle of α ($\mathsf{\varphi }$) = l$\mathsf{\varphi }$ + α₀ (l = 1, 2, 3) are considered. Left circularly-polarized light is adopted as the input excitation source. We use ER and purity to characterize the quality of the generated OAM-carrying vector beams. Figure 5a plots ER as a function of wavelength. One can clearly see the high-quality broadband generation of OAM-carrying vector beams ranging from 1000–2500 nm, i.e., from near-infrared to mid-infrared. For l = 1 and 2, the ER is kept above 20 dB over a 1500-nm bandwidth (1000–2500 nm). For l = 3, ER > 16 dB over a bandwidth of 1500 nm (1000–2500 nm) and ER > 20 dB over a bandwidth of 800 nm (1000–1800 nm) are achieved. Shown in Figure 5b is the wavelength-dependent purity for the OAM-carrying vector beam (l = 3), which is larger than 0.85 over a bandwidth of 1500 nm (1000–2500 nm). The insets depict weight spectra as functions of OAM charge number and polarization order number at 1550 nm. High values of purity are achieved.

In Figure 3a, two concentric rings are used to excite a single OAM-carrying vector beam. Actually, one ring can also enable the generation of one OAM-carrying vector beam. Hence, it is possible to generate multiple OAM-carrying vector beams by forming multiple concentric rings in a gold film, as shown in Figure 6, where each ring contains a rectangular aperture array with each aperture acting as a localized polarizer. In particular, it is expected to selectively generate different orders of OAM-carrying vector beams by shining the circularly-polarized light source onto different concentric rings.

Figure 7a shows ER as a function of wavelength. One can see broadband operation from 1000–1550 nm with an extinction ratio above 20 dB, which indicates high-quality broadband (~500 nm) generation of OAM-carrying vector beams. We further study the fabrication tolerance by evaluating the performance dependence on the initial orientation angle α_i0 of the rectangular aperture. Figure 7b shows the extinction ratio as a function of the initial orientation angle α₂₀ of rectangular apertures in the second ring. The extinction ratio is kept above 20 dB when α₂₀ is varied from −1 to 7 degrees. The obtained range of initial orientation angle around eight degrees implies the good fabrication tolerance of the designed metasurfaces. We use E_1i and E_2i to denote the electric field components on the directions of e_1i ($\mathsf{\varphi }$) and e_2i ($\mathsf{\varphi }$) (similar to e₁ ($\mathsf{\varphi }$) and e₂ ($\mathsf{\varphi }$) in Figure 3b), respectively. We calculate the phase of E_1i and the power components on directions of e_1i ($\mathsf{\varphi }$), e_2i ($\mathsf{\varphi }$), x and y axes, which are represented by P₁, P₂, P_x and P_y, respectively. The spatially-variant polarization state of the OAM-carrying vector beams is also calculated. These results are shown in Figure 7c. Clearly, the power components on the direction of e_1i ($\mathsf{\varphi }$) are much larger than those on the direction of e_2i ($\mathsf{\varphi }$). The phase distributions in Figure 7c indicate that the electric field component E_1i carries OAM with a charge number of m = i (i = 1, 2, 3). The spatial distribution of P_x, P_y and the polarization state confirm that the OAM-carrying vector beams have a polarization order of l = i (i = 1, 2, 3).

Fabrication: The proposed metasurfaces to generate broadband OAM carrying vector beams can be fabricated using the existing fabrication techniques. Figure 8 shows the schematic diagram of the fabrication process of the proposed metasurfaces: (1) using standard electron-beam lithography to generate patterns on the glass substrate; (2) 5 nm Cr and 200 nm Au layers are then respectively deposited on the substrate using electron-beam evaporation; (3) removing the photoresist and undesired metal layers.

2.2. N-Fold OAM Multicasting Using V-Shaped Antenna Array [120]

Design: We design a V-shaped antenna array to realize on-chip multicasting from a single Gaussian beam to four OAM beams. Figure 9 illustrates the concept and principle of N-fold multicasting of OAM beams using V-shaped antennas. At the OAM multicasting side, an input Gaussian beam (l = 0) with a planar phase front at its beam waist is modulated by a V-shaped antenna array, which is specially designed to be equivalent to a complex multi-OAM phase pattern to generate collinearly-superimposed multiple OAM beams. After the modulation, the signal data carried by the input Gaussian beam are duplicated and delivered to N OAM beams, which are distinguishable from each other owing to their different charge values. The phase singularity of OAM beams contributes to the zero intensity distribution at the center of superimposed multiple OAM beams. At the OAM demultiplexing side, those N OAM beams (..., l_i−1, l_i, l_i+1, ...) are distributed to N end users (..., User_i−1, User_i, User_i+1, ...), respectively. For each end user, an inverse spiral phase pattern is used to remove the spiral phase front of the desired OAM beam, leading to a bright spot at the center, which can be separated from other OAM beams by spatial filtering.

To enable on-chip OAM multicasting, a pattern search-assisted iterative (PSI) algorithm is employed to prepare a specific phase pattern for the simultaneous generation of multiple collinearly superimposed OAM beams. Figure 10a displays the calculated phase pattern using the PSI algorithm to generate four collinearly superimposed OAM beams with charge values of l = 1, 4, 7, 10, respectively. In view of the discrete characteristic of the V-shaped antenna, it is necessary to replace the calculated continuous phase pattern with a discrete one. The right image of Figure 10a plots the discrete phase pattern, which discretizes the continuous phase pattern into 32 values along the azimuthal direction. To evaluate the performance degradation caused by the discrete phase pattern on the generated collinearly-superimposed OAM beams, power distributions of OAM channels (i.e., OAM spectra) generated by both continuous and discrete phase patterns are calculated, compared and depicted in Figure 10b. Here we define the crosstalk for all OAM channels by the power ratio of the desired OAM channel (e.g., l = 4) to its neighboring ones (e.g., l = 3 and l = 5). A slight difference is observed for the target multicast OAM channels when using the discrete phase pattern. The crosstalk degradation for all four OAM channels after transmitting through the discrete phase pattern is less than 2 dB. By replacing the discrete phase pattern with the corresponding V-shaped antennas, we design the V-shaped antenna array to facilitate multicasting from a single Gaussian beam to four OAM beams, as shown in Figure 10c,d.

Figure 11a depicts the calculated far-field intensity distribution of the collinearly-superimposed multiple OAM beams generated by the designed V-shaped antenna array, which has a triangular dark center. After demultiplexing, the intensity distributions of OAM channels are shown in Figure 11b,c. For multicasting OAM channels, there appears a bright spot at the beam center (Figure 11c), while for undesired channels, the beam center remains a dark region (Figure 11b). The power distribution of OAM channels generated by the designed V-shaped antenna array is also calculated and displayed in Figure 11d. The crosstalks for all multicast OAM channels are less than −15 dB, as shown in Figure 11d.

Fabrication: The proposed V-shaped antenna array to generate N-fold OAM multicasting can be fabricated using the existed fabrication techniques. Figure 12 shows the schematic diagram of the fabrication process of the V-shaped antenna array: (1) using standard electron-beam lithography to generate patterns on the silicon substrate; (2) 5 nm Cr and 50 nm Au layers are then respectively deposited on the substrate using electron-beam evaporation; (3) removing the photoresist and undesired metal layers.

2.3. Metasurface on Conventional Optical Fiber Facet for Linearly-Polarized Mode Generation [121]

Design: We design and fabricate a metasurface on the facet of conventional G.652 single mode fiber (SMF). Figure 13a shows the concept of LP₁₁ mode generation using the designed metasurface on the facet of G.652 SMF. The insert displays the details and working principle of a metal rectangle resonator: the polarization of incident light is polarized along x, which can be decomposed into two perpendicular components corresponding to the long and short edge of the resonator, respectively; the transmission amplitudes of the response for a resonator in both components are almost the same, while the relative phase retardation is around π, where the linear polarization conversion occurs, resulting in a y-polarized transmitted light. The finite-difference time-domain (FDTD) method is also used to simulate the amplitude and phase response of the proposed metal rectangle resonator and its mirror image. The amplitude responses of y-polarized transmitted light of this resonator and its mirror image are almost the same, while their phase responses have a difference of π. Therefore, this feature can be used to generate a higher-order LP mode in the optical fiber. As depicted in Figure 13a, when an x-polarized Gaussian beam is irradiating on the metasurface on the facet of G.652 SMF at a wavelength of 632.8 nm, it can generate two kinds of transmitted beams with orthogonal polarization. The transmitted beam with x polarization called the normal refractive beam that is not influenced by the metasurface can generate the LP₀₁ mode. Meanwhile, the transmitted beam with y polarization called the abnormal refractive beam, which is influenced by the metasurface, can generate the LP₁₁ mode. Owing to the orthogonal polarization of the generated LP₀₁ and LP₁₁ modes, it is significantly convenient to separate these two modes in the optical fiber.

Fabrication: Figure 13b illustrates the fabrication process of the designed metasurface on the facet of G.652 SMF. First, a 5 nm-thick Cr layer and a 45 nm-thick Au layer are successively deposited on the facet of optical fiber utilizing the electron beam evaporation technique. Then, this is followed by focused ion beam (FIB) etching to shape the appearance of the metasurface.

2.4. Graphene Split-Ring Metasurface-Assisted Terahertz Coherent Perfect Absorption [122]

Design: We present a novel graphene split-ring metasurface displaying a strong plasmonic resonance in the terahertz regime. Figure 14 shows the 2D and 3D structures of the proposed split-ring graphene film for coherent perfect absorption (CPA). For split-ring graphene film with symmetric two side (air, air) conditions (Case 1), as illustrated in Figure 14a, the interference of two counter-propagating incident beams A and B (relative intensity I_A/I_B = 1) on such a film is described by two special cases. When the phase difference $\mathsf{\varphi }$_A − $\mathsf{\varphi }$_B = 0, the CPA occurs. Thus, the graphene reaches the maximum absorption. When the phase difference $\mathsf{\varphi }$_A − $\mathsf{\varphi }$_B = π, Beam A and Beam B pass through one another without mutual disturbance. For split-ring graphene film with asymmetric two side (air, SiO₂) conditions (Case 2), gate-tunable CPA is illustrated in Figure 14b, which is based on the electro-absorption effect of graphene. By electrically tuning the Fermi level of the graphene sheet, corresponding to chemical potential μ_c1, the absorption of split-ring graphene reaches the maximum limit ∼50% for single incident beam. In this situation, when two coherent beams with equal intensities and phases are incident on the graphene from opposite sides, they will interfere with each other, leading to coherent absorption. When the chemical potential of graphene is changed to μ_c2, the absorption of split-ring graphene is tuned to be weak. Thus, Beams A and B pass through each other nearly without loss. Hence, gate-tunable CPA is achievable.

We first study the CPA of split-ring graphene film with symmetric two side (air, air) conditions (Figure 14a). The CPA is enabled by 50% absorption of the single beam and equal intensities and phases of two counter-propagating coherent beams. The chemical potential (Fermi energy) μ_c = 0.3 eV and relaxation time τ = 0.5 ps are initially considered. The transmission (T), reflection (R) and absorption (A) of a split-ring graphene patterned with a periodical array are plotted in Figure 15a. When a single beam is illuminated on the patterned split-ring graphene film at normal incidence, one can see an obvious resonance around 2.91 THz. The strong resonance behaviors are expected to be electric dipolar mode. The excitation of electric dipolar mode results in the enhancement of absorption in the graphene sheet with a maximum of A = 49.92%. While the other parts of the incident beam energy are reflected or transmitted. As shown in Figure 15b, when two coherent beams with equal intensities and phases are incident on the graphene from opposite sides, they will interfere with each other, leading to coherent absorption of 99.69%. Figure 15c shows the normalized coherent absorption in the split-ring graphene as a function of the relative phase difference between two counter-propagating coherent beams. At the resonance frequency of 2.91 THz, the coherent absorption varies continuously from 99.7% to less than 2.1 × 10⁻⁴% as the phase difference changes from 0 to 0.969π, giving a modulation contrast of 56.7 dB.

We then study the gate-tunable CPA of split-ring graphene film with asymmetric two side (air, SiO₂) conditions (Figure 14b). The gate tunable operation relies on the electro-absorption effect of graphene. We consider the same split-ring graphene film on a SiO₂ substrate with a refractive index of n = 1.45, as shown in Figure 14b. Figure 16a,b presents simulated normalized absorption spectra for single-beam surface-normal illumination from the split-ring graphene side and the SiO₂ side, respectively, under different chemical potentials (Fermi energies). By electrically tuning the chemical potential of the split-ring graphene sheet, one can change the resonance center frequency. For illumination from the split-ring graphene side, the maximum absorption keeps ≥ 49.5% when the center frequency varies from 2.29 to 2.71 THz, as shown in Figure 16a. For illumination from the SiO₂ substrate side, the maximum absorption keeps ≥ 49.8%, as shown in Figure 16b.

Figure 17 displays the calculated total absorption spectra of the split-ring graphene under different chemical potentials. An absorption peak of 97.13% at 2.29 THz is seen from Figure 17a for μ_c = 0.24 eV. With the increase of the graphene chemical potential, it shows a blue shift of the resonance peak frequency. When the chemical potential of graphene is tuned to μ_c = 0.35 eV, the absorption at 2.71 THz can still reach 95.6%. As the Fermi level of graphene can be electrically tuned, one can flexibly control the total absorption of the split-ring graphene. The split-ring graphene possesses the minimum absorption when the chemical potential is μ_c = 0 eV, as shown in Figure 5b. For the center frequency at 2.5 THz, the maximum and minimum absorption are 97.5% (μ_c = 2.9 eV) and 1.25% (μ_c = 0 eV), giving a gate-tunable modulation contrast of 19 dB.

Fabrication: The proposed graphene split-ring metasurface can be fabricated using the existing fabrication techniques. Figure 18 shows the schematic diagram of the fabrication process of the graphene split-ring metasurface: (1) transferring the graphene with the polymethyl methacrylate (PMMA) layer onto the surface of the glass substrate; (2) using standard electron-beam lithography to form patterns on the PMMA layer; (3) using the O₂ plasma etching technique to remove the undesired parts of graphene; (4) removing the PMMA layers.

3. Dielectric Metasurfaces

Dielectric metasurfaces are composed of high refractive index dielectric nanoparticles based on the Mie resonances. Several typical types of dielectric metasurfaces and previous relevant research are shown in

Table 2. Several typical types of dielectric metasurfaces and their applications.

	Er-doped Si-rich silicon nitride nano-pillar array for enhanced omnidirectional light extraction and OAM beam generation. Reproduced with permission from [97], Copyright AIP Publishing LLC, 2012.
	Si-based metasurfaces possessing sharp electromagnetically-induced transparency-like resonances in the near-infrared regime. Reproduced with permission from [111], Copyright Nature Publishing Group, 2014.
	Silicon nanobeams antennas function as a flat axicon to generate Bessel beams. Reproduced with permission from [85], Copyright The American Association for the Advancement of Science, 2014.
	Amorphous-silicon nanoridges to realize polarization beam splitting at the pixel-level. Reproduced with permission from [100], Copyright The Optical Society, 2015.
	Silicon nanodiscs to achieve high transmission and full phase control in visible wavelengths. Reproduced with permission from [125], Copyright John Wiley and Sons, 2015.
	Silicon cut-wire array in combination with a silver ground plane to achieve high linear polarization conversion efficiency in the near-infrared band. Reproduced with permission from [126], Copyright American Chemical Society, 2014.
	Silicon nano-pillar array to realize low loss micro-lenses in the near-infrared band. Reproduced with permission from [127], Copyright The Optical Society, 2014.
	Dielectric metasurface with a tailored phase gradient to achieve carpet cloaking at microwave frequencies [128]

: (1) a Er-doped Si-rich silicon nitride nano-pillar array is designed and fabricated using RF magnetron sputtering, electron beam lithography and reactive ion etching methods for enhanced omnidirectional light extraction and OAM beam generation [97]; (2) Si-based metasurfaces are designed and fabricated using electron-beam lithography and reactive ion etching techniques to possess sharp electromagnetically-induced transparency-like resonances in the near-infrared regime [111]; (3) silicon nanobeam antennas are designed and fabricated using low-pressure chemical vapor deposition (LPCVD), electron-beam lithography and reactive ion etching techniques to generate Bessel beams [85]; (4) amorphous-silicon nanoridges are designed and fabricated using standard electron-beam lithography and reactive ion etching techniques to realize polarization beam splitting at the pixel-level [100]; (5) silicon nanodiscs are designed and fabricated using chemical vapor deposition, electron-beam lithography and reactive ion etching methods to achieve high transmission and full phase control in visible wavelengths [125]; (6) a silicon cut-wire array in combination with a silver ground plane are designed and fabricated using chemical vapor deposition, electron-beam lithography and reactive ion etching techniques to achieve high linear polarization conversion efficiency in the near-infrared band [126]; (7) the silicon nano-pillar array is designed and fabricated using electron-beam lithography and reactive ion etching techniques to realize low loss micro-lenses in the near-infrared band [127]; (8) dielectric metasurface with a tailored phase gradient is designed to achieve carpet cloaking at microwave frequencies [128]. All of these previous works are based on manipulating the Mie responses of dielectric nano-particles by altering the dimensions, directions, arrangement of unit structures and even combining multiple resonators with different shapes. Recently, we have also made some progress in dielectric metasurfaces on the basis of previous works.

3.1. OAM Beam Generation Using a Nanophotonic Dielectric Metasurface Array [123]

We design and fabricate a kind of chip-scale dielectric metasurface array on a silicon-on-insulator (SOI) platform. Figure 19 shows the schematic drawing of the proposed dielectric metasurface units, consisting of rectangle or ellipse silicon resonators with various dimensions. The working principle of these proposed dielectric resonators can be explained as follows: the polarization of incident light is along the x axis, which can be decomposed into two perpendicular components corresponding to the long and short axis of the resonator, respectively; the reflection amplitudes of the response for a resonator in both components are almost the same, while the relative phase retardation is around π, where the linear polarization conversion occurs, resulting in a y-polarized reflected light.

OAM beam generation using the dielectric metasurface at 1064 nm: Then, we simulate the amplitude and phase responses of the near-field of reflected light of a rectangle dielectric metasurface unit with various dimensions using the FDTD method at a wavelength of 1064 nm. The simulated results indicate that the response of the proposed resonator can cover a full phase varying over 0–2π and a wide shift of amplitude. It is relatively easy to find out a series of resonators covering full-phase control while maintaining approximately constant amplitude, which is considerably important for OAM beam generation. Then, eight kinds of resonators with different geometric dimensions are chosen to provide an equal-spacing phase shift from 0 to 2π and nearly constant amplitude, as plotted in Figure 20. Figure 21 plots the simulated intensity and phase distributions of OAM beams of topological charge number l = 1–4 generated using the rectangle dielectric metasurface array consisting of the eight chosen resonators. The results indicate that the generated OAM beams match the theoretical ones well.

OAM beam generation using the dielectric metasurface at 632.8 nm: Moreover, we also design the dielectric ellipse metasurface arrays at a wavelength of 632.8 nm. Figure 22 shows the concept of OAM beam generation through a dielectric ellipse metasurface array consisting of a series of resonators covering a full-phase modulation. When a Gaussian beam of vertical polarization is incident to the metasurface, the reflected beam can be transformed into OAM beams of horizontal polarization. In view of the polarization conversation happening on the proposed metasurface, one can easily separate the incident and reflected lights. Figure 23 plots the simulated intensity and phase distributions of OAM beams of topological charge number l = 1–4 generated using the dielectric ellipse metasurface array consisting of the eight chosen resonators at a wavelength of 632.8 nm. The results indicate that the generated OAM beams match the theoretical ones well.

OAM beam generation using dielectric metasurface at 980 nm: We further design, fabricate and test the dielectric ellipse metasurface arrays at a wavelength of 980 nm. Figure 24 shows the experimental results of the fabricated dielectric ellipse metasurface arrays. Figure 24a plots the measured intensity profiles of the generated OAM beams of l = 1–4 using the fabricated metasurface arrays. Figure 24b exhibits the measured intensity profiles of generated OAM beams after being interfered by Gaussian beam. One can clearly see from Figure 24b that the number of helical arms in the distribution of the interference intensity is equal to the topological number 𝑙 of the OAM beam, indicating the favorable results.

Fabrication: The proposed rectangle and ellipse dielectric metasurfaces can be fabricated using the existing fabrication techniques on the SOI platform. Figure 25a shows the schematic diagram of the fabrication process of the proposed rectangle and ellipse dielectric metasurfaces: standard electron-beam lithography to form patterns followed by induced coupled plasma silicon etching. Figure 25b displays the scanning electron microscope (SEM) details of the top view of a fabricated dielectric ellipse metasurface array to generate the OAM beam.

3.2. Bessel Beam Generation and OAM Multicasting Using the Dielectric Metasurface Array [124]

Design: We propose and design dielectric ellipse metasurfaces to realize Bessel beam generation and OAM multicasting at a wavelength of 1550 nm. After the unit structure is simulated by the FDTD method, we choose eight kinds of resonators with different geometric dimensions, as shown in Figure 26a, providing a phase shift from 0 to 2π and a nearly constant amplitude. The method of the phase pattern replacing the axicon is used to generate the Bessel beams. By replacing the phase patterns with the eight chosen resonators, simulated Bessel beams are achieved as plotted in Figure 26b. The purities of the generated Bessel beams with different indices are also analyzed. As shown in Figure 26c, the purities of the generated Bessel beams with different indices of l = −20, −19, ..., 19, 20 are all higher than 0.93.

Moreover, six-fold OAM multicasting using the designed dielectric ellipse metasurfaces is also simulated. The ideal phase pattern to generate collinearly superimposed six OAM beams (l = 1, 4, 7, 10, 13, 16) is plotted in Figure 27a. By replacing the ideal phase pattern with corresponding resonators, we design the dielectric ellipse array shown in Figure 27b. Figure 27d depicts the intensity distribution of the superimposed multiple OAM modes generated by the designed ellipse array, which has a trigonal dark center. After de-multicasting, the intensity distribution of OAM channels is plotted in Figure 27e,f. For multicast OAM channels, there exists a bright spot at the beam center (Figure 27f), while for undesired channels, there is a dark region at the center (Figure 27e). Crosstalks for all OAM channels, defined as the power ratio of the desired OAM channel (e.g., l = 4) to its neighboring ones (e.g., l = 3 and l = 5), are less than −14 dB, as shown in Figure 27c.

4. Discussion

Although metasurfaces composed of plasmonic and dielectric units have only recently emerged as an active area of research, their potential as an approach to realize wavefront shaping devices has already been successfully implemented in flat optics, such as OAM beam generator, lens, wave plates, holograms, etc., for its superiorities of compact and flexible design. For instance, convenient optical elements are based on the refraction, reflection or diffraction of light. Their ability of wavefront shaping relies on propagation through media whose refractive indices can be designed to control the light path. As a consequence, those conventional optical elements suffer from thick materials, large volume, long diffraction distance and even low optical manipulation accuracy, such that it is difficult for them to be compatible with compact systems. Metasurface-based flat optical elements can potentially replace traditional ones. They are able to modulate the wavefront of the light beam by introducing a variety of nano-scaled electric dipole resonators replacing conventional propagation effect and therefore considerably reducing the whole volume of systems. Besides, the responses of metasurfaces rely on the characteristics of unit structures, such as structure dimensions and material types. It is relatively convenient to flexibly control the amplitude, phase and polarization of scattering light.

However, there are still some challenges of metasurfaces. For plasmonic metasurfaces, they suffer from ohmic loss at optical wavelengths. One common solution to loss compensation is to introduce a gain medium in or around the plasmonic material. This solution can partially or fully compensate the loss in metals, but the incorporation of such active materials is very challenging [129]. Another challenge for plasmonic metasurfaces is that noble metals have a large plasma frequency and cannot be adjusted, making tunable or switchable metasurface devices hard to realize [130]. What is more, noble metals are not compatible with the standard semiconductor fabrication technologies, restricting metasurface devices to the proof-of-concept stage only. To deal with those issues, some recent research focuses on new materials platforms defining new, intermediate carrier density materials as the best candidates that have low loss, excellent tuning and modulation capabilities and that are compatible with established semiconductor processing technologies and integration procedures [131,132,133,134]. In addition, metasurfaces are also facing the challenge of being reconfigurable. For metasurfaces, the unit structures must be subwavelength-scaled optical switches with a fraction of a square micrometer footprint and a volume of only approximate 10⁻¹⁹ m³. It is desired for the optical switches to be fast and energy saving. It is challenging for unit structures to satisfy the former two conditions. To solve the problem, one can alter the dimensions of the individual unit structures, or manipulate the near-field interactions between them. The nanomechanical device platform can provide one possible ideal solution [135,136]. There are several impressive works reported recently focusing on tunable and reconfigurable metasurfaces: (1) by combining the Fano resonant metal nanoparticles with a single layer of graphene, obviously electrically-controlled damping is observed in the Fano resonances in the near-infrared regime [137]; (2) by positioning a thin graphene sheet over asymmetric silicon nanobars, the transmission spectrum of the proposed device can be efficiently adjusted at the near-infrared frequencies [138]; (3) a metal nanomechanical metasurface is fabricated on the nanoscale thickness dielectric membrane, and the displacement of the metamolecules can be controlled by light illumination [139]; (4) using differential thermal expansion and Lorentz force, electrothermal tuning and magnetic modulation are achieved in a reconfigurable metal metasurface [140]; (5) using a vanadium dioxide metasurface lens, the amplitude spectrum and the focal intensity can be adjusted by changing the temperature in the terahertz regime [141]; (6) utilizing a heating-induced vanadium dioxide planar metamaterial, the device is able to be switched between capacitive and inductive responses [142]; (7) by combing the metal nanoantenna array with indium tin oxide, the phase and amplitude of the reflected light can be controlled at near-infrared wavelengths [143].

In spite of facing many challenges, metasurfaces are still one of the devices with the most potential applied in beam-steering, spatial light modulator, nanoscale-resolution imaging, sensing and novel quantum optics devices. Besides, the increasing demand for the large capacity of data transmission and fast signal processing promotes eliminating the bottleneck between optical fiber communication networks and electronic data handling. Achieving this goal will require strong and fast nonlinearities for switching light with light and much improved manipulation of the electromagnetic fields with external stimuli, such as electric signals in a compact space. Metasurface-based devices can considerably enhance these functionalities by exploiting the characteristics of metasurfaces [136]. In the future, metasurface-based devices would be developed as tunable, reconfigurable, nonlinear, switchable, gain and quantum metasurfaces, which might be achieved by adopting new materials (such as superconductors or graphene) and hybridizing plasmonics metal nanostructures with other functional materials, such as nanocarbon, organics, nanosemiconductors, organic polymers and phase-change media [130].

5. Conclusions

In summary, we have introduced basic concepts of metasurfaces and reviewed our recent works in metasurface-based devices and their design, fabrication and applications: (1) metasurfaces consisting of multiple concentric rings in a gold film are designed to generate broadband OAM-carrying vector beams in the regime from the near-infrared to mid-infrared band; (2) a metal V-shaped antenna array is designed to realize multicasting from a single Gaussian beam to four OAM beams assisted by the PSI algorithm, achieving a low-crosstalk of near −15 dB; (3) a kind of gold metasurface is designed and fabricated on the facet of a G.652 SMF to generate the LP₁₁ mode at the wavelength of 632.8 nm; (4) a kind of split-ring graphene is designed to realize CPA in the THz regime, achieving a tunable CPA covering from 99.7% to less than 2.1 × 10⁻⁴%; (5) a kind of dielectric metasurface is designed and fabricated on the SOI platform, realizing OAM beam generation; (6) a kind of dielectric metasurface is designed on the SOI platform at a wavelength of 1550 nm, achieving Bessel beam generation with high purity and multicasting from a single Gaussian beam to six OAM beams with a relatively low crosstalk of less than −14 dB. The designed and fabricated metasurfaces in these works have nano-scaled dimensions, and the generated beams carrying OAM have favorable qualities, which are promising for a compact and effective beam-steering system and space-division multiplexing (SDM)-assisted optical communication systems. In the future, the metasurface devices to generate optical beams carrying OAM can be combined with tunable and reconfigurable functions reported in [137,138,139,140,141,142,143], which makes robust spatial light manipulation possible and facilitates more interesting applications.