Optical reflective metasurfaces based on mirror-coupled slot antennas

Abstract. Electrically connected optical metasurfaces with high efficiencies are crucial for developing spatiotemporal metadevices with ultrahigh spatial and ultrafast temporal resolutions. While efficient metal–insulator–metal (MIM) metasurfaces containing discretized meta-atoms require additional electrodes, Babinet-inspired slot-antenna-based plasmonic metasurfaces suffer from low efficiencies and limited phase coverage for copolarized optical fields. Capitalizing on the concepts of conventional MIM and slot-antenna metasurfaces, we design and experimentally demonstrate a new type of optical reflective metasurfaces consisting of mirror-coupled slot antennas (MCSAs). By tuning the dimensions of rectangular-shaped nanoapertures atop a dielectric-coated gold mirror, we achieve efficient phase modulation within a sufficiently large range of 320 deg and realize functional phase-gradient metadevices for beam steering and beam splitting in the near-infrared range. The fabricated samples show (22  %    ±  2  %  ) diffraction efficiency for beam steering and (17  %    ±  1  %  ) for beam splitting at the wavelength of 790 nm. The considered MCSA configuration, dispensing with auxiliary electrodes, offers an alternative and promising platform for electrically controlled reflective spatiotemporal metasurfaces.


Introduction
In the past decade, optical metasurfaces have attracted considerable attention due to their superior capabilities of arbitrarily molding light, thereby challenging conventional bulky optics with ultrathin planar metadevices exhibiting compact footprints and advanced functionalities. [1][2][3][4][5][6][7] Typically, optical metasurfaces are made up of different types of fundamental building blocks called meta-atoms, such as single-layered metallic antennas, 8,9 slot antennas, [10][11][12][13][14][15] metal-insulator-metal (MIM) resonators, [16][17][18][19][20] and all-dielectric antennas, [21][22][23][24] which enable local control over the amplitude, phase, and polarization of optical fields by tailoring the meta-atom geometry and orientation. In terms of the efficiency, MIM meta-atoms, which are composed of a thin dielectric spacer sandwiched between an optically thick metal film and metallic antennas, are found to be superior to other counterparts regarding control of reflected optical fields, a circumstance that has resulted in their numerous technical applications. 6,16 In particular, electrically driven spatiotemporal MIM metasurfaces with dynamic optical responses have been successfully demonstrated by triggering the active media with applied voltages. 7,[25][26][27][28] However, due to the discrete nature of metallic antennas, additional electrodes are necessarily required for their control, which not only complicates the design and fabrication but also affects the performance, hence restricting severely the applications for adaptive photonic systems.
Capitalizing on the concepts of conventional MIM metasurfaces and Babinet-inspired slot-antenna-based metasurfaces, [10][11][12] we design and experimentally demonstrate a new type of optical reflective metasurfaces composed of mirror-coupled slot antennas (MCSAs). By tuning the dimensions of subwavelength-thick rectangular-shaped nanoapertures atop a dielectric silicon dioxide (SiO 2 ) coated gold (Au) mirror, we achieve a phase coverage up to 320 deg for linearly polarized excitation and realize functional phase-gradient metadevices for beam steering and beam splitting in the near-infrared range. The fabricated metasurfaces exhibit (22% AE 2%) diffraction efficiency for beam steering and (17% AE 1%) for beam splitting at the wavelength of 790 nm. The considered MCSA configuration offers an alternative and promising platform for optical reflective metasurfaces, especially for electro-optically tunable metasurfaces, [25][26][27][28][29] by virtue of using electrically connected meta-atoms and thereby dispensing with auxiliary electrodes.

Design of the Meta-atom
The designed MCSA unit cell consists of a rectangular-shaped slot antenna milled in a subwavelength-thick Au film, a middle SiO 2 spacer layer, and an optically thick Au mirror, forming a modified MIM meta-atom [ Fig. 1(a)]. The periodicity of MCSA unit cells is Λ ¼ 240 nm, which is much smaller than the design wavelength of λ ¼ 785 nm to eliminate any free propagating diffraction orders. The thickness of the topmost slot antenna is t m ¼ 50 nm, and the thickness of the bottom Au layer is t sub ¼ 100 nm, which is thick enough to block the transmission. To study the optical response of the MCSA unit cell, we performed full-wave simulations with the commercially available finite-element software COMSOL Multiphysics (version 5.6). In simulations, the refractive index of the SiO 2 spacer layer is assumed to be 1.45, whereas the relative permittivity of Au is described by the Drude model fitted with the Johnson and Christy data, 30 with the plasma frequency being ω p ¼ 1.37 × 10 16 s −1 and the damping frequency being ω d ¼ 1.224 × 10 14 s −1 . The medium above and within slot antennas is chosen to be air with the refractive index of 1. The simulation domain of the unit cell is truncated at the air side with a perfectly matched layer to minimize the reflection. With this model, we calculated the complex reflection coefficients on the top of the slot antenna as a function of L x and L y for normally incident transverse magnetic (TM) polarized excitation (the electric field is polarized along the x axis) at λ ¼ 785 nm.
It is seen [ Fig. 1(b)] that the variation of both L x and L y results in the phase variation for the reflected TM-polarized light in a wide range (up to 320 deg) at a relatively large spacer thickness t s ¼ 130 nm. This variation occurs because of tuning the meta-atom in the vicinity of the mode resonance formed by hybridization between the lateral plasmonic resonance confined in the slot antenna and vertical Fabry-Pérot (FP) resonance between the slot antenna and the bottom Au mirror. In contrast to the well-studied freestanding slot antenna that exhibits limited phase coverage of only 180 deg for the copolarized electric fields, the bottom Au film functions as a reflecting mirror of the FP cavity ( Fig. S1 in the Supplementary Material). Therefore, the incident light will bounce back and forth before being reflected with a sufficiently large phase shift. Close to the plasmonic resonance that is indicated by the pronounced absorption of the TM-polarized incident light, the phase varies strongly, with a range of ∼180 deg [ Fig. 1(b)]. Away from the resonance region, the reflection phase is gradually varied with the lateral dimensions. For example, the reflection contribution of the top slot-antenna layer will be dominating when the slot antennas are narrow in the polarization direction, leading to the incident wave being mostly reflected by the top slot-antenna layer that approximates a continuous Au film [Figs. 1(b) and 1(c) and Figs. 2(a) and 2(c)]. When the slot antennas are wide, the top Au layer becomes more transparent, and the light is mainly reflected by the bottom Au layer. Therefore, the reflection phase changes with the thickness of the middle spacer [ Fig. 1(b) and Figs. 2(a) and 2(b)]. According to the theory of Babinet-inspired plasmonic metasurfaces, 10-12 the phase shift φ xx under TM-polarized excitation should be mainly determined by the length of the slot antenna along the y direction. However, the phase contours of such an MCSA unit cell are not parallel to each other, since the slot dimension along the x axis changes the accumulated charges and finally affects the resonance, especially in the strong absorption regime. The cross sections of the x component of the electric field (color map) and the electric field vectors (arrows) for different cutting planes through the center of the topmost slot antenna, where the electric field is strongly localized within the slot area [ Fig. 1(c)], reveal the localized character of the slot mode. The maximal field strengths are found at the edges perpendicular to the polarization direction of the excitation. This observation is consistent with the studies on the transmission of light through differently shaped holes, where the resonant optical transmission is accompanied by an increased local field within the nanoholes. [31][32][33] Owing to the hybrid plasmonic/FP resonance, the phase modulation for the reflected polarized light thereby depends heavily on the SiO 2 spacer thickness t s , which influences the accessible phase coverage together with the absorption around the resonance of the slot antenna. Figure 2 depicts the phase dependence on the slot dimensions for two different spacer thicknesses. For a thickness of t s ¼ 50 nm, the phase space of around 320 deg is still accessible [ Fig. 2(a)]. In contrast to the case with a thicker spacer of 130 nm, we observe a steeper phase variation in the region of resonance. Specifically, the phase contours become crammed close to each other for a smaller spacer thickness, different from Fig. 1(b). Meanwhile, the absorption becomes stronger with a more pronounced and narrower resonance. Therefore, this thickness is not ideal for a practicable accurate selection of slot-antenna candidates, even though similar phase contours can be found in other regions away from the resonance. The field distribution of the x-component of the electrical field is still strongly localized within the hole volume, as shown in Fig. 2(c). In comparison to the field localization at a 130-nm spacer thickness in Fig. 1(c), a decrease in field strength is observable. Further decreasing the spacer thickness to 10 nm causes a change in the phase distribution and reflectivity as well as in the field localization [Figs. 2(b) and 2(d)]. The resulting phase space is confined to a limited interval of only ∼50 deg [ Fig. 2(b)]. Additionally, the resonant structure does not show a strong localization, where the x component of the electrical field is only confined at the edges of the slot antenna for this 10-nm-SiO 2 configuration [ Fig. 2(d)]. As a result, we choose a 130-nm spacer thickness for the following numerical and experimental discussions, since it provides a sufficiently large phase coverage with more spreading phase contours and higher reflection amplitudes.

Functional Phase-Gradient MCSA Metasurfaces for Beam Steering
To demonstrate the applicability of this kind of MCSA metaatom, we design a phase-gradient metasurface that functions Ebel et al.: Optical reflective metasurfaces based on mirror-coupled slot antennas as a beam steerer. Under the excitation of a TM-polarized wave at normal incidence, the reflected light will be redirected to a specific direction, which is well described by the generalized Snell's law 8 and may be also viewed as a blazed grating where the angle of reflection reassembles the first-order diffraction. 34 By marking the reflection phase through constant phase-contour lines in the complex reflection coefficient map, we select three meta-atoms with a nearly constant phase shift between neighbors and considerably high reflectivities according to realistic fabrication sizes. (The selected dimensions are shown in Table S1 and Fig. S2 in the Supplementary Material) Specifically, the chosen MCSA meta-atoms have a constant phase interval of ∼110 deg between each other. The selected meta-atoms are then placed at a center-to-center distance of Λ ¼ 240 nm to form a supercell. However, the supercell design is nontrivial, since two important factors should be considered.
First, the supercell period should be larger than the incident wavelength to avoid coupling to surface plasmons. [35][36][37] Therefore, each meta-atom should be duplicated to make a bigger supercell, resulting in a decreased diffraction angle. Second, the arrangement of slot antennas within the supercell leads to the intrinsic near-field coupling between differently sized elements since the gradient-wise slot-antennas assembly is different from the array-wise arrangement with the identical unit cells during the design of the MCSA unit cell. Compared to conventional MIM metasurfaces composed of isolated metallic antennas that exhibit weak near-field coupling effects, 38 this MCSA metasurface is strongly affected by the near-field coupling. Since all meta-atoms are naturally connected, the charges induced by the electric fields are not confined anymore, thereby strongly modifying the responses of neighboring size-different elements (Fig. 3). The effect of this near-field coupling on the metasurface performance was studied numerically by considering three different supercells made of double, triple, and quadruple identical  The diffraction efficiencies are obtained by normalizing the reflection into different diffraction orders.
As shown in Figs. 3(b) and 3(c), the supercells consisting of 9 and 12 elements perform well for steering the incoming light to the first diffraction order (green solid line), each reaching a diffraction efficiency of ∼50% at λ ¼ 785 nm. Still, the performance of the 12-element supercell can be considered more advantageous, since the reflectivity into the unwanted diffraction orders, mainly the −2 diffraction order (violet solid line), is more suppressed compared to the 9-element design. For the sixelement design [ Fig. 3(a)], the specular reflection is dominating while the desired diffraction order is relatively low, which can be explained by plotting the electric field distributions within the top x − y plane cut at the center of the slot antenna (right column in Fig. 3). It is clearly observable that the fields within the identical meta-atoms become more similar when the tuple size of the supercell is increased. As a final comment, it should be mentioned that the influence of near-field coupling between neighboring meta-atoms with significantly varied dimensions could also be decreased by inserting more meta-atoms with intermediate reflection phases (instead of the meta-atom multiplication considered above) to provide a smoother phase gradient and better performance (Fig. S3 in the Supplementary Material). However, this design is rather challenging to faithfully reproduce during the fabrication, since the neighbor slot dimensions are too close.

Experimental Demonstration of Functional MCSA Metasurfaces
Following the design, we fabricated the beam-steering MCSA metasurface composed of 12-element supercells on a silicon substrate using thin-film deposition, electron beam lithography, lift-off, and ion-beam etching techniques. First, a 2-nm chromium (Cr) adhesion layer and a 130-nm Au layer are evaporated on a silicon substrate using electron beam evaporation. Subsequently, a 140-nm-thick spin-on dielectric is applied (IC1-200, Futurrex, nominal refractive index of 1.34, hardbaked at 200 deg), which serves as the dielectric spacer layer with an equal optical thickness to the original design. Next, a 2-nm Cr adhesion and a 50-nm Au layer are deposited using electron beam evaporation. We use electron beam lithography (Raith Voyager, 50 kV acceleration voltage, beam current 2.14 nA) to define the structures in a positive tone resist (AR-P 6200.13, Allresist, area dose 160 μC∕cm 2 ). After development, the resist mask is transferred to the underlying Au layer by Ar ion-beam etching (Technics Plasma). The remaining resist layer is removed in NEP at 80°C (N-Ethyl-2-pyrrolidone). The fabricated metasurface has a size of 100 μm × 100 μm, whose scanning electron microscopy (SEM) images are displayed in Fig. 4(b). Generally, the fabricated slot antennas resemble very well the desired shapes and dimensions, despite the inevitable surface roughness and rounded corners. The measurements were performed using a custom-built optical setup (Fig. S4 in the Supplementary Material) within the wavelength interval of 700 to 850 nm in steps of 10 nm. The inset optical image of the diffraction spots in Fig. 4(c) illustrates the beam-steering ability of the fabricated metasurface at the wavelength of λ ¼ 790 nm, nearest to the design wavelength. To determine the overall performance quantitatively, we measured the corresponding diffraction efficiencies of different orders from m ¼ −2 to +2. In the measurement, four fabricated metasurfaces with identical recipes were measured independently to calculate the mean value and corresponding standard errors of the mean. From Fig. 4(c), it is evident that the total reflectivity is close to 40%, and the diffraction efficiency of the þ1 order is (22% AE 2%) at λ ¼ 790 nm. Further, results show that beamsteering capability is pursued in a broadband spectrum ranging from 700 to 800 nm with zeroth order below 10% and higher diffraction orders suppressed below 5%. Compared to the averaged reflectance of the three selected MCSA meta-atoms (∼47%) at λ ¼ 790 nm, the measured diffraction efficiency is reduced by half, which is mainly ascribed to the increased dissipation loss from the 2-nm-thick Cr adhesion layer as well as the surface scattering and grain boundary effects of the deposited Au layers. To validate our assumptions, we increased the damping rate of Au by a factor of 3 in the simulation and obtained reasonably matched simulation results with the measurements. For further validation of the MCSA concept, an additional metasurface was designed and fabricated that enables splitting the incoming TM wave into ±1 diffraction orders with equal intensity, which is made up of a quadruple arrangement of the two slot antennas with the phase difference of ∼180 deg (Fig. 5). This leads to a supercell consisting of eight elements with a diffraction angle of θ r ¼ 24 deg at the design wavelength of λ ¼ 785 nm. 8 The SEM images of part of the fabricated sample are shown in Fig. 5(b). Similarly, the fabricated beam-splitting metasurface samples were characterized by measuring the diffraction efficiencies into different orders in Fig. 5(c). We again observe a reasonable agreement between the simulated and measured results in a wide wavelength range. At the measured wavelength of λ ¼ 790 nm, (17% AE 1%) of the incident wave is split into the AE1 diffraction orders, whereas the total reflectivity almost reaches ∼20%. Due to increased ohmic losses in metallic layers (as compared to the tabulated values) especially at shorter wavelengths, the total reflectivity in the simulation and experiment exhibits different dependences on the optical wavelength. As a final comment, we emphasize that the efficiencies of MCSA metasurfaces could be further improved using single-crystalline metallic materials in the fabrication process to reduce the ohmic loss. 39

Conclusion
We have demonstrated a new type of reflection optical metasurface based on MCSAs, which combine the advantages of high efficiencies and ease of fabrication from conventional MIM metasurfaces as well as the electrical connection from Babinet-inspired slot-antenna-based metasurfaces. By tuning the lateral dimensions of the slot antennas and optimizing the spacer thickness, a phase coverage of 320 deg can be achieved for the copolarized reflected light. With this platform, we have implemented functional phase-gradient metasurfaces that perform beam steering and beam splitting in the near-infrared wavelength range. Reasonably good working performance, in terms of efficiencies and operation bandwidths, was achieved by alleviating the near-field coupling between adjacent differently sized meta-atoms. We believe that this newly established reflective metasurface configuration with electrically connected individual elements offers an alternative and promising platform for electrically controlled reflective metasurfaces.

Code, Data and Materials Availability
The simulated and experimental data that support the works of this study are available from the corresponding authors on reasonable request.