Hybrid bilayer plasmonic metasurface efficiently manipulates visible light

Abstract

Metasurfaces operating in the cross-polarization scheme have shown an interesting degree of control over the wavefront of transmitted light. Nevertheless, their inherently low efficiency in visible light raises certain concerns for practical applications. Without sacrificing the ultrathin flat design, we propose a bilayer plasmonic metasurface operating at visible frequencies, obtained by coupling a nanoantenna-based metasurface with its complementary Babinet-inverted copy. By breaking the radiation symmetry because of the finite, yet small, thickness of the proposed structure and benefitting from properly tailored intra- and interlayer couplings, such coupled bilayer metasurface experimentally yields a conversion efficiency of 17%, significantly larger than that of earlier single-layer designs, as well as an extinction ratio larger than 0 dB, meaning that anomalous refraction dominates the transmission response. Our finding shows that metallic metasurface can counterintuitively manipulate the visible light as efficiently as dielectric metasurface (~20% in conversion efficiency in Lin et al.'s study), although the metal's ohmic loss is much higher than dielectrics. Our hybrid bilayer design, still being ultrathin (~λ/6), is found to obey generalized Snell's law even in the presence of strong couplings. It is capable of efficiently manipulating visible light over a broad bandwidth and can be realized with a facile one-step nanofabrication process.

Keywords: Metasurface; high efficiency; plasmonics; visible light, coupling.

Fig. 1. High-efficiency beam bending of visible light with a complementary bilayer metasurface.

(A) Working principle of the designed structure. The light spots on the screen in the foreground are the combined real photo images of anomalous light deflection for different wavelengths. Inset: Sketch of the subunit cell, which consists of one layer of gold nanoantennas on the top and its Babinet-inverted pattern at the bottom, separated by conformal HSQ pillars. L and W are the length and width of the nanoantenna arms, respectively; θ is the angle between two arms; T is the thickness of the gold film; H is the height of the HSQ pillar; and D is the nominal space between the top and bottom layers. (B) Schematic representation of the fabrication procedure. (i) A 100-nm-thick layer of HSQ is spin-coated onto a SiO₂ substrate. (ii) Patterning using EBL. (iii) A 30-nm-gold film is deposited on the sample by electron beam evaporator. (iv) Top view of SEM image of the bilayer metasurface. The supercell (yellow) comprises six V-shaped nanoantennas. The supercell repeats with a periodicity of 900 nm along x axis and 150 nm along y axis. SU, subunit.

Fig. 2. Performance of the bilayer plasmonic metasurface.

(A) Experimental results of the conversion efficiency and extinction ratio. (B) Simulated cross-polarized E_x component of the transmitted field of a bilayer metasurface at 770 nm, which is the peak value position of the conversion efficiency in simulation. The geometrical parameters used in the simulations are obtained by measuring the fabricated sample with SEM and AFM techniques. The lengths of each antenna and the angles between the two arms for unit cells 1 to 3 are as follows: L_i = 130, 125, and 120 nm, and θ_i = 70°, 90°, and 110°, respectively; width of the nanoantennas W = 55 nm, thickness of the gold film T = 30 nm, and height of the HSQ pillar H = 100 nm. Unit cells 4 to 6 are constructed by rotating the symmetry axis of unit cells 1 to 3 90° clockwise. A sketch of the supercell consisting of six subunits is shown at the bottom of the field pattern, where the actual sample would be placed. (C) Relationship of transmittance of anomalous light on the wavelength and the anomalous refraction angle. False-color map indicates the experimentally measured intensity of the anomalous refracted beam. Inset: Optical image of the transmission spots at a wavelength of 720 nm.

Fig. 3. Comparison of three different types of metasurfaces, showing different sensitivities to the subperiodicity (that is, the distance between neighboring cells).

For ease of comparison, in these simulations, the same parameters were used for all the three types of metasurfaces. (A to C) Dependence of the conversion efficiency on the subperiodicities for three types of metasurfaces as the subperiodicities increase from 150 to 180 nm. (D to F) Dependence of the extinction ratio on the subperiodicities for three types of metasurfaces as the subperiodicities increase from 150 to 180 nm.

Fig. 4. Simulated transmittance with the optimized structure.

The parameters were defined as follows: For the length of each antenna and the angle between the two arms for units 1 to 3, L_i = 140, 130, and 125 nm, and θ_i = 70°, 90°, and 110°, respectively; W = 50 nm, T = 30 nm, and H = 100 nm. Unit cells 4 to 6 are constructed by rotating the symmetry axis of unit cells 1 to 3 90° clockwise.

Fig. 5. Field and current distributions for one selected subunit (that is, SU2 in Fig. 1B, iv) under antisymmetric excitation.

(A) z components of electric and magnetic field distributions on the top and bottom layers of the bilayer metasurface, respectively. The displacement current density J at resonance on both arms of the HSQ pillar is represented by vector arrows, where green and orange indicate whether the z component of the displacement current (J_Z) is toward −z and +z directions, respectively. (B and C) Transverse electrical current (J_s) and equivalent transverse magnetic current (J_m) of the bilayer metasurface. The position of the V-shaped nanoantenna on the top and the Babinet-inverted aperture on the bottom are indicated by the white outlines. The amplitudes are represented by the color of the arrows with the same relative scale for J_s and J_m, respectively.