High-Q Quasi-Bound States in the Continuum in Terahertz All-Silicon Metasurfaces

Abstract

Bound states in the continuum (BIC)-based all-silicon metasurfaces have attracted widespread attention in recent years because of their high quality (Q) factors in terahertz (THz) frequencies. Here, we propose and experimentally demonstrate an all-silicon BIC metasurface consisting of an air-hole array on a Si substrate. BICs originated from low-order TE and TM guided mode resonances (GMRs) induced by (1,0) and (1,1) Rayleigh diffraction of metagratings, which were numerically investigated. The results indicate that the GMRs and their Q-factors are easily excited and manipulated by breaking the lattice symmetry through changes in the position or radius of the air-holes, while the resonance frequencies are less sensitive to these changes. The measured Q-factor of the GMRs is as high as 490. The high-Q metasurfaces have potential applications in THz modulators, biosensors, and other photonic devices.

Keywords: all-dielectric metasurface; bound state in the continuum; guided mode resonance; terahertz.

Figure 1

(a) Schematic diagram of an all-silicon metasurface consisting of two air-holes array. (b) Unit cell of a symmetric metasurface. The periods in the x and y directions are Λ_x = 2Λ and Λ_y = 2Λ. (c,d) The unit cell of an asymmetric metasurface is represented by Δr or Δd, respectively.

Figure 2

(a) Dispersion curves of ten related TE and TM eigenmodes for symmetrical metasurface when r = 55 μm, where k is the propagation constant, and the inset shows the first Brillouin zone of the lattice. (b) Near-field distribution of electric (E_y) and magnetic (H_y) eigenmodes in the x-z plane at Γ-point, in which the black dotted box represents the air-hole. Cross-sections of the near-field distributions are all taken from the A–A section in Figure 1b. (c,d) Transmissions of metasurface at different parameters Δr and Δd when illuminated by THz wave polarized along the y or x direction at normal incidence. The Roman letters are used to represent each resonance. (e) Electric (E_x) and magnetic (H_x) near-field distributions of two low-Q guided modes TE 9 and TM 10 at Γ-point in the y-z plane from B–B cross-section are shown in Figure 1b.

Figure 3

(a–d): Transmissions of the metasurface at y- and x-polarized THz incidence when Δd varies from 0 to 25 μm or Δr from 0 to 35 μm. (e) Q-factors of resonances I, III, V, and VII vs. α_d. (f) Q-factors of resonances II, IV, VI, and VIII vs. α_r.

Figure 4

(a–d) Simulated and measured transmissions of metasurfaces under y- and x-polarized THz incidence. (e,f) Microscope images of the fabricated metasurfaces, where Δd = 25 μm and Δr = 25 μm, respectively.

Figure 5

Resonance frequency with respect to Δd or Δr for different Λy when Λ_x = 150 μm. (a) Resonance V. (b) Resonance VI.