Fig. 2: RSP-BIC principle and experimental verification.
From: Optical control of resonances in temporally symmetry-broken metasurfaces
a, Sketch of the unit cell geometry, which consists of two crystalline silicon rods with lengths l1 and l2 and widths w1 and w2, respectively. The dipole moments p1 and p2 along the x axis are equal, resulting in a total dipole moment ptot = 0 for an out-of-phase mode, indicating a SP-BIC condition. b, When w1 is increased, the symmetry is broken (quasi-BIC), and ptot ≠ 0, allowing the mode to couple to the far field. c, Increasing l2 restores the symmetry, returning the system to ptot ≈ 0, the RSP-BIC condition. d, Numerical transmittance spectra of the SP-BIC mode as w1 is varied from 95 nm to 185 nm (left). Tuning l2 from 175 nm to 275 nm for fixed w1 = 185 nm sharpens the mode until it disappears at the RSP-BIC (marked by the grey circle) (right). e, γrad, obtained from TCMT fitting, converges to zero at the SP-BIC and RSP-BIC conditions. f, SEM images of the crystalline silicon metasurface corresponding to the cases shown in a–c. g, Optical images of the two gradient metasurfaces. Left, a w1 gradient. Right, an l2 gradient. h, Experimental spectra matching the numerical results in d. i, Fitted experimental data for γrad, corresponding to the results in e. Exp., experimental; Sim., simulated. Scale bars, 50 nm (f), 20 μm (g).
