Fig. 1. Design and simulation results of rotation-induced plasmonic chiral quasi-BICs. (a) Schematic of the plasmonic unit cell, with one of the paired Au bricks rotated by an angle of θ. Here, θ is defined as positive (negative) when the brick is rotated in the clockwise (counterclockwise) direction. (b) Calculated Q factors as a function of rotation angle at the corresponding eigenwavelengths. (c) Correlation between Qrad and sin2(θ); an inverse correlation is achieved between Qrad and sin2(θ) after fitting. (d), (e) Simulated cross-polarized reflection spectra under (d) LCP and (e) RCP excitations. (f) Calculated CD spectrum.

Fig. 2. Normalized electric field distributions and current flows in metasurface unit cells with rotation angles of θ=10° and 50° under (a) LCP and (b) RCP excitations at corresponding resonance wavelengths of 948 nm and 934 nm, respectively. The electric field distributions and current flows are extracted from the top surfaces of the Au bricks (top row) and the middle planes of the SiO2 spacer (bottom row).

Fig. 3. Experimental results of rotation-induced plasmonic chiral quasi-BICs. (a) SEM images of fabricated samples with rotation angles θ of 0°, 30°, and 50°. (b) Measured and (c) simulated cross-polarized reflection spectra RRL under LCP incidence for metasurfaces with different rotation angles. (d) Measured and (e) simulated cross-polarized reflection spectra RLR under RCP incidence for metasurfaces with different rotation angles.

Fig. 4. Plasmonic chiral BIC metasurfaces for tunable CP light absorption. (a) Simulated and (b) measured absorption spectra for the metasurfaces with different rotation angles under LCP excitation. The black dots indicate the wavelength with CD of ∼0.35 in both simulation and experiment.

Fig. 5. Simulation results of the plasmonic metasurface as an HWP. (a) Schematic of the plasmonic HWP without any rotation. (b) Simulated co- and cross-polarized reflection spectra under LCP excitation. (c), (d) Simulated (c) reflection and (d) phase retardation of the plasmonic HWP under linearly polarized excitations.

Fig. 6. Simulated CD spectra for metasurfaces with varying SiO2 spacer thicknesses and rotation angles.

Fig. 7. Simulated CD spectra for metasurfaces with varying Au antenna thicknesses and rotation angles.

Fig. 8. Simulated CD spectra for different rotation angles with varying dimensions of the right gold brick. (a), (b) CD spectra when the length l of the right brick is (a) 300 nm and (b) 320 nm. (c), (d) CD spectra when the width w of the right brick is (c) 100 nm and (d) 140 nm.

Fig. 9. Simulated CD spectra for different rotation angles with varying dimensions of the left gold brick. (a), (b) CD spectra when the length l of the left brick is (a) 300 nm and (b) 320 nm. (c), (d) CD spectra when the width w of the left brick is (c) 100 nm and (d) 140 nm.

Fig. 10. SEM images of metasurfaces with rotation angles θ of 10°, 20°, 40°, and 60°.

Fig. 11. Experimental setup for optical characterization.

Fig. 12. Plasmonic chiral BIC metasurfaces for tunable CP light absorption. (a) Simulated and (b) measured absorption spectra for the metasurfaces with different rotation angles under RCP excitation.

Fig. 13. (a) Simulated and (b) measured CD spectra with different rotation angles.