Fig. 1. All-optical modulation of polarization in the chiral quasi-BIC metasurface. (a) Schematic of modulation of polarization by femtosecond laser pumping. Reflected light changes from LCP to elliptical polarization (EP), owing to the dynamics of the photoexcited carriers in the α-Si chiral metasurface. (b) The evolution of polarization ellipses and polarization states on the Poincaré sphere after introducing the refractive index change (Δn = −0.04 and Δk = 0.03). (c) Top and side views of the unit cell of the chiral metasurface. The unit cells consist of two slanted α-Si elliptical cylinders. The photoresist above and the silica substrate below ensure rotational symmetry in the x–z plane. The geometric parameters are p = 400 nm, h = 205 nm, l = 200 nm, s = 100 nm, d = 180 nm, θ = 10°, and β = 23°.

Fig. 2. The evolution of eigenpolarization. (a) Side view and top view of the unit cell with different deformation parameters of θ = 0° and β = 0°, θ = 0° and β = 23°, and θ = 10° and β = 23°. (b) Near-field electric field distribution in the designed metasurfaces. Polarization maps in the vicinity of the Γ point of structures with deformation parameters of (c) θ = 0° and β = 0°, (d) θ = 0° and β = 23°, and (e) θ = 10° and β = 23°.

Fig. 3. (a), (b) Simulation results of the angle-dependent LCP and RCP transmission spectra. The Γ points are indicated with arrows. (c) Simulated reflection spectrum of normally incident LCP light on the sample. (d), (e) Experimentally measured angle-dependent LCP and RCP transmission spectra. (f) Experimental measured reflection spectrum of normally incident LCP light on the sample.

Fig. 4. (a) Reflection spectra with the delay time set as 0 and −0.8 ps, respectively. (b), (c) Spectral–temporal maps of the RCP and LCP components for the chiral metasurface under normal illumination with a 400 nm pump beam. The dashed lines indicate the resonant wavelength. (d) The dependence of ΔS₃ on the time delay.