Fig. 1. (a) Illustration of a unit cell of the metasurface with the geometric parameters and asymmetry parameters. (b) SEM image (top view) of a representative Si metasurface on a SiO2 substrate (without PMMA/dye). (c) The top panel represents the top view of one unit cell of the metasurface, and the bottom panel is the side view of the sample structure. (d) Normalized PL emission (red) and extinction (1-transmission) (blue) of a layer of dye molecules in PMMA.

Fig. 2. Measured and simulated extinction circular dichroism of metasurface S0 with a ∼300  nm thick PMMA layer on top. (a), (b) Measured extinction spectra as a function of the incident wavevector parallel to the surface of the metasurface for right-handed and left-handed circularly polarized light, respectively. The four distinct modes observed in the extinction measurements are indicated in (a). (c) Experimental dispersion of the circular dichroism. (d), (e) Simulated extinction dispersion for right-handed and left-handed circularly polarized light, respectively. (f) Simulated dispersion of the circular dichroism.

Fig. 3. Dispersion of photonic modes in the investigated metasurface, calculated with the guided-mode expansion. The symmetry points in the 2D Brillouin zone are Γ=(0,0), X=(2πax,0), Y=(0,2πay). The inset shows a zoom of the region around k=0, with the dominant dipolar character of the modes and the symmetry labels explained in Appendix B.

Fig. 4. Near-field distribution of the electric field for the four studied modes, numerically calculated with COMSOL. The color scale represents the magnitude of the electric field, |E|2, and the arrows represent the electric field direction and magnitude. The projection planes, xy and xz, are chosen to better illustrate the character of each mode: xy-even modes are plotted on the xy plane, while xy-odd modes are projected onto the xz plane, in both cases containing the centers of both disks.

Fig. 5. Measured circular dichroism in the emission of dye coupled to metasurfaces. (a), (b) Experimental dispersion of the PL emission for right- and left-handed circularly polarized light from a PMMA/dye (25% dye, in mass fraction) layer with a thickness of ≈200  nm on top of a substrate. (c) DCP of the PL emitted from this layer. (d), (e) Experimental dispersion of the right- and left-handed circularly polarized PL emission from a PMMA/dye layer on top of the metasurface. (f) DCP of the PL emitted from this layer.

Fig. 6. (a)–(e) Experimental dispersion of the DCP from the metasurfaces S1 to S5 covered with the PMMA/dye film. Along with the increase of size, distance, and lattice from S1 to S5, the position asymmetry parameter increases but the size asymmetry parameter is nearly constant.

Fig. 7. Experimental RCPL (orange curves) and LCPL (blue curves) intensities at a wavelength of 620±10  nm as a function of the emission angle along y for metasurfaces S1 (a), S2 (b), S3 (c), S4 (d), and S5 (e).

Fig. 8. (a) Schematic illustration of the Fourier setup for extinction measurements. (b) Schematic illustration of the Fourier setup for PL measurements. Obj_1 is a 60× or 100× objective lens and Obj_2 is a 40× objective lens. LP1 is a linear polarizer and QWP1 is a quarter-wave plate (Thorlabs FR600QM). LP2 is a linear polarizer and QWP2 is a quarter-wave plate (Thorlabs AQWP05M-600). LPF is a long-pass filter (550 nm). BPF is a bandpass filter (Thorlabs FBH620-10). The excitation source for the PL is a pulsed laser with a central wavelength of 400 nm.

Fig. 9. Schematics of dipole orientation for py, pz, my, and mz modes.

Fig. 10. Schematic structure with unit cell, and character tables of the corresponding point group, for (a) the size-detuned structure with point group C2v, supporting two BICs, and (b) the size- and position-detuned structure with point group C1v. The symmetries of electric (p) and magnetic (m) dipole components are indicated. Panel (b) shows also the compatibility relations for the reduction of symmetry from C2v to C1v.

Fig. 11. Optical microscope image (a) and PL image (b) of a corner of S0 with the dye/PMMA layer on top. The off-the-array region corresponds to the dye/PMMA layer on top of the substrate.

Fig. 12. Experimental dispersion of the RCPL (a), LCPL (b), RCPL enhancement (c), and LCPL enhancement (d) from the PMMA/dye films on samples S1 to S5. The horizontal white dashed lines in (a) and (b) indicate the center wavelength position of the bandpass filter (FBH620-10, Thorlabs) used for the measurements shown in Fig. 7.

Fig. 13. Simulated intensities for RCPL (orange curves) and LCPL (blue curves) at a wavelength of 620 nm as a function of the angle along y for metasurfaces that represent the trends of samples S1–S5, like in Fig. 7. They are obtained from sample S0 by scaling the 2D parameters (d, L, and a) with scale factors s=0.95 (a), s=0.99 (b), s=1.04 (c), s=1.12 (d), and s=1.16 (e).

Fig. 14. Simulated angles of the photonic mode M4 (py) as a function of the scaling factor. The data are extracted from Fig. 13. The orange and blue curves represent RCPL and LCPL, respectively. The wavelength is fixed at 620 nm.