Fig. 1. (a) Schematic of the Jones matrix inverse design algorithm for freeform meta-polarizers optimization; (b) four kinds of freeform polarizers that construct a (top) micropolarizer array and the (bottom) corresponding polarization basis.

Fig. 2. (a) Schematic of the Poincaré sphere location of the target RCP state at the North Pole; (b) iterative evolution process of the four Jones matrix elements for the circular polarizer; inset, diagrams for the initial structure and the evolved final structure; (c) iterative evolution process of the CD and the FoM; inset, iterative evolution of the freeform structure; (d)–(f) transmission spectra of (d) all Jones matrix elements, (e) CD spectrum and (f) ER spectrum of the optimized freeform pattern. The structure parameters of the initial pattern are a period of 600 nm, a height of 500 nm, and a diameter of 360 nm.

Fig. 3. (a), (c), (e) Iterative evolution process of the four Jones matrix elements and (b), (d), (f) polarization dichroism of the metasurface-based elliptical polarizer at three locations of the Poincaré sphere. (a), (b) ψ₀ = 0°, χ₀ = − 9.7°; (c), (d) ψ₀ = 60°, χ₀ = − 9.7°; and (e), (f) ψ₀ = 120°, χ₀ = −9.7°. The structure parameters of the initial patterns are a period of 600 nm, a height of 500 nm; the length and width of the three elliptical polarizers are 310 nm × 230 nm, 330 nm × 250 nm, and 300 nm × 210 nm, respectively.

Fig. 4. (a), (e), (i) Schematic of the finally optimized meta-atom structure; (b), (f), (j) transmission spectra in terms of all Jones matrix elements; (c), (g), (k) polarization dichroism spectrum and ER spectrum; (d), (h), (i) of the freeform elliptical polarizer for polarization parameters. (a)–(d) ψ₀ = 0°, χ₀ = −9.7°; (e)–(h) ψ₀ = 60°, χ₀ = −9.7°; (i)–(l) ψ₀ = 120°, χ₀ = −9.7°.

Table 1. Comparison of the Circular Polarizers with the Reported Studies on Theoretical Circular Polarization ERs