Fig. 2. Topology-optimized structure element.

(A) Schematic of the optimized structure. It consists of freeform amorphous silicon (a-Si) nanostructures patterned on top of a glass substrate. The a-Si thickness is H = 1500 nm. The center-to-center distance between neighboring elements is U = 600 nm. (B) SEM image of a fabricated sample. Scale bar, 1 μm. (C) The arrows represent the angle-dependent eigen-polarization states ($\mid {\mathrm{\lambda }}_{\text{eig}}^{+}(\mathrm{\theta })⟩$) of the device. Different colors correspond to different angles of incidence (θ). As one varies the angle, the device can be continuously tuned between linear and elliptical birefringence. (D) Angle-dependent polarization generation. For a fixed incident polarization (∣in⟩ = ∣H⟩), the output polarization state (∣out(θ)⟩ = J(θ)∣H⟩) changes continuously from right circular polarization through horizontal linear polarization to 45^∘ linear polarization for varying angle of incidence.