The bottom surface texture of the absorbing material is computationally optimized. This diagram is a schematic of one unit cell; there are periodic boundary conditions along the y–z and x–z planes.

(a) Initial AE as a function of frequency, at normal incidence and (b) top-down view of the surface texture; the colors show the height of the absorbing material (from the AR coating to the bottom dielectric, as seen in Fig. 1).

(a) Final AE as a function of frequency, at normal incidence and (b) top-down view of the surface texture; the colors show the height of the absorbing material (from the AR coating to the bottom dielectric, as seen in Fig. 1).

Reciprocal $(k$-) space representation for the final texture seen in Fig. 3. The blue pie slices represent the phase of the complex exponential Fourier coefficients.

Absorption enhancement (AE) for the texture in Fig. 3, plotted as a function of frequency at normal incidence, averaged over both polarizations, (blue) and angle averaged (green).

Absorption enhancement factor (AE) averaged over frequency and polarization, as a function of incident angle ${\theta}$ in the x–z plane (blue) and the y–z plane (red).

(a) Initial AE as a function of frequency, at normal incidence and (b) top-down view of the surface texture, for a symmetric texture with a slight perturbation along the diagonal.

(a) Final AE as a function of frequency, at normal incidence and (b) top-down view of the surface texture, showing broken mirror symmetry, from almost symmetric initial conditions seen in Fig. 9.

Reciprocal space representation for the texture with broken mirror symmetry in Fig. 10. The blue pie slices represent the phase of the complex exponential Fourier coefficients.

Optimizations were carried out at periodicities from 50 to 800 nm, in increments of 50 nm. For each periodicity, at least three optimizations were completed for randomly chosen initial starting noise. The AE_{FOM} (absorption enhancement for the worst performing frequency and polarization at normal incidence) is plotted for each optimization.

AE _{FOM} (minimum absorption enhancement at normal incidence) plotted for 100 randomly generated textures of 710-nm periodicity. For comparison, the AE_{FOM} for the optimized texture in Fig. 6(b) is shown by the dotted red line.

Absorption enhancement (AE) as a function of frequency at normal incidence for the optimized texture from Fig. 6(b) (blue) compared with the median random texture from Fig. 14 (green). Lines are averaged over the two orthogonal polarizations.

Absorption enhancement (AE) as a function of frequency, angle averaged, for the optimized texture from Fig. 6(b) (blue) compared with the median random texture from Fig. 14 (green).

AE _{FOM} (minimum absorption enhancement at normal incidence) plotted for 11 randomly generated textures of 2300-nm periodicity. For comparison, the AE_{FOM} for the optimized texture in Fig. 6(b) is shown by the dotted red line.

A top-down view of the surface texture, for the randomly generated texture with periodicity of 2300 nm = 10 ${\lambda}_{n = 3.5}$, with median AE_{FOM} (minimum absorption enhancement at normal incidence).

Absorption enhancement (AE) as a function of frequency at normal incidence for the optimized texture from Fig. 6(b) (blue) compared with the median random texture with 2300-nm periodicity from Fig. 18 (green). Lines are averaged over the two orthogonal polarizations.

Absorption enhancement (AE) as a function of frequency, angle averaged, for the optimized texture from Fig. 6(b) (blue) compared with the median random texture with 2300-nm periodicity from Fig. 18 (green).