Design and fabrication of NP-PCAs. (a) Schematic diagram of the NP-PCA. The circular image is a partial magnification of the nanopillars, the period of the unit cell: p = 350 nm, the height of the unit cell: h = 230 nm. (b) Microscopy photo of the fabricated PCA with the nanopillar arrays. (c) Hierarchical fabrication procedure for the NP-PCAs with SI-GaAs nanopillars. PR: photoresist. The purple color of PR is positive electron beam resist and the red color of PR is positive photoresist S1813.