Fig. 6

3D subwavelength structured photonic crystals fabricated by SC-PPI lithography. a Schematic of 3D woodpile structure with lattice constant a and axial period √2a. The SEM image (b) and the magnified SEM image (c) of the woodpile structure fabricated by SC-PPI lithography with a feature size of 58 nm, and a lattice constant of a = 250 nm, under a writing speed of 1 mm s^− 1. For comparison, the smallest lattice constant of 3D woodpiles printed using PPI lithography was reported to be 275 nm with a writing speed of 0.1 mm s^− 1 (ref. [22]). d Reflection-mode optical micrographs of the 3D woodpile structure