Fig. 1: Schematic of a full-color second harmonic generator made of TP-HMMs.

The inset is an enlarged view of its unit cell. The alternating metal and dielectric materials are Au and ZnO, with the thickness being marked as tm and td, respectively. The HMM width is linearly increased from wt to wb, and the periods along the x- and y-directions are equal and are denoted as p.