The polystyrene beads suspension was spread evenly by dispensing 90μL of the solution on the cleaned substrate and spin-coated (100 rpm for 60s, 280 rpm for 35s, 700 rpm for 40s, 1200 rpm for 15s). The polystyrene beads size was reduced to 300nm diameter using PECVD plasma etching (50 sccm O2, 40 mTorr, 30W, 7 min). A 45nm of silver film was thermally deposited onto the surface (0.2Å/s).