The effect of etching time was studied by etching Au NP (size G 1 ) coated and bare silica substrates in 50 vol. % HF solution for different periods from 5 to 30 min (in steps of 5 min). Similar experiments were conducted for the substrates coated with Au NPs of different sizes G 2 to G 4 . The effect of HF concentration was studied by varying the HF concentration (in vol %) as 10, 20, 30, 40, and 50%. Both bare and Au NP (G 2 ) coated silica were etched with these concentrations for 30 min. Post etching, Au NPs were completely removed by treating with Aqua regia (HCl (35%) and HNO 3 (65%) in 4:1 volume ratio) for 1 h. The transmittance spectra (in the visible region) were recorded for the films in different stages.