Etching experiments were also carried out on silica substrates coated with Au NPs of increasing average size, from G 1 to G 4 respectively. Transmittance at 550 nm is calculated and plotted for different etching times, as shown in Figure (a). The variation of surface roughness with etching time, for different-sized Au particles, is also plotted in Figure Etching was carried out in 50% HF for times ranging from 0 to 30 min. All measurements were taken post-NP removal. and surface roughness are compared with that of bare silica etched under the same experimental conditions.