Au NPs of varying sizes were coated on the silica substrates and the corresponding SEM images are shown in Figure . Initially, substrates coated with Au NPs of size 14.7 ± 1.9 nm (G 1 ), were etched with 50% HF solution for different times. The transmittance spectra of the etched substrates, before and after Au NP removal, are shown in Figures ) and S2(b) respectively. For comparison, the spectra of the etched bare silica substrates are also shown in Figure ). To compare the different samples, the observed transmittance at 550 nm (middle of the visible spectrum) is plotted against the etching time in Figure . The surface roughness of the etched substrates, after Au NP removal, is analyzed and plotted against the time of etching in Figure . The data for bare silica is also included for comparison. From the plots (Figure and Figure ), it can be seen that the transmittance exhibits a decreasing trend with etching time. This is true in the case of both bare and NP-coated substrates and is due to the formation of different surface morphologies on the substrate due to etching. It can be seen from Figure (a) that the transmittance of the samples with Au NPs is systematically lower than the samples post-NP removal. This is because the presence of Au NPs on the substrate leads to increased absorption of a portion of the visible light (corresponding to the plasmonic resonance of Au) which results in a lower transmittance than that of the etched substrate post-NP removal. The observed increase in surface roughness with etching time indicates the extensive formation of surface structures on the substrate. To visualize the morphologies of these structures, SEM was used, and the data is presented in Figures ). The surface morphologies that formed on bare silica for the same time periods are also shown in Figure . SEM images reveal the formation of micron-sized pit structures on the substrates after etching. Three different structures are reported for the etching of Au NP-coated silica: small circular pits, elongated pits, and arrays of elongated pits. In contrast, for the etching of bare silica, pits of only two distinct surface structures are observed, as seen in Figure . Some of them are small, pseudo-spherical in shape while others are elongated. Thus, the arrays of elongated pits are reported only with the Au NP-mediated silica etching. The horizontal pit dimension, i.e., the width, is measured using ImageJ software, and multiple pits are measured for each etching time to obtain the average and standard deviation, and the data is plotted in Figure . It is observed that the average pit width increases with etching time. In the case of Au NP masked etching (as shown in Figure ), the elongated pits are initially (at 5 min etching time) far apart and have the smallest pit width, and can be treated as individual pits. With the increase in etching time (even at 10 min), as more material is removed, these elongated pits come closer and by 15 min, they form an array. Beyond 20 min, shorter arrays of larger pit widths are observed and the entire silica surface is covered with a distribution of small and elongated pit structures. Comparing the bare and NP-mediated etched silica, it is clear that the overall pit widths are longer when etching in the presence of NPs. Also, the arrays of elongated pits are seen only in the case of Au NP mediated etching and not seen in the case of bare silica etching (compare Figure and Figure ). Upon increasing the etching time, more etched structures are formed on the surface, with increased pit dimensions, which results in an overall decrease in transmittance. This can be attributed to the increased scattering of light by these etched structures. The increase in surface roughness with etching time can also be attributed to the increase in the surface density as well as the width of these etched structures.