(a) PAS signal amplitude of samples C, C-Au and the substrate (without Au), excited at normal incidence and modulated with 54Hz, 408 Hz and 625 Hz chopper frequency; the excitation is at normal incidence with linear polarization. All the signals are normalized to the incident laser power at the corresponding wavelength. (b) Normalized PAS signal for both nanostructured samples is obtained by subtracting the substrate signal from the NW signal, and normalizing each spectra to its maximum. The resonant modes appear at higher frequencies, and their bandwidth shrinks. (c) PAS difference between the NWs and the substrate for Sample C and Sample C-Au agree with the NW modes predicted by simulations.