(a) Samples investigated are Si₃N₄–Si (Si substrate with 1,022 Si₃N₄), Au–Si₃N₄–Si (60 nm Au on the top of Si₃N₄–Si), and Nano-Au–Si₃N₄–Si (elliptical nanohole array fabricated in Au on Si₃N₄–Si). (b) Photo-acoustic spectroscopy set-up, enriched with a microscope, allows for independent tuning of parameters: polarization, power, modulation frequency, excitation beam diameter and position; inset: photograph of the set-up. (c) Simulations and raw PAS amplitude spectra measured at 81 Hz for Si₃N₄–Si and Au–Si₃N₄–Si: measured absorption peaks well agree with the calculated reflectivity dips.