(a) Photo-acoustic spectra of Si₃N₄–Si and Nano-Au–Si₃N₄–Si at 54 Hz and 408 Hz, normalized to the bare Si spectra obtained at the same frequencies. (b) Experimental absorption difference between Nano-Au–Si₃N₄–Si and Si₃N₄–Si, normalized to maximum, at 54 Hz and 408 Hz. (c) Simulations of absorption differences between Nano-Au–Si₃N₄–Si and Si₃N₄–Si, where the partical absorption was integrated in over 10 μm or 5 μm of Si depth and including the upper plasmonic layers for the nanostructured sample. (d) Absorption confinement differences between Nano-Au–Si₃N₄–Si and the substrate over first 10 μm of depth, at the absorption difference dip at 755 nm and peak at 894 nm. Insets plot the electric field intensity around nanohole.