(a) Photo-acoustic spectroscopy, allows for tunable polarization, power, modulation frequency, incidence angle; inset: schematic of the two samples. Both have diameter of 165 nm and height of 4.6 μm; Sample C-Au has two out of six sidewalls covered by 20 nm Au. (b) Simulation of a single NW of Sample C, in the air and standing on the Si substrate, and the 3D distribution of the absorption density at the resonance of 780 nm. The Si substrate widens the leaky waveguide mode resonance. (c) Simulation of a single NW covered by Au, and the 3D distribution of the absorption density at 850 nm. Here, the substrate strongly influences absorption as its parts contain absorbing material.