Nanomechanical Plasmon Spectroscopy of Single Gold Nanoparticles.

We experimentally demonstrate the effect of the localized surface plasmon resonance (LSPR) of a single gold nanoparticle (AuNP) of 100 nm in diameter on the mechanical resonance frequency of a free-standing silicon nitride membrane by means of optomechanical transduction. We discover that a key effect to explain the coupling in these systems is the extinction cross section enhancement due to the excitation of the LSPR at selected wavelengths. In order to validate this coupling, we have developed a fixed wavelength interferometric readout system with an integrated tunable laser source, which allows us to perform the first experimental demonstration of nanomechanical spectroscopy of deposited AuNPs onto the membrane, discerning in between single particles and dimers by the mechanical frequency shift. We have also introduced three-axis mechanical scanners with nanometer-scale resolution in our experimental setup to selectively study single nanoparticles or small clusters. Whereas the single particles are polarization-insensitive, the gold dimers have a clearly defined polarization angle dependency as expected by theory. Finally, we found an unexpected long-distance (∼200 nm) coupling of the LSPR of separated AuNPs coming out from the guided light by the silicon nitride membrane.