Plasmonic enhancement of photoacoustic strain-waves on gold gratings

In this paper, we report on the time-dependent strain-wave-induced changes in the reflection and diffraction of a gold plasmonic grating. We demonstrate efficient excitation of strain waves using enhanced absorption at and around the surface plasmon polariton resonance. In addition, we observe that the strain-wave-induced changes in the reflection and diffraction of the grating show an approximately quadratic dependence on pump fluence when probed at a wavelength of 400 nm. We tentatively attribute this non-linear behavior to strain-induced nonlinear changes of the interband transition energy. Using a model that calculates the permittivity of the gold taking into account the d to s/p interband transition, we deduce that the interband transition energy would have to change by about 0.013 eV to account for the measured changes in reflection.