Also the lattice expansion can be detected optically. Due to the expansion
of the lattice, the electron density decreases, which manifests itself, through
a change in the refractive index of the particle, as a red shift of the plasmon
resonance. Since an acoustic vibration is a periodic expansion of the lattice, it
can be detected optically as a periodic oscillation of the electron density, and
thus as a periodic red shift of the resonance