Ultrafast relaxation of electrons probed by surface plasmons at a thin silver film

We present a novel subpicosecond optical transient-reflection method, which employs the surface-plasmon resonance of a thin silver film as a very sensitive probe to study small transients of the effective electron and lattice temperature. By measuring under surface-plasmon resonant and nonresonant conditions, we can discriminate between lattice and electron heating. We find a change of the lattice and electron temperature of respectively, T₋=0. 10 (5) K and T₄=30--90 K for an intensity of 180 W cm^-2 and an electron-phonon energy relaxation time of 670 (70) fs.