Ultrafast pump-probe surface plasmon resonance spectroscopy of thin gold films.

A time-resolved reflection pump-probe method was combined with a surface plasmon resonance technique in Kretschmann geometry for the investigation of ultrafast light-induced processes in thin films. Transient changes in the gold layer's reflectivity were observed when the layer was excited by 3 ps duration pulses with photon energy exceeding the interband transition and by probing with photon energy close to the interband transition. Comparison of the experimental and modeling results has shown that the imaginary part of the dielectric function of gold increases linearly during excitation, whereas the real part remains unchanged. The decay of the light-induced changes has two components. The first component is faster than the pulse duration, and the second is much longer than 1.5 ns; they are related to cooling of the electron plasma and lattice, respectively.