Nonlinear energy absorption of femtosecond laser pulses in noble metals

We use calorimetry to determine the energy absorption of femtosecond (fs) laser pulses as a function of incident fluence for Ag, Ag alloys (Ag-Cu and Ag-Pt), and Pt. At low fluences, the measured absorption agrees well with reflectivity data derived from ellipsometry measurements. For Ag and Ag-Cu, the absorbed energy increases nonlinearly with the incident fluence for fluences larger than approximately half of the melting threshold. Near this threshold, the absorption increases by a factor of 3-4. Similar nonlinear absorption is not observed in Pt or Ag-Pt. We propose that the nonlinear absorption is caused by the excitation of d-band electrons below the Fermi surface. For pulse widths longer than 850 fs, the observed nonlinear absorption in Ag diminishes, indicating that diffusive transport and not ballistic transport is the major mechanism of cooling at this excitation level.