Plasmon-enhanced two-photon absorption of infrared femtosecond laser pulses in thin gold films

The transmission of femtosecond laser pulses of the near-infrared range (wavelength of 1030 nm) through nanocrystalline gold films with a variable thickness (15-90 nm) has been experimentally studied for various intensities of laser radiation in the range of 1-10 TW/cm$^2$. A dip in the transmission coefficient at the wave-length of pump pulses at moderate radiation intensities is due to two-photon transitions from $d$ bands with a high density of states to $s$ and $p$ bands with a low density of states and their saturation at higher intensities. The cross section is estimated for two-photon absorption in gold films enhanced by plasmon resonance of gold nanocrystallites at the wavelength of the second harmonic of exciting radiation, which is compared to values for gold films and various plasmon nanostructures known for other spectral ranges.