Nonlinear optical properties of gold with intense femtosecond laser excitations above the damage threshold

Abstract

The nonlinear optical properties of gold thin films have been studied under the irradiation of destructive laser pluses. The transmissivity and the reflectivity of a 100-nm-thick gold film are measured with femtosecond laser irradiations up to 106 J/m2 fluence, which is much higher than the damage threshold of gold. The nonlinearity is calculated with Fresnel loss equations. The real part of the complex refractive index is calculated to have a peak at ∼10 kJ/m2, which is close to the damage threshold. The complex third-order nonlinear susceptibility χ3 is estimated to be (4.95 − 2.02i) × 10−21 m2/V2, which agrees with the trend in previous reports that it decreases as the pulse duration decreases. This nonlinearity is further studied with a plasmon-photon exchange (PPE) model, where laser induced plasmons and absorbed photons are strongly correlated. The result of the simulation with the PPE model is in good agreement with the experimental transmissivity above the damage threshold. The model predicted an interaction time between photons and plasmons to be about 500 fs. This model is found to be essential in understanding such nonlinear optical processes under both extremely high and conventionally low laser irradiations.