Nonthermal component in heat-induced structural deformation and phase transition in gold

Using an ultrafast optical pump-probe technique, we study the dynamics of the heat-induced structural deformation and phase transition in gold at early time delays (within 1 ps after excitation) by monitoring both the fundamental and second-harmonic generation (SHG) signals of the probe beam. The ratios of the time-resolved fundamental and SHG signals with respect to low-intensity reference curves reveal clear changes immediately following pump excitation for intensities above 0.9X the melting threshold that is absent for lower intensities. These observations indicate a disordered lattice due to electronic heating and suggest that a nonthermal component exists in the heat-induced structural deformation and phase transition in gold.