Optical Constants of Ultra-Short-Pulse Laser Heated Metal

The complex refractive index (n + ik) of ultra-short-pulse laser heated gold metal was measured with a new ellipsometric pump-probe technique. Two ratios of four different probe-beam polarizations were used to determine s- and p-reflectivity and their phase difference. In the early stage of heating, only the imaginary part of the dielectric constant ε(Im[ε] = 2nk) increased while real part of e (Re[ε] = n²-k²) was almost constant. This agrees with the Drude model. We observe a pause in the rise of Im[ε] at the boiling temperature. Beyond this point, Re[ε] started to change and the observed parameters disagree with the Drude theory. It was found that the parameters of the expanding plasma follow a unique trajectory in n, k space, even though time variation of Re[ε] and Im[ε] are different for various pump intensities. This means the gold remains in a unique state such as the neutral groundstate in this intensity range.