Solid to Plasma Transition in Fs-Laser-Irradiated Fe: Collapse of the Spin-Orbit Gap

Abstract

Quantitative measurements of the optical conductivity of iron under earth core conditions are important in modelling geomagnetism¹. We approximate such conditions transiently² by exciting an Fe, and a control Al, surface in a vacuum or helium environment with 620 nm, 120 fs FWHM laser pulses with 10⁵ peak-background contrast ratio at .6 ps focussed to peak intensities 10¹¹ < I < 10¹⁵ W/cm² on target. Figs. 1 and 2 present p- and s- polarized self-reflectivity R _p,s (θ, I) for constant incident angle θ and peak intensity I, respectively. Geometric correction for the dependence of spot shape on θ has been made in plotting the data. Using very linear pulse energy reference monitors, reproducibility to within ∆R/R ≤ 0.1% was achieved. This allowed measurement of very slight reflectivity changes, as shown in the inset of Fig. 1. This figure also shows that the reflectivities of Fe and Al in the solid to plasma transition region (10¹³ – 10¹⁵ W/cm²) approach each other as I increases, consistent with their similar total conduction electron densities (n _e ≃ 1.8 × 10²³ cm⁻³) when the Fe d-electrons are included. This suggests qualitatively that progressive unbinding of the d-electrons dominates the changes in optical properties of Fe in this regime. Fig. 2 provides evidence that a density gradient develops during the laser pulse. It is well known³ that θ _min the angle at which the minumum p-polarized reflectivity occurs, shifts toward smaller angles as the density gradient scale length grows. Although the largest angle of incidence attainable in the experiment was smaller than θ _min under these conditions, the data clearly indicate a shift toward smaller values, seen for Fe in Fig. 2 as a crossing of the p-polarized data sets.