Optical Properties in Nonequilibrium Phase Transitions

Broadband Dielectric Function of Nonequilibrium Warm Dense Gold

“…2 eV show similar transient and quasi-steady-state behaviors as described above. A single ε(ω) plot is presented for 1.2-4 ps since no significant change in ε(ω) is observed in the interval, consistent with the quasi-steady state found in a previous study [22].…”

“…This is due to the breakdown of the uniform slab assumption as target disassembly gives rise to gradients in the expanding foil. The cut-off point at 2.38 eV corresponds to a probe delay of 1.9 ps that is consistent with the disassembly time observed in the earlier experiment [22]. Accordingly, dielectric function measurement using a frequency chirped source offers a new means of probing hydrodynamic disassembly of a heated solid.…”

“…The minimum energy required for d-p transitions is ∼ 2.3 eV. In an earlier study at an excitation energy density of 4.0×10 6 J/kg [22] it was found that the dielectric function at 1.55 eV showed an initial transient consisting of a decrease (increase) in the real (imaginary) part to a minimum (maximum) value in ∼ 600 fs. This was followed by an increase (decrease) to a quasi-steady-state value in ∼ 900 fs.…”

“…Time zero again corresponds to the onset of observed changes in R*, T* at 1.55 eV. For ∆ of 2.2×10 6 and 4.7×10 6 J/kg, the 1.4-2 ps probe delay falls completely within the quasi-steady state duration [22] allowing the dielec- tric function to be determined over the entire spectral range of 1.55-2.6 eV. However, at 1.7×10 7 J/kg no solution to the Helmholtz equations can be found for ε(ω) from the R*, T* data above 2.38 eV.…”

“…Troullier-Martin's optimized pseudopotential is used to represent the ionic core potential with a plane wave expansion truncated at a cutoff energy of 35 Ry. The lattice constant is kept fixed at 7.657 au since earlier measurements indicate the absence of significant expansion during the quasi-steady state of the gold foil [13,22]. ε 2 (ω) is then calculated using the Kubo-Greenwood formula [25] where the atomic core region of wavefunctions are replaced by atomic orbitals to avoid complication arising from the non-locality of the pseudopotential [26].…”