Broadband dielectric function of non-equilibrium warm dense gold Page: 4 of 6
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2
1-
0.8- (a)
%0.6 --
c 0.4
N
- 0.2
0
800
5 600
200
- (b)
0
1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0
Photon energy (eV)
FIG. 1: (a) Supercontinuum spectrum and (b) frequency chip
in the supercontinuum probe.
source (Fig. 1(b)) is measured using the Kerr optical-gate
technique [19]. To remove the effect of chirp in measure-
ments, spectral data are binned in 10-nm intervals and
appropriate temporal shifts are applied using Fig. 1(b).
The reflected and transmitted spectra of the supercontin-
uum probe are recorded with two intensified CCD cam-
eras. Spatially and frequency resolved reflectivity R* and
transmissivity T* in the heated region are determined
using in-situ calibration from the unheated region of the
target. These are used to solve the Helmholtz equations
for a gradient-free dielectric slab in accordance with the
Idealized Slab Plasma concept [20] to yield both the real
part, ei(w), and the imaginary part, e2(w), of the dielec-
tric function.
Figs. 2(a) and (b) show the temporal evolution of e(w)
of gold at an excitation energy density of (2.9 0.3) x 106
J/kg. The data have been corrected for frequency chirp
as described above. Time zero corresponds to the on-set
of changes in R* and T* from their room temperature
values. This is found to be the same for all frequencies
in the supercontinuum spectrum after chirp correction.
Also included in the figure are tabulated data of gold at
room temperature [21]. The minimum energy required
for d-p transitions is ~ 2.3 eV. In an earlier study at
an excitation energy density of 4.0 x 106 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. The quasi-steady
state then lasted for another ~ 4 ps before apparent tar-
get disassembly occurred. The time steps in Fig. 2 are
chosen to span a similar duration. The new data at 1.550
-10
-30
-40
20
15
10
5
0
40
30
W20
10
0(a)
Cold (Ref. [21])
150 fs
400 fs
600 fs
800 fs
-12-4 ps
(b) -1.6 1.8 2 2.2 2.4
Photon Energy (eV)2.6
FIG. 2: (a) Ci(w) and (b) C2(w) at different times, and (c)
e2 (w) displayed with an off-set of +5 along the y-axis between
time steps.
eV show similar transient and quasi-steady-state behav-
iors as described above. A single e(Y) plot is presented for
1.2-4 ps since no significant change in E(w) is observed in
the interval, consistent with the quasi-steady state found
in a previous study [22].
For the observed spectral range, ei(w) appears rela-
tively featureless. However, intra-band and inter-band
(d-p) components are clearly discernable in e2(w) below
and above ~ 2.3 eV. They also show substantial enhance-
ments over their room-temperature values. Furthermore,
by displaying e2 (L) at different time steps with an offset
(Fig. 2(c)), it can readily be seen that the intra-band
component shows good agreement with best-fitted Drude
functions [23] that assume frequency-independent elec-
tron collision time and density, except for a small region
around 1.6 eV at the peak of the transient at 800 fs. The
fitting parameters are given in Table 1.
To examine the dependence of E(w) on the excitation
energy density AE, we use measurements made on the
quasi-steady state. The results are presented in Fig. 3.
These are not corrected for frequency chirp and the probe
delay varies from 1.4 ps at 1.55 eV to 2 ps at 2.6 eV. Time
zero again corresponds to the onset of observed changes
in R*, T* at 1.55 eV. For AE of 2.2x 106 and 4.7x 106
J/kg, the 1.4-2 ps probe delay falls completely within
the quasi-steady state duration [22] allowing the dielec-
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Ping, Y.; Hanson, D.; Koslow, I.; Ogitsu, T.; Prendergast, D.; Schwegler, E. et al. Broadband dielectric function of non-equilibrium warm dense gold, article, April 25, 2006; Livermore, California. (https://digital.library.unt.edu/ark:/67531/metadc889993/m1/4/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.