Comparisons of x-ray sources generated from subpicosecond laser-plasma interaction on clusters and on solid targets

This is an investigation of the difference between the x-ray emission of nanometer cluster targets and that of solid targets, when they are irradiated by a subrelativistic laser pulse. Special effort is made to provide comparable conditions and observables and to emphasize the specific role of target geometry. The behavior of the x-ray-emission level with respect to the laser duration shows a clear difference between target types. The x-ray duration is observed to be one order of magnitude shorter with clusters than with solid targets. The observations are reproduced by numerical simulations that corroborate the effect of the faster density and temperature decrease of clusters on the x-ray emission, as well as the ability to produce subpicosecond x-ray pulses for time-resolved applications.

Figure 1

Schematic top view of the experimental setup: (a) solid target geometry and (b) with a cluster jet target. A $\sim 100$-mJ femtosecond laser pulse with adjustable duration is focused on different x-ray conversion targets. The induced x-ray emission is collected and spectrally resolved along two axes. More details are provided in the text.

Figure 2

Some $K$-shell spectra measured at different laser durations in (a) a NaCl solid target ($94\pm 6$ mJ on target) and (b) an Ar cluster jet ($100\pm 3$ mJ) operated at an upstream pressure of 52 bars. Spectra are vertically shifted for clarity and corresponding baselines are drawn.

Figure 3

Intensity of the $\text{He}\alpha$ line as a function of the laser pulse duration. The Ar (circles) and Cl (squares) data are fitted by arbitrary functions described in the text (dotted lines).

Figure 4

Laser durations $\mathrm{\Delta }{t}_{\text{opt}}$ that optimize the x-ray emission, as a function of the cluster average radius $R$. Values are reported for Ar and Xe clusters irradiated with similar laser fluence. Here $R$ is evaluated from the indicated upstream pressure (see the text for details). The Ar dotted line is obtained from the empirical law reported in Ref. [17]. The Xe dotted line is a linear fit of the data obtained for up to 17 bars.

Figure 5

Some $M$-shell spectra measured at different laser durations in (a) a CsI solid target ($82\pm 5$ mJ on target) and (b) a Xe cluster jet ($95\pm 5$ mJ) operated at an upstream pressure of 17 bars. Spectra are vertically shifted for clarity and corresponding baselines are drawn. The copper $L3$ absorption edge is indicated at 0.9325 keV (dashed vertical line).

Figure 6

Evolution of the x-ray emission near the copper $L3$ absorption edge with the laser pulse duration. The Xe (circles) and CsI (squares) data are fitted by arbitrary functions described in the text (dotted lines).

Figure 7

X-ray emission from the Xe cluster jet, measured near the copper $L3$ absorption edge as a function of the upstream pressure. The values up to 17 bars are fitted linearly (dotted line).

Figure 8

(a) The XANES spectra registered near the $L3$ edge of a 80-nm-thick copper sample. The cold spectrum (300 K) is obtained without the pump pulse. The hot spectrum is recorded at a delay of 10 ps after the sample is heated by a $6-\mathrm{J}/{\mathrm{cm}}^2$ pump pulse. The observed pre-edge is evaluated by spectral integration (shadowed area). (b) Evolution of the pre-edge with the pump-probe delay, when using the $M$-shell emission from the Xe cluster jet and the CsI solid target, as the x-ray probe. The associated lines are best fits described in the text. The result expected from a two-temperature model is plotted for comparison.

Figure 9

Spatiotemporal maps of the electron temperature $T_e$ resulting from one-dimensional hydrodynamic calculations of the argon target heated with the same laser fluence as in the experiment reported in Fig. 2 for (a) the 100-nm-thick Ar foil heated with a 5-ps FWHM laser pulse duration and (b) the 10-nm-radius Ar sphere (cluster) with 500-fs duration. The initial density is the solid one. Note the different scales for the $x$ axis (time) and the $y$ axis (spatial coordinate and along the foil thickness and the cluster radius). The $T_e$ values are indicated by the common color scale.

Figure 10

The $K$-shell x-ray spectra postprocessed from the two simulations of Fig. 9, after spatial and temporal integration, compared with experimental spectra registered in similar conditions for (a) the NaCl solid target and (b) the Ar cluster. The laser durations are indicated in the figures.

Figure 11

Comparison between calculated and measured $M$-shell x-ray emission from the CsI solid target (experiment) and the Xe solid foil (calculation for photon energy $\sim 980$ eV), as a function of the laser duration. (a) X-ray-emission level. The calculated values are adjusted by a constant factor that accounts for the unknown number of emitting ions in the plasma. (b) X-ray-emission duration.