Observation of Subpicosecond X-Ray Emission from Laser-Cluster Interaction

We present the first experimental evidence of the subpicosecond duration of x-ray pulses emitted from laser-irradiated clusters, demonstrating the suitability of such a debris free target for ultrafast x-ray science applications. The $K$-shell emission ($\sim 3\mathrm{keV}$) from large Ar clusters ($6\times {10}^5$ to $4\times {10}^6$ atoms) is time resolved, when irradiated by ultrashort (40 fs to 5 ps) and intense laser pulses (${10}^{15-17}\mathrm{W}/{\mathrm{cm}}^2$). The observations are supported by hydrodynamical and collisional-radiative calculations, that reproduce the extremely short x-ray pulse duration.

Figure 1

X-ray emission spectrum recorded by the broadband spectrometer. 275 Å average radius Ar clusters are irradiated by 2.0 mJ, 320 fs FWHM, linearly polarized laser pulse. The laser intensity is $7.5\pm 2.5\times {10}^{15}\mathrm{W}/{\mathrm{cm}}^2$. $25\mu \mathrm{m}$ Be and six other filters, each composed by $6\mu \mathrm{m}$ Mylar and $0.3\mu \mathrm{m}$ Al, are set up in front of the detector.

Figure 2

Temporal profile of the x-ray pulse (with arbitrary origin). 275 Å average radius Ar clusters are irradiated by 2.0 mJ, 320 fs FWHM, linearly polarized laser pulse. Full line: x-ray measurement. Dashed line: 0.8 ps rms instrumental response as measured on the same setup with third harmonic generated from 40 fs laser pulses.

Figure 3

X-ray emission duration measured as a function of the laser pulse duration. Ar clusters with 180 Å (circles), 275 Å (squares), and 350 Å (triangles) average radius are irradiated by 2.0 mJ, linearly (black) and circularly (white) polarized laser pulses. Full line: guiding line equating the x-ray pulse duration to the laser pulse duration. Dashed line: time-integrated $K$-shell x-ray emission from 275 Å Ar clusters.

Figure 4

Laser energy absorption measured in the cluster jet as a function of the laser pulse duration. Ar clusters with 180 Å (circles), 275 Å (squares) and 350 Å (triangles) average radius are irradiated by 2.0 mJ, linearly polarized laser pulses.

Figure 5

Black squares: x-ray emission duration calculated as a function of the laser pulse duration from a 275 Å average radius Ar cluster. Full line: guiding line equating the x-ray pulse duration to the laser pulse duration. White squares and dashed line: time-integrated $K$-shell x-ray emission deduced from the calculation.