Determination of the Ion Temperature in a Stainless Steel Slab Exposed to Intense Ultrashort Laser Pulses

We present an effective approach to determine the amount of energy absorbed by solid samples exposed to ultrashort laser pulses, thus, retrieving the maximum temperature attained by the ion lattice in the picosecond time scale. The method is based on the pyrometric detection of a slow temperature fluctuation on the rear side of a sample slab associated with absorption of the laser pulse on the front side. This approach, successfully corroborated by theoretical calculations, can provide a robust and practical diagnostic tool for characterization of laser-generated warm dense matter.

Figure 1

(a) Sketch of the experiment. (b) Schematic description of the “image sources” method applied to an infinite sample slab. The locations of the real source ($•$) and the pyrometer detection center ($\star$) as well as the nearest image sources ($\circ$) are indicated.

Figure 2

IR pyrometer voltage signals recorded on the rear side of the stainless steel slab (thickness: $5\mu \mathrm{m}$) as a function of time after absorption of a 200 fs laser pulse on the front side. Measurements have been carried out at three different laser pulse energies ($E_p$): 2.9, 2.0, and 1.2 mJ. The curves for $E_p=2.0\mathrm{mJ}$ and $E_p=1.2\mathrm{mJ}$ have been shifted, respectively, by $-0.4$ and $-1.2\mathrm{mV}$ to ameliorate readability of the figure. The pyrometer response function for a delta-like excitation is shown within the inset.

Figure 3

Simulated temperature profile [Eq. (2), $E_a=15.8\mu \mathrm{J}$, $s=38\mu \mathrm{m}$] on the rear side of a stainless steel foil, as a function of time and radial distance from the center of the laser axis.

Figure 4

The pyrometer voltage shown in Fig. 2 for 3 different laser fluences is fitted by the theoretical curve $V(t)$ (details within the text). The curves for $E_a=15.8\mu \mathrm{J}$, and $E_a=8.1\mu \mathrm{J}$ have been shifted, respectively, by $-0.4$ and $-1.2\mathrm{mV}$ to ameliorate readability of the figure.