Silicon Mirrors for High-Intensity X-Ray Pump and Probe Experiments

An all-x-ray pump and probe capability is highly desired for the free-electron laser community. A possible implementation involves the use of an x-ray mirror downstream of the sample to backreflect the pump beam onto itself. We expose silicon single crystals, a candidate for this hard-x-ray mirror, to the hard-x-ray beam of the Linac Coherent Light Source (SLAC National Acceleration Laboratory) to assess its suitability. We find that silicon is an appropriate mirror material, but its reflectivity at high x-ray fluences is somewhat unpredictable. We attribute this behavior to x-ray-induced local damage in the mirror, which we have characterized post mortem via microdiffraction, scanning electron microscopy, and Raman spectroscopy. We demonstrate a strategy to reduce local damage by using a structured silicon-based mirror. Preliminary results suggest that the latter yields reproducible Bragg reflectivity at high x-ray fluences, promising a path forward for silicon single crystals as x-ray backreflectors.

Figure 1

Schematic view of the experimental configuration.

Figure 2

SEM image of a crater in $500\text{−}\mu \mathrm{m}$-thick silicon (100) induced by an LCLS shot at full fluence (a). Bragg signal measured via XRMD along a lineout across the crater (b). Intensity is normalized to the single-crystal silicon reflectivity $R_0$.

Figure 3

Bragg signal measured via XRMD across five craters generated by LCLS shots at variable x-ray fluence: 0.8 (E), 1.4 (D), 2.6 (C), 5.5 (B), and 10.0 (A) $\mathrm{kJ}/{\mathrm{cm}}^2$. Intensity is normalized to the single-crystal silicon reflectivity $R_0$. Additionally, a baseline is removed from the data.

Figure 4

Radius of the damaged area around a crater as a function of x-ray fluence with the corresponding linear fit.

Figure 5

SEM image of pillar structure: Pillars are $100\mu \mathrm{m}$ wide and separated by $100\mu \mathrm{m}$. (Inset) Enlarged view of the LCLS-exposed pillar and neighboring pillars; the red arrow indicates the direction of XRMD lineout shown in Fig. 6, while the orange dots represent the locations of Bragg reflectivity measurements shown in Fig. 6.

Figure 6

XRMD lineout across three pillars neighboring the LCLS-exposed pillar (a); Bragg peak measured on the four pillars neighboring the LCLS-exposed pillar (b).