Shockless compression of silicon using high-power laser*

Shockless compression experiments for single-crystal silicon were performed using a high-power laser. Silicon is a typical interesting material exhibiting polymorphism, and the phase diagram is not understood well. For instance, Si is predicted to undergo a metallization transformation under compression. A polyimide reservoir target (75 $\mu$m) was irradiated with the GEKKO/HIPER laser ($\lambda =$ 0.35 $\mu$m), the reservoir plasma expanding to a vacuum gap ($\sim$ 200 $\mu$m) and colliding with a Si sample (20-30 $\mu$m). The rear-surface of Si was observed with 2-channel velocity interferometer system (VISARs) and a streaked optical pyrometer. The change of reflectivity from the VISARs may indicate phase transitions under the continuous quasi-isentropic compression. We also for the first time recovered the Si target shocklessly compressed by high-power laser. The structure of the recovered sample was analyzed using a large synchrotron radiation facility.