Modification and Dynamics inside Diamond by Femtosecond Laser Double-Pulse Irradiation

We investigated modifications and the temporal evolution of the morphology in the photoexcited region inside diamond single crystal after photoexcitation by tightly focused fs laser pulses. We found that double-pulse irradiation with 200-400 fs laser pulses was able to induce a permanent modification in the photoexcited region, while single-pulse irradiation with 100-300 fs laser pulses was not. The modification by double-pulse irradiation became larger for a longer time-difference between the two pulses in the range from 1 ps to 10 ps. The morphology change observed by a pump-probe optical microscope showed that the photoexcited region by double-pulse irradiation was smaller than by single-pulse irradiation, even for the same total pulse energy, with the ampli-tudes of the laser induced-stress waves being the same. This observation suggests that the photo-induced plasmas by double-pulse irradiation was localized in a smaller region than that by single-pulse irradiation, and the difference in plasma distribution could be the origin of the permanent modification.