Thermal and shock induced modification inside a silica glass by focused femtosecond laser pulse

The femtosecond laser induced structural modification was studied by the detection of a laser induced pressure wave using a transient lens (TrL) method. The TrL signals observed at various excitation pulse energies showed that there were two thresholds of the pulse energy for the modification. Above the higher threshold, a pressure wave was observed clearly, and the amplitude of the pressure wave increased with increasing excitation pulse energy. In addition, Raman spectra at the laser irradiated region showed that the compact silica ring structures increased in the photoexcited region above the higher threshold, which suggested that the photoexcited glass was densified by a shock due to a pressure wave generation. In the energy region between the lower and higher thresholds, a pressure wave was not detected but a temporally constant refractive index change was observed. From the amplitude of the change, the temperature elevation just after the photoexcitation in this energy region was estimated to be about 1400 K. We interpreted that fast cooling from the high temperature is responsible for the modification between the lower and higher thresholds. The two modifications identified in this study should correspond to two types of damage inside a silica glass which have been observed previously. This is the first study that elucidated the difference of two types of modification from the time-resolved observation of the dynamics.