Infra-red microscopy study of laser-driven phase transformations in fused silica

Vitreous silica is the material of choice for transmissive and diffractive optics in high-power laser chains. The intensity delivered by high-power lasers for inertial confinement fusion (ICF) is limited by optical breakdown in the silica [1]. Laser-induced damage (LID) in fused silica windows is often observed to initiate at the rear surface. The LID threshold (LIDT) defines the fluence where permanent damage is observed at the surface of the fused silica window. Once damage is initiated, it propagates at sub-threshold fluence upon repetitive irradiation [2]. At higher fluence, damage initiates at the front surface. Damage at the rear surface is explosive and is characterized by extensive material cracking. At the front surface the laser beam drills a smooth-walled channel and redeposited material is observed around the drilled channel (Fig. 1) [2].