Photothermal analysis of sub-micrometric scale defects in laser damage studies

Laser-induced damage has long been widely acknowledged as a localized phenomenon associated with the presence of defects such as nodules, scratches, fractures, polishing or cleaning residues, impurities, contaminants, metal or dielectric inclusions, etc. Destructive investigations in ultra pure fused silica have led to the conclusion that defects, typically a few nanometers in size, were responsible for laser damage initiation. The understanding of damage phenomena requires the development of more sophisticated, non destructive tools with both high spatial resolution and high sensitivity, to detect defects as small as possible. Photothermal microscopy has been widely employed to characterize optical absorption, thermal properties of optical materials and for mapping defects. This technique has been coupled and compared with scattering mapping for studying laser damage processes before and after irradiation. Furthermore the capability of collinear photothermal deflection to reach sub-micrometric resolution by reduction of the pump beam diameter has been theoretically explored and experimentally demonstrated on specially prepared absorbing targets. A photothermal microscope based on photothermal deflection of the transmitted beam and well-suited for multi-scale studies of absorbing defects in thin films has been coupled with an experimental set-up allowing damage threshold measurement at the same wavelength. We present an overview of these developments in the field of photothermal microscopy and scattering mapping in connection with laser damage.