Laser-induced damage mechanisms were investigated for an ultra-broadband chirped mirror, as part of a systematic
study of few-cycle pulse laser-induced damage threshold (LIDT) of widely-used ultra-broadband optics,
in vacuum and in air, for single and multi-pulse regimes (S-on-1). Microscopic analysis of damage morphology
suggests that three different damage mechanisms occur across the fluence range 0.15-0.4J/cm2, while no ablation
was yet observed. The three regimes resulted in shallow swelling (< 10 nm tall), tall blistering (~ 150 nm
tall), and annular blistering (damage suppressed at highest intensity, forming a ring shape). Descriptions of the
potential mechanisms are discussed.