Laser damage prediction under composite multi-wavelength irradiation based on field-thermal-stress coupled model

An analytical model for laser-induced damage under composite dual/multi-wavelength irradiation is established for a multilayer film. It is a field-thermal-stress coupled model that can predict the damage types under varying ratios of the fundamental harmonic (1ω) energy and its complementary third harmonic (3ω). Both the theoretical and experimental results revealed that the film damage is asymmetric on the front and rear surfaces. At a fixed total laser energy, the front surface primarily exhibited field-induced damage when the 1ω energy ratio reached 95%. However, as this ratio decreased to 92% or below, the front surface damage evolved into nanoscale fracture-type damage, dominated by field-stress coupling. In contrast, regardless of the energy ratio, damage sites on the rear surface consistently manifested as micrometer-scale remelted structures, dominated by thermal-stress coupling. The theoretical predictions agree well with experimentally observed damage morphologies and Raman spectral changes, validating the applicability and reliability of the model.