Femtosecond-laser-induced damage initiation mechanism on metal multilayer dielectric gratings for pulse compression

The femtosecond-laser-induced damage behaviors of metal multilayer dielectric gratings (MMDG) for pulse compression are explored. The grating ridge of this type of MMDG consists of a layer of HfO2 sandwiched between two SiO2 layers. The initial damage position is on the HfO2 layer of the ridge which opposite to the laser beam direction. A theoretical model is constructed to explain the femtosecond-laser-induced damage initiation mechanism on the MMDG, and the model can simulate the evolution of the electron density in the conduction band and the change of the dielectric constants of HfO2 and SiO2 in the sandwiched grating structure. The dramatic increase in the imaginary part of the dielectric constant of the middle HfO2 layer indicates that it strongly absorbs laser energy, resulting in damage to the MMDG. The experimental results and theoretical calculation agree very well with each other.