The Mechanism Of Laser-induced Damage In Porous Films

The development of laser technology toward higher power density makes a corresponding higher demand for the resistance of optical films to laser radiation. Generally, porous films have good performance in resistance to laser-induced damage (LID). So to study the mechanism of LID in porous films is of valuable and significant for the development of films with high LID threshold.Several methods are experimented to prepare porous films. Thermal effect of porous films is analyzed and mechanism of stress release is presented. Also the effect of the change of band structure induced by dimensional effect on electron ionization in porous films irradiated by short pulse laser is discussed briefly. The main contents are outlined as follows:Porous films with different structure are prepared by sol-gel method and anodic oxidation method respectively. E-beam PVD method is also used to deposit porous films and the results show that further study should be done on this subject. The dimensional effect and the effect of porous structure on heat conduction in porous films irradiated by longer pulse width laser are analyzed by means of Fourier's law, Boltzmann transport theory and phonon phenomenological theory.The temperature distribution and evolution is analyzed by means of transient heat conduction models with features of short pulse time of laser and low effective heat conductivity of porous films. The temperature rise is much higher in porous films than in solid films due to the comparely low heat conductivity, which means more important effect of temperature rise on the LID damage in porous films than in solid ones.The distribution of thermal stress in porous films is described by means of thermal stress theory and elasticity theory. Aim to avoid the stress damage initialized irrespectively by phase transformations or inclusions, corresponding stress release mechanisms are presented to explain the reasons why porous films have better resistance to stress damage than solid films. The pressure of vapor due to local sharp temperature rise usually resulting in a drum break can be eliminated by the mechanism of pore release pressure which means that the pressure is eliminated by release of vapor through the pores of films. The thermal stress induced by inclusions can be partly or fully eliminated by micro-deformation or local micro-damage, which is called stress release mechanism of micro-deformation and micro-damage.The change of band structure may be initiated by micro-dimension of porous films, which may further effect the electrons ionization under irradiation of shorter pulse laser. The preliminary discussion on that is presented in this paper.