Experimental Investigation In Laser Induced Damage To Diamond-Like Carbon Film

The study on laser-induced damage to optical films was an important project in laser technology field. It is a foundation of the development of laser technology on the one hand and an important part of study on interaction between laser and materials on the other hand. So, studying the laser-induced damage properties of optical films is necessary not only for testing the performance of optical films, but also for the research of the mechanism of interaction between laser and materials.Diamond-like carbon (DLC) film is a new material for optical coatings, which has been attended in electronics, biology and military fields because of its excellent properties. It is usually used as a protective and antireflective optical coating because it has both higher optical transmittance on wide spectra and good resistance to chemical attack. As a kind of optical film, the laser-induced damage property is one of the most important parameters.We investigated the laser-induced damage properties of DLC film experimentally on the basic of having analyzed the mechanism and experimental technology of that. At first, the method of He-Ne laser scattering as a diagnosis technology for film damage detecting was assured. Then, the pulsed 1064nm/248nm laser and CW 10.6μm laser were used as the testing source. The obtained conclusions are as follows:1) The 0% probability laser-induced damage thresholds of DLC films (film thickness of 100~200nm) on Si substrate were investigated. Under the pulsed 1064nm laser (effective pulse duration 10ns, effective spot area 4.9mm~2) irradiation, the single pulse damage threshold is about 0.5J/cm~2 and the multi-pulse (20 pulses, repetition rate 1Hz) damage threshold is about O.U/cm~2. The multi-pulse damage threshold is obviously lower than that of single pulse. In the case of the pulsed 248nm laser (effective pulse duration 28ns, effective spot area 1.2mm~2) irradiation, both the single pulse threshold and multi-pulse (20 pulses, repetition rate 1Hz) threshold are about O.U/cm~2. And under the condition of CW 10.6μm laser (irradiation time 2~10s, effective spot area 0.1mm~2) irradiation, the threshold is about 5.0×10~4W/cm~2.2) The position of laser incidence on target is an important factor for damage threshold in the experiment of CW laser irradiation with the small laser spot and finite flat target. It was found that the damage occurred easier on the edge of the target.3) To our knowledge, the main damage mechanism of DLC film irradiated by 1064nm laser is inclusion absorption, and the reason for the higher single pulse damage threshold than multi-pulse damage threshold is the accumulation of slight damage initiated by inclusion. Under the pulsed 248nm laser irradiation, inclusion absorption probably is not a main reason for damage because of the existence of intrinsic absorption of film and substrate. So the multi-pulse effect was not obvious. The further investigation and analysis are needed for better understanding. In the case of CW 10.6μm laser irradiation, the damage of film was caused mainly by the graphitization even vaporization of DLC after the irradiated laser energy was absorbed by the substrate and coating.