The application of photoacoustic technique in studying the laser-polymer interaction

Since, the ablation of organic materials by photo decomposition process using UV lasers have many different applications in fields such as optoelectronics, microelectronics and surgery, therefore an special attention is given to it. The time resolved information together with available mathematical models can provide a better understanding about ablation. As the ablation of materials begin a few nanoseconds after arrival of photons, thus it would be useful if one can study the ablation process in a time scale shorter than the laser pulse duration. In this article, the possible application of PVDF piezoelectric transducer to study the photoacoustic reaction of polymers against excimer lasers is described. Time resolved signals in two areas will be useful I- The thermoelastic region below threshold which gives some information about optical characteristics of target and also thermal relaxation time of excited states, II- Ablation time scale above threshold which provide us information regarding the nature of ablation mechanism. The experiments have shown that PET and polyimide polymers began to ablate within (4-6)ns of ArF pulse width at a fluence higher than 100 mJcm-2. This produced a short acoustic pulse of 20 ns with a maximum pressureof 107 pa (100 atm). Ablation of PMMA using the same laser began within 8 ns. These results show that the polymers with high absorption coefficients are photochemically decomposed and those with low absorption coefficients such as PET at 308 nm, the ablation is brought about by thermal effects