503 Angular dependence of coupling between focused laser pulses and a transparent material studied by time-resolved imaging technique

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Short and intense laser pulse can process the surface and the inside of transparent materials by focusing the pulse at desired position. Processing of such a transparent material is considered to be applicable to a lot of practical applications. Details of such process are, however, still unclear and are subjects of further investigations. Here we report on dynamical observation about interaction of fundamental radiation (1064nm) of the Q-switched Nd : YAG laser to the surface of PMMA observed by an imaging system with nanosecond time resolution. The system uses fundamental radiation of a Q-switched Nd : YAG laser as a processing laser and second harmonic radiation (532nm) of another Nd : YAG laser as illuminating light. We observed stress waves which propagate into the material and into the atmosphere by shadowgraph, schlieren and photoelastic method. We distinguished three different types of waves propagating into the bulk samples. The contrast of photoelastic images depends on laser pulse energy. Based on this result, we try to examine the effects of incidence angle of the laser to coupling between laser radiations and the material and found that magnitude of stress wave correlates with laser fluence.
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