Photoacoustic and photothermal beam deflection as a probe of laser ablation of materials

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Abstract
Photoacoustic and photothermal laser-beam deflection were applied as diagnostics of the pulsed ultraviolet (UV) laser ablation of a polymer polyethyleneterephthalate. Here, a continuous-wave (cw) laser beam is passed parallel to the sample, but displaced from it by a few hundred micrometers. A density gradient caused by the pulsed UV laser heating or ablation of the sample deflects the cw laser beam. This deflection is measured directly using a position-sensitive detector. A quantitative model of the photothermal deflection at low fluence was developed which fits the data very well. This enabled a new method of measuring the thermal diffusivity of the fluid in contact with the sample. Distortion of the photothermal and photoacoustic signal as the excimer fluence is raised through the ablation threshold allowed the determination of the threshold. Also, the velocity of the ablation products was measured through a time-of-flight analysis and found to be dependent on the laser fluence used, the nature of the gas above the sample, and the distance above the sample at which the velocity is measured. The beam deflection in a vacuum is used to measure the ablation product velocity.