Abstract:
Laser-generated ultrasonic waveforms in graphite/polymer composites were detected with a reflection-mode confocal Fabry--Perot (CFP) interferometric system. The CFP was operated in reflection mode for maximum bandwidth (rise times <40 ns). The observed waveform amplitudes are linear in laser power below an ablation threshold. Waveforms at higher power are interpreted as superpositions of thermoelastic (TE) and ablatic (AB) components. The AB wave is formed when the generating laser vaporizes surface material and causes the formation of a hot, glowing plasma. Direct photodetection of the ablation products indicates that the ablation process lasts more than an order of magnitude longer than the generation-laser pulse (5 ns). The ultrasonic waveforms, interpreted with the aid of a theoretical model of the CFP-detection system, are consistent with the assumption of two concurrent generation processes. The maximum amplitude of the ablatic component is reached later than that of the thermoelastic component because of the longer time scale of the ablatic generation.