Energy absorption of intense Nd:YAG laser pulse at the surface of aluminum was measured as a function of laser energy fluence φ by pbotoacoustic method using a piezoelectric transducer. Luminescence spectra from laser induced plume were measured. A Q-switched Nd:YAG laser which generate pulses of 8 ns duration was used.
Pbotoacoustic signal intensity I increased according to the laser pulse energy at low fluences. At a constant pulse energy, the I was a function of the energy fluence φ and had a threshold φth, When the φ was lower than the φth, the I was constant and corresponded to the absorption coefficient of the metal. Above the φth, it approached the maximum I/max and then began to decrease with further increase in the φ. If we represent the relative change in I at different pulse energies as the normalized intensity I/Imax. a universal curve was obtained.
Below the φth, the process was normal photon absorption by the metal. Damage left on the irradiated surface became evident above the φth, Above the φth and below the Imax. surface temperature increased high enough to form melt and the photo-absorption coefficient would increase during one laser pulse. A shock wave would be formed at the surface and contribute to the increase of the I. Higher φ than at the Imax some processes which inhibit the energy absorption would occur and the I decreased.