Sound waves generated by nanosecond and femtosecond laser ablation on different metals

Abstract

The photoacoustic shock waves generated by laser ablation have been studied extensively. However, the sound waves with frequencies below 20 KHz, which originate from the decaying of the photoacoustic shock wave in ambient air, have not been studied according to our knowledge. Researchers may have noticed the sounds generated by the laser ablation on different metals but never looked into this phenomenon in details. In this work, femtosecond and nanosecond laser beams were focused onto the surfaces of four different metals, and a microphone was employed to collect the signals of the generated sound waves. Experimental results show that the amplitude of the acoustic signal generated by the nanosecond laser increases slowly, then rises rapidly, and finally becomes saturated with increasing laser energy, while the strength of the acoustic signal generated by the femtosecond laser increases approximately linearly with increasing laser energy. The dominant frequencies of the sound waves are constant with changing pulse energy and are closely related to the ionization energies of the four metals.