Laser-plasma sound sources in atmospheric air: A systematic experimental study

Abstract

Laser-plasma sound sources are increasingly utilized in technological and scientific applications as attractive alternatives to conventional electromechanical or piezoelectric transducers. This work presents a systematic experimental study of laser-plasma sound sources generated in atmospheric air by strong, short and ultrashort laser pulses. Emphasis is given on the dependence of their acoustic emission on the characteristics of the optical radiation. In the experiments, nanosecond and femtosecond laser pulses with different energies and optical wavelengths are focused in atmospheric air using lenses of different focusing power. The produced acoustic pulses are captured via a system with high dynamic range and broad frequency response in the far field of the plasma source. The acoustic signals are analysed both in the time and frequency domains to extract their primary characteristics, particularly the total acoustic energy, peak pressure, spectral magnitude and spectral peak frequency. The results are systematically examined and compared with respect to the characteristics of the exciting laser pulses. The experimental observations are supported by theoretical calculations of the generated plasma density via rate equations that account for the main ionization and electron loss mechanisms. The presented analysis provides a frame for controlling the acoustic characteristics of laser-plasma sound sources, which is particularly important for applications in the fields of acoustic measurements, sound reproduction and biomedicine.