Filament Induced Acoustic Spectroscopy:Sensing Molecular Aerosols*

Filamentation is a dynamic, intense propagation regime of the ultrashort pulses due to the periodic and dynamic competition between Kerr Self focusing and plasma defocusing. Filaments so formed confine and propagate within them, fs light pulses over large distances, with peak intensities > 10¹³ Wcm^-2, within a mm spot size. This intense plasma channel radiates energy into its surrounding atmosphere in the form of optical emissions, RF emissions, THz emissions as well as intense acoustic shockwaves. The periodic spatially varying plasma radiates acoustic pulses proportional to the plasma density and unique to the medium in which they are generated, hence can be exploited for molecular sensing applications. In this report, we present our work on the generation and characterization of acoustic emissions from filaments in Tryptophan aerosols. The acoustic pulses generated in ambient air had acoustic emissions as a broad peak centered at 120 kHz, whereas the filament generated in tryptophan aerosol had multiple emission frequencies at 75kHz, 90kHz, 120kHz and 140kHz. The spectral intensity and peak over pressures were dependent on molecule concentration. We propose this as a proof of concept, “Filament Induced Acoustic Spectroscopy” technique for standoff sensing of molecular aerosols.