Evaluation of Sonic Emissions from Femtosecond Filaments

Quasi-static light filaments with variable lengths and properties were generated in the lab, by propagating the femtosecond laser pulses in ambient air1 under varying focusing conditions. The temporal dynamics and high intensity associated with filaments has attracted interest from many research domains2, thereby demanding a holistic characterization of their structure and emissions. Here, we present our work on acoustic interrogation of a femtosecond filament with a visible plasma channel length of 1cm generated using 300 mm focusing lens. The spatial structure of filament comprises of a series of periodic breakdown plasma regions, which emit acoustic waves proportional to plasma intensity into the ambient air3. An amplified calibrated microphone and an optical fiber coupled USB spectrometer were used to perform an acoustic and optical scan of the entire spatial extent of the filament. As we scan the microphone from the start to end of the filament, we observe that the acoustic peak pressures rise till the center then decreases. The maximum acoustic peak pressure at the center of the filament is 2.3 kPa (161 dB). From optical scan of the filament, the optical emission spectrum is obtained along the length of the filament, which helps in estimating the plasma electron density4, which is ~ 18.6×1016 cm−3. The acoustic and optical emission scans help us in precisely mapping the spatial extent and the intensity profile of the filament plasma channel. We also calculated the acoustic spectral composition from the time domain pulses and observed that the acoustic central peak frequency is 120 kHz at middle of the filament, and changes to 60 kHz at the beginning and end of the filament, indicating a change in plasma properties along the length of the filament.