The propagation of femtosecond laser filaments in air with continuously varying pressures

The characteristics of the filaments generated by propagating a femtosecond Gaussian beam in a 2 m long gas cell with continuously varying pressures at different focal distances are numerically investigated. The simulated results indicate that the onset distance and the length of the filaments depend sensitively on the variation of pressure. These effects are enhanced with the increase of the focal length for the initial power P-in = 5P(cr). The main reason is that some parameters (the group velocity dispersion, the nonlinear refractive index, multiphoton ionization rate and the neutral atom density) are variational with the gaseous pressure. And the stability of the filament is determined by the combined effect of the pressure and the focal distance. In addition, we discuss the differences of the filament characteristics between the fixed pressure and continuously varying pressures. Through the analyses of the temporal dynamics and supercontinuum spectra, we find that maintaining a large pressure (1 atm) and changing to a large pressure (such as 0.3 similar to 1 atm) are beneficial to the propagation of the filament and the broadening of the spectra. It is interesting that, for fixing the initial energy E-in = 1 mJ, although the critical power of laser pulse is higher at the low pressure, the filament length at the continuously varying pressure p = 0.3 similar to 1 atm is still longer than that of p = 1 similar to 0.3 atm as the focal distance increases. This research is of great significance to remotely detect atmospheric pollutant components.

Automated summary

This study numerically investigates the characteristics of filaments generated by propagating a femtosecond Gaussian beam in a gas cell with continuously varying pressures, revealing the sensitivity of filament onset distance and length to pressure variations. The study highlights the importance of pressure and focal distance in determining filament stability, as well as the benefits of maintaining or changing to higher pressures for filament propagation and spectrum broadening. Additionally, the research discusses differences in filament characteristics between fixed and continuously varying pressures and their implications for remote detection of atmospheric pollutants.