Non-equilibrium temperature dynamics in a femtosecond filament using hybrid coherent anti-Stokes Raman scattering

This study investigates the non-equilibrium temperature dynamics in air within a femtosecond laser plasma filament using coherent anti-Stokes Raman spectroscopy (CARS). Hybrid femtosecond/picosecond CARS is demonstrated for measuring the ro-vibrational CARS spectra for a range of delays relative to the femtosecond filament. Ro-vibrational CARS fitting is used to extract the vibrational temperature temporal evolution from the experimental data by differentiating between the hot plasma region and the colder surrounding gas. This also allows for measurements of gas filament number density based on the intensity of the ro-vibrational spectrum. The rotational temperature dynamics are inferred from the measured density and calibrated using spontaneous Raman measurements.