Measurements of fluence profiles in femtosecond laser sparks and superfilaments in air
We investigate the nonlinear propagation of multiterawatt femtosecond laser pulses at 800 nm wavelength in air, under different external focusing conditions. We profile the laser beam in the vicinity of the nonlinear focus using a technique based on the dependence of the single-shot ablation threshold for gold on the angle of incidence of the laser beam on the sample. Under very tight focusing conditions (f number ∼15) we observe the propagation regime reminiscent of the nanosecond optical breakdown.