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. No clear individual filaments are formed across the beam, and the estimated peak intensity surges to at least 200TW/cm2. As the external focusing is loosened to f number ∼125, we observe the transition to the multifilamentation regime. Distinct individual filaments are formed before the linear focus while the peak intensity reaches ∼80TW/cm2. Once formed, the filaments do not coalesce into a single or few superfilaments as they pass through the focus zone. Our experimental observations are supported by numerical simulations.