Supercontinuum emission from tightly focused femtosecond pulses in air - Beyond intensity clamping

We present the evolution of supercontinuum emission (SCE) from tightly focused fs laser pulses propagating in air. 45 fs laser pulses at 806 nm, 10 Hz repetition rate, from Ti:Sapphire laser (Thales Laser, Alpha 10) with a nanosecond contrast ratio better than 10^-6: 1 are focused in air by a lens to an f/12 focusing geometry in one case, and by an off-axis parabolic mirror leading to an f/6 focusing in another. The laser input power is varied in the range of 10 - 90 P_Cr and 6 - 60 P_Cr in the f/12 and f/6 focusing geometries, respectively, where the critical power for selffocusing in air is P_Cr = 3 GW for 806 nm. The effect of the tight focusing condition on the SCE spectrum and the dependence on the input laser polarization are studied. Within the input power range used in the study, the blue edge (the maximum positive frequency shift) of the SCE spectrum is found to decrease continuously when the laser energy is increased. This result is in contrast with previous measurements of SCE in condensed matter and gases with loose focusing geometry, for which a constant blue edge was interpreted as due to intensity clamping. We propose a model, which show that for tight focusing conditions, external focusing prevails over the optical Kerr effect annihilating plasma defocusing and self-focusing, thereby giving access to a new propagation regime featured by an efficient laser energy deposition in fully ionized air and intense 10¹⁵ W/cm² pulses at the focus.