Onset of nonlinear self-focusing of femtosecond laser pulses in air: Conventional vs spatiotemporal focusing

We report an experimental comparison of critical intensities of nonlinear self-focusing in air with conventional focusing and spatiotemporal focusing schemes. Our results show that the conventional focusing with the focal lens completely filled with the incident beam allows for a higher peak intensity near the focus against the nonlinear self-focusing than the spatiotemporal focusing scheme. This is because in the high-numerical-aperture condition, the focal spot will have a compact size, which results in a high focal intensity. Meanwhile, the Rayleigh length of the focused beam will be substantially shortened, which efficiently postpones the onset of self-focusing.