  We report on an experimental comparison on 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 the strongest axial
intensity confinement against the self-focusing effect. This is because that 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.

Optics