  We present a detailed experimental investigation, which uncovers the nature
of light bullets generated from self-focusing in a bulk dielectric medium with
Kerr nonlinearity in the anomalous group velocity dispersion regime. By high
dynamic range measurements of three-dimensional intensity profiles, we
demonstrate that the light bullets consist of a sharply localized
high-intensity core, which carries the self-compressed pulse and contains
approximately 25% of the total energy, and a ring-shaped spatiotemporal
periphery. Sub-diffractive propagation along with dispersive broadening of the
light bullets in free space after they exit the nonlinear medium indicate a
strong space-time coupling within the bullet. This finding is confirmed by
measurements of spatiotemporal energy density flux that exhibits the same
features as stationary, polychromatic Bessel beam, thus highlighting the
physical nature of the light bullets.
