Superluminal-subluminal orbital angular momentum femtosecond laser focus.

The interplay between the frequency chirping of a broadband laser pulse and the longitudinal chromatic aberration of a focusing optic introduces the superluminal or subluminal behavior to a laser focus. In this paper, we present an analytic expression for an electric field describing a superluminal or subluminal femtosecond laser focus with orbital angular momentum. The analytic expression for a superluminal or subluminal laser focus is obtained through a diffraction integral, in which the focal length is replaced by a time-dependent focal length under the paraxial approximation, and the Fourier transformation. The speed and pulse duration of a laser focus are determined by the total group delay dispersion and a chromaticity parameter defined by the longitudinal chromatic aberration of a dispersive focusing optic. It is shown that it is possible to generate a several femtosecond superluminal orbital angular momentum laser focus in the focal region.