Phase-front transformation of a first-order Bessel beam in Raman-resonant four-wave mixing

Abstract

We have studied the evolution of the phase front of an apertured first-order Bessel beam (J1) generated by a diffractive axicon during free-space propagation and its transformation in a Raman-resonant four-wave mixing process in hydrogen gas. It is demonstrated that the order of the phase singularity (vortex) of a Bessel beam can easily be detected by interferometry at the boundaries of the beam propagation range. With a J1 beam and a Gaussian beam at the Stokes and the pump frequencies and vice versa, anti-Stokes light is generated as a singular, conical beam. In both cases the observed phase front agrees with theoretical predictions. In particular, the transformation of the topological charge is consistent with the conservation of angular momentum.