Direct observation of Gouy phase shift in a propagating optical vortex.

Direct observation of Gouy phase shift on an optical vortex was presented through investigating the intensity profiles of a modified LG_p;m beam with an asymmetric defect, around at the focal point. It was quantitatively found that the rotation profile of a modified LG_p;m beam manifests the Gouy phase effect where the rotation direction depends on only the sign of topological charge m. This profile measurement method by introducing an asymmetric defect is a simple and useful technique for obtaining the information of the Gouy phase shift, without need of a conventional interference method. In addition, the 3-dimernsional trajectory of the defect was found to describe a uniform straight line.