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Directly generating vortex beams in the second harmonic by a spirally structured fundamental wave

Based on nonlinear wave mixing, we experimentally propose a scheme for directly generating optical orbital angular momentum (OAM) by a spirally structured fundamental wave interacting with a nonlinear medium, in which the nonlinear susceptibilities are homogenous. In the experiment, the second-harmonic generation of a fundamental wave carrying positive (negative) integers and fractional OAM states was investigated. This study presents a convenient approach for dynamic control of OAM of vortex beams, which may feature their applications in optical manipulation and optical communication.