OAM content and propagation dynamics of integer-order Gamma vortices

We clarify, both theoretically and experimentally, the conceptual difference between a general vortex beam and an orbital angular momentum (OAM) beam regardless of carrying the same integer topological charge. We use a phase-only spatial light modulator to generate various non-canonical vortices, i.e., Gamma vortices, whose phase structures are described mathematically by the Gamma function. To further explore the OAM content of Gamma vortices, we perform a mode decomposition in the basis of OAM eigenstates. The measured OAM spectra indicate that each Gamma vortex is a coherent superposition of high-dimensional OAM states, but its average OAM value remains the same as the topological charge. In addition, resulting from the different Gouy phases among the OAM states, we find that the Gamma vortex carrying a high-order topological charge will generally decay into several unit-charge vortices and these vortices deviate unsymmetrically from the center of the light beam. Our work may find potential applications where the structured phase engineering is desired.