Amplitude Non-Uniformity of Millimeter-Wave Vortex Beams Generated by Transmissive Structures

We investigate the origin of non-uniformity in an orbital angular momentum (OAM) beam generated by a transmissive structure in the millimeter-wave frequency range. This work includes the study of amplitude non-uniformity including modal composition analysis using the angular spectrum method (ASM) for OAM beams generated by spiral phase plates (SPPs) and flat-surface metamaterial structures (MSs). The dependency of the axial dimension, over which the OAM beam transforms from a Gaussian beam, on the amplitude non-uniformity of both the structures has been rigorously analyzed. For experimental verification, we designed a perforated planar dielectric slab to convert the Gaussian beam into an OAM beam of order 2 with improved amplitude uniformity. Within the manufacturing limitations, the design of the transmissive MS was fabricated and tested to obtain optimum amplitude uniformity in the generation of OAM beams and to compare with simulation results.