  In view of wide applications of structured light fields and plasmonic
vortices, we propose the concept of compound plasmonic vortex and design
several structured plasmonic vortex generators. This kind of structured
plasmonic vortex generators consists of multiple spiral nanoslits and they can
generate two or more concentric plasmonic vortices. Different from
Laguerre-Gaussian beam, the topological charge of the plasmonic vortex in
different region is different. Theoretical analysis lays the basis for the
design of radially structured plasmonic vortex generators and numerical
simulations for several examples confirm the effectiveness of the design
principle. The discussions about the interference of vortex fields definite the
generation condition for the structured vortex. This work provides a design
methodology for generating new vortices using spiral nanoslits and the advanced
radially structured plasmonic vortices is helpful for broadening the
applications of vortex fields.
