  Here, we demonstrate a simple direct maskless laser-based approach for
fabrication of back-reflector-coupled plasmonic nanorings arrays. The approach
is based on delicate ablation of an upper metal film of a metal-insulator-metal
(MIM) sandwich with donut-shaped laser pulses followed by argon ion-beam
polishing. After being excited with a radially polarized beam, the as-prepared
MIM configuration of the nanorings permitted to realize efficient nanofocusing
of constructively interfering plasmonic waves excited in the gap area between
the nanoring and back-reflector mirror. For optimized geometric parameters of
such MIM structure, substantial enhancement of the electromagnetic near-fields
at the center of the ring within a single focal spot with the size of
0.37$\lambda^{2}$ can be achieved, which is confirmed by Finite Difference Time
Domain (FDTD) calculations, as well as by detection of enhanced PL signal from
adsorbed organic dye molecules. The simple large-scale and cost-efficient
fabrication procedure used, along with relatively good tolerance to excitation
beam misalignment, make the proposed structures promising for realization of
various nanophotonic and biosensing platforms that utilize cylindrical vector
beam as a pump source.
