Axial superresolution of focused radially polarized light using diffractive optical elements

Particle swarm optimization (PSO) was used to design binary phase- and amplitude diffractive optical elements (DOEs) that axially superresolve a tightly focused radially polarized beam. The Pareto front was generated for a fitness-value space that describes the superresolution that can be achieved with the DOEs and radially polarized light versus an allowed upper bound in side lobe intensity. Fitness values for designs obtained via the PSO-, simplex-, and simulated-annealing methods are compared to show that PSO yields globally optimized solutions. Performance of binary phase/amplitude DOEs with different numbers of zones is compared. Binary amplitude DOEs give slightly better superresolution, whereas binary phase DOEs offer the advantage of higher power efficiency. The potential of using DOEs for 4Pi microscopy to further reduce the side lobes and achieve better superresolution is discussed.