Abstract

Plasmonic spirals have attracted considerable research attentions recently with their intriguing capability in generating optical near-field vortices [1-3]. Under radial circularly polarized optical excitations of a single wavelength, surface plasmon waves can be analytically expressed for an observation point (R, θ) on the spiral plane as ${\stackrel{\rightharpoonup }{E}}_{spp,s}\left(R,\theta \right)\propto \widehat{z}{J}_s(k_{spp}R)\exp \left[js\theta \right]$, where k_spp is the wave-vector of the surface plasmon wave, J_s the s^th-order Bessel function, and the index s denotes the topological charge of the surface plasmon vortex [1]. For a right hand spiral (RHS) with 2π winding, surface plasmon vortices with s=0 and 2 can be selectively created using left-hand circularly polarized (LHCP) and right-hand circularly polarized (RHCP) input excitations of the same wavelength, respectively. Exploiting this unique property, plasmonic spirals have been proposed to act as nanoscopic polarization analyzers [1,2]. In another recent study, switching between higher-order vortices with topological charge difference of two (s=3 and 5) have also been demonstrated using LHCP and RHCP excitations of the same wavelength [3]. In this paper, the possibility to generate and switch between rational surface plasmon vortices are numerically demonstrated using a Raman frequency comb [4]. The feasibility in differentiating the resulting vortices through optical near-field measurements are assessed. Adaptive near-field control of a plasmonic spiral is also investigated.

© 2011 IEEE