Subwavelength Focusing of Bloch Surface Waves

Microsized spheres can focus light into subwavelength spatial domains, a phenomenon called photonic nanojet. Even though well studied in three-dimensional (3D) configurations, only a few attempts have been reported to observe similar phenomena in two-dimensional (2D) systems. This, however, is important to take advantage of photonic nanojets in planar optical systems. Usually, surface plasmon polaritons are suggested for this purpose, but they suffer notoriously from rather low propagation lengths due to 2 3 4 5 6 7 8 9 intrinsic absorption. Here, we solve this problem and explore, numerically and experimentally, the use of Bloch surface waves sustained by a suitably structured all-dielectric media to enable subwavelength focusing in a planar optical system. Since only a low-index contrast can be achieved while relying on Bloch surface waves, we perceive a new functional element that allows a tight focusing and the observation of a photonic nanojet on top of the surface. We experimentally demonstrate a spot size of 0.662 lambda in the effective medium. Our approach paves the way to 2D all-dielectric photonic chips for nanoparticle manipulation in fluidic devices and sensing applications.