Whispering-gallery modes promote enhanced optical backflow in a perforated dielectric microsphere.

Optical energy flow inside a dielectric microsphere is usually co-directed with the optical wave vector. At the same time, if the optical field in a microsphere is in resonance with one of the high-quality spatial eigenmodes (whispering-gallery modes- WGMs), a region of reverse energy flow emerges in the shadow hemisphere. This area is of considerable practical interest due to increased optical trapping potential. In this Letter, we consider a perforated microsphere with an air-filled pinhole fabricated along the particle diameter and numerically analyze the peculiarities of WGM excitation in a nanostructured microsphere. A pinhole isolates the energy backflow region of a resonant mode and changes a perforated microsphere into an efficient optical tweezer. For the first time, to the best of our knowledge, a multiple enhancement of the energy backflow intensity in the pinhole at a WGM resonance is revealed and we discuss the ways for its manipulation.