Optofluidic Near-Field Optical Microscopy: Near-Field Mapping of a Silicon Nanocavity Using Trapped Microbeads

By analyzing the thermal motion of fluorescent dielectric microbeads trapped in the near-field of a silicon nanocavity, we investigate the influence of the bead's size and the trapping laser power on the shape of the optical trap and the ``effective'' trap stiffness. We demonstrate that the trapping potential is proportional to the subwavelength patterns of the electromagnetic near-field intensity distribution for unexpectedly large Mie particle sizes. More especially, we show that mapping the trapping potential experienced by a 500 nm diameter bead reveals the nanopattems of the cavity resonant mode. This result highlights how photonic force microscopy in nanotweezers can provide an elegant way to image evanescent fields at the nanoscale via the thermal motion of optically trapped fluorescent microprobes.