Particle-trapped near-field scanning optical microscopy: scattering and depolarization

摘要

Particle-trapped near-field scanning optical microscopy utilises a laser-trapped dielectric or metallic particle as a near-field scatterer to probe the high spatial frequency information from a sample. Scattering and depolarization by a trapped particle in an evancescent wave are twoimportant issues in such an imaging system. These two issues are addressed in this paper. The strength of scattered evanescent waves was measured for particles of different sizes (0.1 mu m to 2 mu m in diameter) and different materials (polystyrene, gold and silver). It has been found that the signal strength of scattered evanescent waves increases appreciably with the size of a particle. AS a result, image contrast is improved significantly with laser-trapped metallic particles of large size. It has also been found that the depolarization of scattered evanescent waves under s polarised illumination is stronger than that under p polarized beam illumination, and that image contrast of the evanescent wave interference pattern can be improved by fa factor of 3 with a parallel analyser under s polarized beam illumination. This result suggests that less depolarized scattered evanescent photons carry more information of an object and should be utilised for the imaging in particle-trapped near-field scanning optical microscopy.