Near-field optical manipulation by using evanescent waves and surface plasmon polaritons

Abstract

Near-field optical manipulation has emerged as an alternative technique for trapping and manipulation of small particles (micro- and nano-objects) due to the localization of the field to typically less than 100nm from an interface. It has been demonstrated that near-field photonic forces can move microparticles located near an interface. The surface field couples to microparticles in close proximity, where the near-field wave can be converted to a propagating wave, via photon tunnelling across the gap from prism to microparticles. We have extended this method to observe both guiding and trapping of microparticles in unison over an extended area of 1mm² in a system we call Lensless Optical Trapping. Our method makes use of a patterned evanescent field that produces well defined individual potential wells where objects immersed in water can be laterally trapped and longitudinally guided along the propagation direction of the evanescent waves. Stable trapping in an array was achieved by using two counterpropagating beams, balancing the forces involved in the optical trapping. Our method permits full control of speed, direction and position of thousands of microparticles simultaneously. This will be useful for future lab-in-a chip cell sorting. Evanescent electric field can be increased until two orders of magnitude by using a metallic surface. We also present the first demonstration of an enhancement of the accumulation of nanoparticles by optically exciting surface plasmons. Convection and thermophoresis effects are also involved in this accumulation and the role of the thermal and optical forces involved will be discussed in detail.