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Near field laser tweezers in biophotonics

Lasers have opened up numerous opportunities for biomedical sciences. Optical tweezers are one of the key biophotonic techniques. They employ the forces of radiation pressure of light to trap and manipulate microscopic particles, and have enormous applications in various disciplines ranging from physics to biology. The trapping volume of the far field optical tweezers is diffraction limited with an elongated axial size. While one deals with very small biological specimens like single cells or molecules, a reduced trapping volume is desirable, which would ideally be provided by a near field trap. Near-field trapping employs the evanescent field to manipulate microparticles, and hence reduces the axial trapping volume down to tens of nanometres. In this lecture, I will introduce a near-field trapping technique using focused evanescent illumination produced by a high numerical aperture objective, obstructed by an opaque disk whose size satisfies total internal reflection condition.