As discussed in Chapter 6, the trapping volume of a far-field laser trapping geometry is approximately three times larger in the axial direction than that in the transverse direction. Such trapping volume elongation leads to a significant background and poses difficulties in the observations of nano-particle dynamics. In this chapter, we deal with near-field optics using focused evanescent illumination. The recent development of near-field optical tweezers is reviewed in Section 8.1. Section 8.2 introduces the new concept of near-field laser tweezing with a focused evanescent field. This technology is characterised both experimentally and theoretically in Section 8.3. Section 8.4 presents the utilisation of a femtosecond laser beam in a near-field optical trap. Finally, some discussions on this new method are given in Section 8.5.

Near-field optical tweezers

Near-field laser trapping or tweezers means that radiation force that is used for trapping and manipulating a micro-object results from the interaction with an evanescent wave. Recently, a new trapping modality based on the evanescent wave illumination, also called near-field illumination, has been proposed and demonstrated. This trapping technique results in a significantly reduced trapping volume due to the fact that the strength of an evanescent wave decays rapidly with the distance from the surface at which the field is generated. In this section, the near-field trapping mechanism based on the different ways to generate a localised near-field is reviewed.