6 - Trapped-particle near-field scanning optical microscopy

Summary

The aim of this chapter is to provide a comprehensive understanding of trapped-particle near-field scanning optical microscopy (NSOM). The principle of optical trapping and laser tweezers is briefly explained in Section 6.1. Section 6.2 summarises the motivation of using a laser-trapped microsphere as a probe in NSOM. The basic principle of trapped-particle NSOM is described in Section 6.3. Two major aspects of this technique, laser trapping performance and near-field Mie scattering of dielectric and metallic particles, are discussed in Sections 6.4 and 6.5, respectively. Experimental results on image formation in trapped-particle NSOM are described in Section 6.6. In Section 6.7, some prospects for the future development of this technique are put forward.

Optical trapping and laser tweezers

Photons carry momentum. When the change in momentum occurs upon reflection, refraction, transmission and absorption of a light beam, the rate of change of momentum results in a force being exerted on an object. The origin of this force can be understood from Newton's laws. A light ray that is refracted through a dielectric particle changes its direction due to the refraction process. Since light carries momentum, a change in light direction implies that there must exist a force associated with that change. The resulting force, manifested as a recoil action due to the momentum redirection, draws mesoscopic particles toward the highest photon flux in the focal region. This recoil is unnoticeable for refraction by macroobjects such as lenses, but it has a substantial and measurable influence on mesoscopic refractive objects such as small dielectric particles.