Fig. 2.10 — Focusing a paraxial light beam

Chapter 2 — Ray Optics

If one shines a laser beam on a large transparent object, such as the prism shown in Fig. 2.1, a fraction of the power of the beam will be reflected back and the rest will go through and exit from the other side. Since the directions of the reflected and transmitted beams are different from that of the incoming beam, the mechanical momentum associated with the light beam changes and, by Newton’s action-reaction law, a force acts on the object. As long as one deals with objects that are much larger than the wavelength of light, which is typically around one micrometre for optical tweezing applications, the behaviour of the laser beam can be accurately described by considering it as a collection of light rays and employing the tools of geometrical optics. One can also make use of geometrical optics for the calculation of optical forces and obtain accurate results when dealing with relatively large objects, such as cells and large colloidal particles, whose size is typically significantly larger than one micrometre. In this Chapter, we will therefore study optical forces by using the geometrical optics approach, as this permits us to intro- duce in a more intuitive way many concepts that give insight into the mechanism by which microscopic particles can be optically trapped and that will be treated more rigorously in Chapters 3, 5 and 6.

2.1  Optical rays

2.2  Optical forces

2.3  Scattering and gradient forces

2.4  Counter-propagating beam optical trap

2.5  Optical tweezers

2.6  Filling factor and numerical aperture

2.7  Non-uniform beams

2.8  Non-spherical objects and windmill effect



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