Fig. 4.1 — Optical beams and optical components

Chapter 4 — Optical Beams and Focusing

The key enabling factor for the rise of optical manipulation was the invention of the laser. In fact, lasers provide easy access to intense electromagnetic fields in the form of optical beams. Coherent optical beams propagate approximately along a straight line over large distances. Furthermore, their propagation can be controlled by standard optical elements, e.g., lenses and mirrors, and by more exotic devices, e.g., axicons and gratings. Perhaps more importantly for the sake of optical manipulation, optical beams can be focused producing the high intensity optical fields and strong optical field gradients required to exert appreciable forces on microscopic and nanoscopic particles. The most common example, featured in Fig. 4.1 is a Gaussian beam, i.e., a laser beam characterised by a Gaussian intensity profile. This beam is the most commonly employed to generate an optical tweezers by focusing it to a tight spot using a high-numerical-aperture objective lens. However, other kinds of beams can also be used, e.g., Hermite-Gaussian beams, Laguerre-Gaussian beams, Bessel beams, cylindrical vector beams, to obtain different kinds of focal fields, e.g., focal fields with a dark central spot. In this Chapter, we will review the basic the- oretical notions needed to understand optical beams, including, in particular, their properties, propagation and focusing.

4.1  Propagating electromagnetic waves

4.2  Angular spectrum representation

4.3  From near field to far field

4.4  Paraxial approximation
4.4.1  Gaussian beams
4.4.2  Hermite–Gaussian beams
4.4.3  Laguerre–Gaussian beams
4.4.4  Non-diffracting beams
4.4.5  Cylindrical vector beams

4.5  Focusing

4.6  Optical forces near focus

4.7  Focusing near interfaces
4.7.1  Aberrations
4.7.2  Evanescent focusing



Leave a Reply