Calculation of Optical Forces in Optical Tweezers

Optical trapping involves the utilization of a tightly focused laser beam to confine microscopic particles within a liquid medium. A laser beam that has been collimated becomes tightly focused at the focal point of a high numerical aperture (NA) objective lens. When this focused beam encounters a microscopic particle with a matching refractive index at the focal point, the particle experiences a force of pico-newton magnitude. Consequently, if the force distributions are balanced at the focal point, the microscopic particle becomes trapped. This technique is referred to as optical trapping. In a more advanced setup called an optical tweezer arrangement, one or more microscopic particles can be trapped and manipulated in three-dimensional space. Optical trapping forces can be assessed by employing a suitable theoretical framework to estimate the trapping force across different conditions. Various optical force calculation models have been documented to date. This chapter aims to offer a concise survey of various optical force calculation models pertinent to optical trapping. Moreover, researchers are also making tremendous efforts to calculate the trapping force on trapped particles experimentally. A few experimental methods are also discussed in this chapter.