Optical Manipulation. Theory of Optical Trapping Forces: A Review.

Abstract

Optical trapping is a technique that uses focused laser light to confine, manipulate, and apply optical forces to a variety of microscopic objects including dielectric, metallic, and biological samples. In this paper, we review the basic principles of optical trapping and summarize some of the key results derived from ray-optic and wave-optic theories for trapping forces and their parametric dependence. The application of these theories to the study of low-index microparticles and different trapping geometries are also described.

Quantitative Evaluation of Optical Forces by Single Particle Tracking in Slit-like Microfluidic Channels

“…Consequently, the gradient force is accurately evaluated from the PTV results based on the equation of motion of particles in viscous liquids. Furthermore, the gradient force is quantitatively calculated for a particle whose size is larger than the wavelength of light according to Ashkin’s theory, − reproducing experimental conditions. On the basis of previous studies, − the Langevin dynamics simulation is performed to treat a many-body problem, where dielectric microparticles are exposed to a converged laser beam.…”

“…In contrast, for the case of ϕ ≫ λ, the phenomena of rays that are reflected and refracted at a particle surface is treated as momentum transfer. In this regime, optical forces are evaluated based on geometrical optics . Since Ashkin , demonstrated optical tweezers for microparticles, many researchers have contributed to develop novel methods utilizing the optical manipulation of microobjects. − For instance, laser trapping techniques are applied to microfluidic devices, where micrometer-sized objects were applied to pumps or valves in the channels and were used for a switch to select a channel branch. , Recently, assembled structures of microparticles were formed at liquid–solid interfaces using polarized laser beams .…”

“…In this regime, optical forces are evaluated based on geometrical optics. 22 Since Ashkin 23,24 demonstrated optical tweezers for microparticles, many researchers have contributed to develop novel methods utilizing the optical manipulation of microobjects. 25−28 For instance, laser trapping techniques are applied to microfluidic devices, where micrometer-sized objects were applied to pumps or valves in the channels and were used for a switch to select a channel branch.…”

Optical trapping in micro- and nanoconfinement systems: Role of thermo-fluid dynamics and applications

“…Let us consider the ray of an incident angle θ with the incident momentum per second n 1 P∕c, where n 1 and c are the index of refraction of fluid and the speed of light, respectively. The optical force exerted on the sphere is expressed by the sum of power PR and the multiply refracted rays with powers of PT 2 , PT 2 R,., and PT 2 R n , where R and T denote refraction and transmission coefficients, respectively [11]. As a result, optical forces along the optical axis and perpendicular to the axis are optical scattering force F scat and gradient force F grad , respectively, as follows:…”

“…η and ζ result in the direction of the gradient and scattering forces, respectively. Reprinted with permission from [11]. Copyright 1996, The Laser Society of Japan.…”