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Introduction to Optical Tweezers
For centuries, people have been exploring the nature of light. In essence, light is an electromagnetic wave, but with a wave-particle duality. For a long time, people are more concerned with the photon energy, until the 19th century, Maxwell’s electromagnetic theory with rigorous demonstration of the light show that photon not only has energy, but also has momentum. In 1986, Ashkin first proposed the use of highly focused laser beam to form a single beam gradient force trap to bound particles, which is the optical tweezers.
Interaction of light with particles is the result of interactions between charged particles and electromagnetic fields of the dielectric light in nature. Theoretical study of optical tweezers is still under exploring. Today’s mainstream theoretical model is divided into two types: electromagnetic models (Electromagnetics Model: EM Model)  and the geometrical optics model (Ray-Optics Model: RO Model) . Electromagnetic model is based on Maxwell equations and electromagnetic polarization, applicable to Rayleigh particles, i.e. particle diameter is much smaller than the wavelength of light (<102). A small-sized particle produces an electric dipole field in the light, which moves close to light focus point gradually in a gradient field intensity of light. Geometrical optics model is based on geometrical optics and photon momentum conservation, suitable for Mie’s particle that the particle size is much larger than the wavelength of light (>102). Large particles equivalent to a microlens in the optical field, the force generated by a series of catadioptric change the direction of the photon momentum, so that the particles move toward the maximum light intensity that is the focus spot. It is worth mentioning that this effect is nearly impossible to observe on a macro-lens, only when the light intensity is large enough, then a significant effect will be produced in microscopic particles.
For a typical cell of about 5 ^m in diameter, it becomes a Mie’s particle if visible laser beam is used, thus a geometrical optics model is applied to calculate and analyze optical trapping force. In geometrical optics model, light rays pass through a series of refraction to generate a gradient force, with its direction pointing toward beam focusing center; whereas scattered light produced by the reflection of light rays off cell surface delivers a scattering force to the cell, pointing to the direction of the light propagation. Analysis of scattering force and gradient force constitutes a mechanical analysis of optical tweezers.
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