Optical Tweezers ( Originally called as ”Single beam gradient optical trap” ) is one kind of scientific instruments, which is using high numerical aperture objective to focus laser beam. By using the laser beam to attracting/exclusion objects, the photon scattering transfers the energy to them, thus the light can actually hold and moving objects, like the tweezers. For studying the theory of optical trap, the size of particles and wavelength are important. They are both in the same order, and usually used micron-transparent particles in the biological, materials, and physics research area. In recent years, optical tweezers has been successfully used in learning a variety of biological systems, such as cell sorting, cell cooling, and cell motility, even for grabbing DNA or chromosomes.

As laser light through the objective lens, there will be the strongest electric field gradients in the center of focal plane, causing the attractive force to nearby objects. If the attracted objects are not in the beam center, the light through objects will be refracted because of the different refraction factors, and objects will hence be moved by the reactive force. The light density in center is higher than that in edge, and the bigger attractive force pull objects to beam center; thus, the particles are be trapped by the laser light.

The illustration of condense laser beam density. The illustration of particles being pushed by refracted light.