Optical Trapping

Optical trapping/tweezers are tools whereby a piconewton force is exerted on microscopic dielectric objects and is used to physically hold, manipulate, and/or repulse the material in question by utilizing highly focused laser beams.  Mainly employed in the arena of bioresearch, dielectric objects are attracted along the strong electric field gradient to the region of the strongest electric field, which is located at the beam waist.  Once the particle is trapped in the beam waist, it can be translated to another location, as if it were picked up by tweezers.

Due to the ability to translate on the sub-nanometer scale, optical tweezers are used to study single molecules which have been attached to the dielectric particle. For example, biological research into the properties of DNA and associated proteins has been advanced significantly due to the advent of optical trapping and tweezing. To probe the biomolecules of interest, many of the optical trap setups utilize the fundamental wavelength of 1 μm continuous wave lasers. These single frequency, linear polarized lasers are essential because many biological samples, aqueous in nature, have a low absorption coefficient at this wavelength, which minimizes the damage to the biological material. Other wavelengths, such as green (0.51-0.55 μm) and near-IR (1.15-1.8 μm), are also used.