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Laser tweezers, also known as optical tweezers, use laser beams to trap microscopic or nanoscopic particles with precise 3-dimensional positioning. The laser beams take advantage of a phenomenon called refractive index mismatch. We see this whenever we look at a straw in a glass of water. On tiny scales, the subtle bending of light by the particle results in momentum being conveyed to it, projecting a tiny attractive or repulsive force. The outcome is extremely fine-grained precision and control over a single particle in the beam, control on sub-nanometer scales.
The laser tweezers only work when the material used is dielectric, meaning an insulator that is averse to electromagnetic fields. A focused laser in the tweezers generates an electromagnetic field in the form of condensed light. The laser approach can be used to levitate bacteria, viruses, and even single atoms and molecules. For many applications, tiny samples are attached to a slightly larger microscopic bead. Multiple laser tweezers can even be used to pull on parts of a molecule, stretching it out and allowing scientists to observe how it snaps back. This is incredibly helpful in elucidating their subtle chemical properties.
The phenomenon of optical scattering by microscopic particles was first reported by Bell Labs scientist Arthur Ashkin in 1970. Then, in 1986, scientist Steven Chu and others wrote a paper on the topic, and improved the systems greatly. Dr. Chu went on to apply the laser tweezers in a wide variety of useful areas, including the cooling of atoms by stopping them in place, and won the 1997 Nobel Prize in Physics for his hard work.
Laser tweezers have been invaluable in the study in tiny features in biological machines, such as the ubiquitous biological motors that drive motion in the cell. This contributes to the emerging science of nanotechnology, and expands our knowledge of biology greatly. Laser tweezer-based probing of the cytoskeleton of cells helped scientists create a high-resolution map of the cell, with more detail than other approaches could have produced. Laser tweezers continue to be a hot area of research, with intrepid teams at Berkeley, Stanford, MIT, and many other universities exploiting the investigative possibilities that the technology has to offer.
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