Optical tweezers

Optical tweezers (originally called "single-beam gradient force trap") is a scientific instrument that uses a highly focused laser beam to provide an attractive or repulsive force (typically on the order of piconewtons), depending on the refractive index mismatch to physically hold and move microscopic dielectric objects. Optical tweezers have been particularly successful in studying a variety of biological systems in recent years. The detection of optical scattering and gradient forces on micrometer sized particles was first reported in 1970 by Arthur Ashkin, a scientist working at Bell Labs. Years later, Ashkin and colleagues reported the first observation of what is now commonly referred to as an optical tweezers: a tightly focused beam of light capable of holding microscopic particles stable in three dimensions. One of the authors of this seminal 1986 paper, United States Secretary of Energy Steven Chu, would go on to use optical tweezing in his work on cooling and trapping neutral atoms. This research earned Chu the 1997 Nobel Prize in Physics along with Claude Cohen-Tannoudji and William D. Phillips. In an interview, Steven Chu described how Ashkin had first envisioned optical tweezing as a method for trapping atoms. Ashkin was able to trap larger particles (10 to 10,000 nanometers in diameter) but it fell to Chu to extend these techniques to the trapping of neutral atoms (0.1 nanometers in diameter) utilizing resonant laser light and a magnetic gradient trap (cf. Magneto-optical trap). The most basic optical tweezer setup will likely include the following components: a laser (usually Nd:YAG), a beam expander, some optics used to steer the beam location in the sample plane, a microscope objective and condenser to create the trap in the sample plane, a position detector (e.g. quadrant photodiode) to measure beam displacements and a microscope illumination source coupled to a CCD camera. An Nd:YAG laser (1064 nm wavelength) is a common choice of laser for working with biological specimens. This is because such specimens (being mostly water) have a low absorption coefficient at this wavelength. A low absorption is advisable so as to minimise damage to the biological material, sometimes referred to as opticution. Perhaps the most important consideration in optical tweezer design is the choice of the objective. A stable trap requires that the gradient force, which is dependent upon the numerical aperture (NA) of the objective, be greater than the scattering force. Suitable objectives typically have an NA between 1.2 and 1.4. Optical traps allowed these biophysicists to observe the forces and dynamics of nanoscale motors at the single-molecule level; optical trap force-spectroscopy has since led to greater understanding of the stochastic nature of these force-generating molecules. Optical tweezers have proven useful in other areas of biology as well. For instance, in 2003 the techniques of optical tweezers were applied in the field of cell sorting; by creating a large optical intensity pattern over the sample area, cells can be sorted by their intrinsic optical characteristics. Optical tweezers have also been used to probe the cytoskeleton, measure the visco-elastic properties of biopolymers, and study cell motility. Researchers have also worked to convert optical tweezers from large, complex instruments to smaller, simpler ones, for use by those with smaller research budgets.

A generic optical tweezer diagram with only the most basic components

Further information

1. Ashkin A., «Acceleration and Trapping of Particles by Radiation Pressure», Phys. Rev. Lett. 24, 156 (1970).
2. Ashkin A., Dziedzic J. M. & Yamane T., «Optical trapping and manipulation of single cells using infrared laser beams», Nature 330, 769 (1987).
3. Letokhov V. S., et. al. Cooling and trapping of atoms and molecules by a resonant laser field. Opt. Commun. 19, 72 (1976).
4. Macdonald M. P., Spalding G. C. & Dholakia K., «Microfluidic sorting in an optical lattice», Nature 426, 421 (2003).
5. Hu Z, Wang J, Liang J, «Manipulation and arrangement of biological and dielectric particles by a lensed fiber probe», Optics Express, 12, 4123 (2004).
6. Grover S. C. «Automated single-cell sorting system based on optical trapping» J Biomed Opt. 6, 14 (2001).
7. Kawata S. and Sugiura T. «Movement of micrometer-sized particles in the evanescent field of a laser beam» Opt. Lett. 17, 772 (1992).
8. Schmitz C., Spatz J., & Curtis J., «High-precision steering of multiple holographic optical traps» Optics Express, 13, 8678 (2005).
9. Статья Optical tweezers из Wikipedia, свободной энциклопедии. Доступно под лицензией Creative Commons Attribution-Share Alike.

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