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Optical Tweezers
a technique for contact-free manipulation of microscopic particles using light


Optical tweezers exploits the energy and momentum of light to manipulate microscopic dielectric objects. When these forces are managed and controlled, they can be used to trap and move minute objects as desired. For example, dielectric objects are attracted to the region of strongest electric field in a beam of focused light. The stream of photons scattering from the surface of an object, in addition, exerts 'radiation pressure' in the direction of the beam and, in a tightly focused beam an opposite force balances this pressure. Refraction or reflection of light can be used to change the direction and momentum of light and with the help of computer controlled lenses, mirrors, and accousto/electro-optical devices, precise manipulation of the object can be achieved.


Optical tweezers has been used to manipulate a variety of materials in both biological and industrial environments, such as cells, viruses, organelles, colloids and metal particles. Optical trapping is highly sensitive capable of detecting movement of dialectric particles in the sub-nanometer scale. It is possible to study individual molecules by attaching them to beads and manipulating the bead in the laser trap. This method is widely used to study the physical properties of DNA and in the study of molecular motors. Both movement and small forces exerted within a system can be measured.


A basic setup for optical micromanipulation is likely to include high numerical aperture oil-immersion objectives to trap particles suspended in an aqueous medium, laser (most commonly an infrared laser in live cell applications to avoid damage to the cell), optics used to manipulate the beam location, position detector, and illumination source coupled to a CCD camera.


The modular structure of Nikon's Eclipse Ti inverted microscope system with five output ports readily incorporates capability for laser tweezers. Laser tweezers can be mounted in the top position, for example, while simultaneously providing TIRF, DIC or epi-fluorescence observation by simply switching the light source.


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