Optical tweezers are used to trap and manipulate small objects using focused laser light. For the technique to work, the objects need to be transparent to the wavelength of the light being used and have a refractive index less than the medium in which they are suspended.

Despite these restrictions this method is extremely useful. Most biological cells are transparent and can be suspended in a medium with lower refractive index than themselves. This means that single cells can be picked up and moved using optical tweezers. Whilst the restraining force exerted by the laser is only small, it is larger than the propulsive force of most bacteria, allowing them to be individually dragged about against their will. Obviously, in order to see what you're holding, the tweezers are used in tandem with an optical microscope.

The tweezers work by trapping the object at the focus of the laser. If the object moves away from the focus in any direction it feels a restorative force directing it back towards the focal point. It is only at the focus that the forces on the object from the incident photons are in balance. These forces arise from the conservation of momentum. When an incident photon is refracted by the surface of the object, because it changes direction the photon must have transferred some momentum to the object in order to keep the total momentum constant. (Please note that photons do posses momentum even though they are massless.)

Another way of looking at the way this works is to consider the interaction between the intense electric field set up at the focal point and the dipoles induced by this field in the object. The forces felt by the object are proportional to both the gradient of the electric field and the polarizability of the object. This explains why the focus needs to be so sharp, so the gradient becomes large - to this end a microscope lens is used to achieve a short focal length.

--->-- | \       /
      |*| \     /
      |*|  \   /
     |***|  \ /
     |***|   X    Object is held at focus.
     |***|  / \
      |*|  /   \
      |*| /     \
--->-- | /       \

Laser     Focus


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