Light Tweezer

In science fiction movies we often see light beams shot out from handheld light guns supplying a tremendous impulse to knock over an enemy storm trooper approaching along the direction of the beam. By Newton’s third law, the light gun itself should have experienced an equivalent recoil! We know that a ray of light has energy and linear momentum, so its impingence on any surface will produce a slight backward movement of that surface. However, we would like to know whether a light beam can be used to physically move a tiny object, such as a small one-celled animal, in a direction perpendicular to the beam.

Answer

Yes. A focused laser beam can exert a trapping force perpendicular to the beam direction of 2 × 10^–12 Newtons or more to keep cells confined in a microscope at the optical axis. The intensity gradient across the light beam is the source of the force.

In the simplest geometry, consider a semitransparent object with a diameter greater than the wavelength of the incident light but smaller in size than the diameter of the incident light beam. Let the light source be a parallel beam of light rays all of the same frequency, such as in a laser beam focused to the point f by a symmetrical lens. The object tends to focus the light rays somewhat, changing the direction of the light rays. The sideways recoil of the object occurs to simply conserve the linear momentum. If the light beam has an intensity gradient, brighter in the center than near the edge, the object will receive a net push back toward the optical axis in the center. There must also be a recoil of the object in the direction of the original light beam, which usually is taken up by the apparatus and Earth because the object is on a horizontal platform. A one-celled paramecium remains well trapped in a microscope via this light tweezer technique, begun at Bell Labs in the 1970s.

When the object is smaller than the wavelength of the incident light, a more detailed analysis is required to understand the 3-D trapping and the quantum interference effects.

Optical tweezers have been widely used for several decades in applications as diverse as experiments on molecular motors in biology and the movement of Bose-Einstein condensates in physics. The capabilities of single optical tweezers have been greatly improved and extended by the development of tailored beams and by schemes for generating large numbers of trapping sites and shapes simultaneously.

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