Background Information

What are Optical Tweezers?

Optical Tweezers are a device that can trap neutral particles using a combination of optical forces including radiation pressure, resulting from a tightly-focused laser beam. Optical Tweezers can be used to perform such delicate tasks as in-cell organelle manipulation without any consequent cellular damage.

What are some applications of Optical Tweezers?

Optical tweezers have numerous applications and potential applications in physics, biology, medicine, and in industry. Physicists and biologists interested in measuring the force with which two objects bond have used optical tweezers to pry the objects apart. The force needed to break the bond is then equal to the forceof the bond itself. Optical tweezers have been used to trap various microbes and viruses, and, in conjuction with laser scissors, optical tweezers have been used to manipulate cells at the organelle and chromosomal levels. Optical tweezers also promise to serve as a great tool for industry, as they can be used to sort and select small objects noninvasively.

Optical Trapping Theory

In an optical trap, there are two primary optical forces: radiation pressure and the gradient force. Radiation pressure force is imparted to a particle by the absorption, scattering, emission, or reradition of light. Its magnitude is F=P/c, where P is the fixed power level of the light, and c is the speed of light. When a ray of light hits a neutral particle and reflects off it, by the law of conservation of momentum, a force is imparted to the particle that is equivalent to F=P/2. The scattering force is part of any system in which light hits a neutral particle and works against any type of particle trap that may be created.

In order to trap a particle, the system must be arranged so that another force, of equal or greater magnitude than the scattering force, manifests itself. This force, known as the gradient force, is most responsible for the trapping or neutral microscopic particles. Its magnitude is F=Qn P/c, where Q is the efficiency of the tra, P is the fixed power level of the light, n is the refractive index of the medium in which the particle is contained, and c is the speed of light.

The gradient force can be described in terms of a hypothetical optical tweezers set-up. In order to trap a particle in the longitudinal direction, the light must be brought to a focus just above the center of the particle. The light rays enter the particle at a high angle of incidence. Because the particle has a higher index of refraction than its surrounding medium, Snell's law shows that the light rays refract within the particle and exit the particle closer to the verticle direction. Since the light rays have been refracted downward and towards the verticle axis of the particle, conservation of momentum requires that the particle gain a momentum "kick" upwards and outwards - towards the focus of the laser light.

Trapping in the laternal direction works in a similar manner. Since the focus is the point at which the light is brightest, there is more power per unit area at this point than at any other point in space. Thus, brightness of light decreases in a gradient as one movers away from the focus. The light rays that hit a particle in such a gradient have different brightnesses. Any light ray that hits the particle will be refracted inward and slightly upward. The particle gain a momentum "kick" downwards and outwards. It should be noted that rays of greater brightness have a higher momentum associated with them. Thus, the net force imparted to a particle will be in a direction opposite that of the direction in which the brighter ray of light was refracted. Due to this force, the particle will move towards the focus. The particle becomes trapped in three dimensions such that the particle's center is just below the laser focus.

Note: this information is taken from Sandra Nudelman's paper for Intel.