Laser trapping of microparticles

To trap a microparticle, it is necessary that the optical force acting on it should be strong enough to compensate for the gravitational force. A simple estimate shows that this condition is easy to satisfy by means of a focused CW laser beam. Let us consider as an example a laser beam with a power of 1W focused onto a 1 /jm sphere around 1 g/cm in density and let the sphere scatter aU of the incident light isotropically. The acceleration of the particle under the action of such a beam is of the order of 

When n 1, the polarizability a. is positive, and the gradient force pulls particles into the high-intensity regions of the beam. 

Ashkin and Dziedzic (1987) gave an impressive demonstration of the optical trapping of dielectric particies in experiments with a biological particle (tobacco mosaic virus) in water by means of a single-beam gradient trap formed by an argon laser 0.1-0.3 W in power. The rodlike tobacco mosaic virus is 3000 nm long and 200 nm in diameter and has a refractive index of 1.57. In this experiment, Ashkin and Dziedzic 

Optical tweezers have also been used to transfer various types of photon angular momentum to macroparticles, as was done long ago with the photon spin and atoms (Chapter 4). It is not out of place here to note that the photon angular momentum is, first, associated with the spin for circularly polarized light and has a magnitude of h = h/2Tt per photon. Second, there is an orbital angular momentum that is associated with the inclination of the wavefront an the laser beam (Allen et al. 1999). Unlike 

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