Crystal polarizes light with 73% efficiency

By blocking one polarization component, an ordinary linear polarizer converts unpolarized light to polarized light with a maximum theoretical efficiency of 50%; actual linear polarizers do less well than this. Researchers at the University of Rochester (Rochester, NY), however, have built a polarizer that works at greater than 50% efficiency—and, at first glance, seems to violate the second law of thermodynamics.

The device is based on a photorefractive crystal within which two interfering laser beams create a grating that causes the first order of the first beam to constructively interfere with the zeroth order of the second and the first order of the second to destructively interfere with the zeroth order of the first. The result is that both beams are combined. The two beams can result from an unpolarized beam split into two polarized components with one component rotated 90°. Theoretical efficiency of the device is 100%, and actual efficiency has reached 73%. Future photorefractive materials may do the same with incoherent light. The researchers presume that the decrease in entropy caused by the device is offset by an increase of entropy either in the crystal or in the beam intensity. Contact Robert Boyd at [email protected].