Different Kinds of Polarization

Posted on 2008-04-28. Permanent link: http://www.rp-photonics.com/spotlight_2008_04_28.html

Author: Dr. Rüdiger Paschotta, RP Photonics Consulting GmbH

Ref.: encyclopedia articles on polarization of laser emission, polarization waves, nonlinear polarization

Did you know that the term “polarization”, as used in photonics, can have entirely different meanings?

The more common meaning refers to the spatial direction in which the electric field e.g. of a laser beam is oriented. Laser beams are often linearly polarized, i.e. there is one direction perpendicular to the beam axis in which the electric field oscillates, while the magnetic field oscillates in a direction which is perpendicular to both the beam axis and the electric field.

An entirely different thing is the polarization of a transparent medium through which light propagates. The electric field of a light beam generates an electric polarization, and the magnetic field a magnetic polarization. The latter is usually very weak, since transparent media are normally either paramagnetic or diamagnetic, not ferromagnetic. In an isotropic medium, the electric polarization has the same direction as the electric field of the propagating light; in anisotropic media, these directions can differ. In any case, the propagating electric wave is then accompanied by a polarization wave in the medium, traveling with the same phase velocity.

Particularly in the context of nonlinear frequency conversion, both types of polarization come together. The polarization of the input beam(s) determine the direction of the nonlinear polarization. The resulting nonlinear polarization wave(s) then radiate light at new optical frequencies. Of course, the polarization state of that light is determined by the direction of the nonlinear polarization of the medium.