Use of q-plate as a coupler.

A coupler is a device that allows interconversion of angular momentum of the optical beam among spin and orbital parts, without absorbing any angular momentum. In this paper, we demonstrate that a -plate with =1 can act as a coupler for any of the polarization distribution that can be represented by a point on a Poincare sphere (PS), or on a higher-order PS, or on a hybrid-order PS. A -plate with any -value can act as a coupler for polarization distribution that can be represented by an equatorial point on any of these spheres. It is also possible to find a -plate with an appropriate -value that can act as a creation or annihilation operator that raises or lowers the polarization singularity index. The -plate being a coupler and made of half-wave plate segments, the singularity index change is concomitant with the helicity inversion of C-points. Interconversion of bright and dark C-points is possible by using a -plate.