Phase matching of a nonlinear interaction by adjustment of a propagation direction.
Critical Phase Matching is a technique for obtaining phase matching of some nonlinear process in a birefringent crystal. The interacting beams are aligned at some angle(s) to the axes of the index ellipsoid. In almost all cases, there are one or two waves polarized along one axis of the index ellipsoid, while another one or two waves are polarized at some variable angle with the plane spanned by the other two axes. Adjustment of the propagation angle affects the Refractive Index of the extraordinary beam, whereas the ordinary index stays constant. For some angular position, phase matching may be achieved.