Nonlinear optical properties and superluminal propagation in the ruby

In this review article, we summarized the study of the weak light nonlinear properties associated with group velocity change in the ruby. The ruby could be reduced to a two-level system, in which population oscillations could lead to a subluminal result. With the modulated TEM₀₀ beam introduced into the ruby, self-phase modulation together with Fraunhofer diffractions and nondegenerate two-wave coupling between different spatial frequency components of the beam occurs at the same time. Population oscillations, self-phase modulation together with Fraunhofer diffractions and nondegenerate two-wave coupling mechanism competes with each other, which brings a new phenomenon of self-superluminal group velocity propagation in the ruby.