Inverted zones in the axial regions on the group velocity surface of a biaxial crystal.

It is well-known that both the phase velocity surface and the ray velocity surface of a biaxial crystal have two layers that touch each other at only four points. However, we show that the two layers of the group velocity surface of a biaxial crystal penetrate through each other and form four inverted zones. Inside an inverted zone, the slow and fast light pulses are carried by the fast and slow modes of rays, respectively. The border of an inverted zone passes through the optic ray axis and intersects with the cone of internal conical refraction. Numerical examples of the orthorhombic crystal KNbO₃ and the monoclinic crystal Sn₂P₂S₆ are given.