Group delay in indefinite media

We present here a theoretical analysis of the group delay for a light beam transmitted through indefinite media, for which not all of the principal elements of the permittivity and permeability tensors have the same sign. When the permittivity and permeability tensors are not simultaneously diagonalizable, but rather there is an angle (called axis angle) between two tensors' corresponding principal axes, it is found that the axis angle, slab's thickness, incident angle and frequency of light have considerable influences on the group delay. When these parameters mentioned above are varied, a transmitted light pulse in such media can experience a positive or negative group delay and hence a subluminal or superluminal propagation. The results might be helpful for the development of delay lines and the control of the dispersive effects for systems.