The group indices of refraction and the group velocities of the e-ray and the o-ray waves in α-quartz

A spectroscopic method was proposed for direct determination of the group velocity and the group index of refraction of the medium. By evaluating the wavelengths at successive interference fringes shown in transmission spectrum, the group velocity and the group index of refraction, and their dependences on the wavelength of the incident light can be determined. This method was applied to evaluate the group velocities and the group indices of refraction for the extraordinary (e-ray) and the ordinary (o-ray) waves in α-quartz in a wide infrared wavelength range. Anomaly in both group index of refraction and the group velocity of the e-ray results from the interaction of the e-ray and the o-ray waves in α-quartz.