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Definition: the ratio of the vacuum velocity of light to the group velocity in a medium
In analogy with the refractive index, the group index (or group refractive index) can be defined as the ratio of the vacuum velocity of light to the group velocity in the medium:
The group index is used, e.g., to calculate time delays for ultrashort pulses propagating in a medium, or the free spectral range of a resonator containing a dispersive medium.
For crystals or glasses, the group index in the visible or near-infrared spectral range is typically larger than the ordinary refractive index, which determines the phase velocity.
Figure 1: Refractive index (solid lines) and group index (dotted lines) of silica versus wavelength at temperatures of 0 °C (blue), 100 °C (black) and 200 °C (red).
See also: group velocity, refractive index
