Calculation of the refractive index of lead-tin-telluride for infrared devices

The refractive index
N
of lead-tin-telluride is calculated for frequencies near the fundamental absorption edge. The contribution to
N
from electron states near the band-edges is calculated more exactly than in previous works and a new expression for the contribution from the high frequency region is proposed. Good agreement is obtained between our calculations and experimental results. The change of
N
due to gain in injection laser is also calculated and it is shown that it cannot be neglected.