Text this: The refractive index of Pb(1-x)Sn(x)Te, Pb(1-y)Sn(y)Se, and PbS(1-x)Se(x) and theoretical calculations for Pb(1-x)Sn(x)Te and Pb(1-y)Sn(y)Se 10.6 micron heterostructure injection lasers.