The authors present experimental as well as theoretical data for the linear optical response of symmetrically strained Ge4Si6 (001) superlattices. Ab initio calculations show that they have a direct gap. The complex dielectric function has been measured ellipsometrically. Agreement of the experimental second-derivative spectrum d2 epsilon 2/d omega 2 with the theory is obtained, both for the shape and position of bulklike transitions and for new superlatticelike transitions, after including lifetime effects in the calculated curves through Lorentzian convolution.
 

Physical Review Letters, 65 1933-6, 1990.