We present experimental as well as theoretical data for the linear-optical response of symmetrically strained ${\mathrm{Ge}}_4$${\mathrm{Si}}_6$ [001] superlattices. Ab initio calculations show that they have a direct gap. The complex dielectric function has been measured ellipsometrically. Agreement of the experimental second-derivative spectrum ${\mathit{d}}^2$${\mathrm{\epsilon }}_2$/d${\mathrm{\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.

• Received 24 July 1990

DOI:https://doi.org/10.1103/PhysRevLett.65.1933