  An accurate approach to calculate the optical response of periodic structures
is proposed. Using the genuine superlattice eigenfunctions and energy
eigenvalues, the eigenfunctions parity symmetries, the subband symmetries and
the detached surface energy levels, we report new optical-transition selection
rules and explicit optical-response calculations. Observed transitions that
were considered forbidden, become allowed and interesting optical-spectra
effects emerge as fingerprints of intra-subband and surface states. The
unexplained groups and isolated narrow peaks observed in high resolution
blue-laser spectra, by Nakamura et al., are now fully explained and faithfully
reproduced
