  Radiation-induced coupling between A and B excitons in ZnO is theoretically
studied. Considering the center-of-mass motion of excitons in bulk and thin
film structures, we reveal the eigenmodes of an exciton--radiation coupled
system and the ratio of each excitonic component, which is determined from
diagonalization of the self-consistent equation between the polarization and
the Maxwell electric field. In particular, in a nano-to-bulk crossover size
regime, the large interaction volume between multipole-type excitonic waves and
radiation waves causes radiative coupling between excitons from different
valence bands, which leads to an enhancement of the radiative correction. The
results presented in this study are in striking contrast with the conventional
view of the optical response of excitons in ZnO, where A and B excitons are
independently assigned to their respective spectral structures. It is also
clarified that the density of each excitonic component is of a significant
value for attribution of nonlinear optical signals.
