We investigate the infrared optical transitions between excitons of the yellow, green and blue series in the cuprous oxide Cu$_{ \mathrm{ 2}}$O. We show that in many cases the dipole approximation is inadequate and, in particular, that it breaks down in yellow-blue transitions even for moderate principal quantum numbers of $n \approx 10$. The interband matrix elements of the transition operator needed for the evaluation of the excitonic transition strengths are derived from known as well as from fitted band parameters.
Categories: cond-mat.mes-hall