Interseries transitions between Rydberg excitons in Cu2O

We investigate the infrared optical transitions between excitons of the yellow, green, and blue series in the cuprous oxide ${\mathrm{Cu}}₂\mathrm{O}$. 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 $n\ensuremath{\approx}10$. 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.