# Interseries transitions between Rydberg excitons in Cu$_{\mathrm{2}}$O

## TL;DR

This paper studies infrared optical transitions between different Rydberg exciton series in Cu₂O, revealing the limitations of the dipole approximation and deriving transition matrix elements from band parameters.

## Contribution

It provides a detailed analysis of interseries excitonic transitions in Cu₂O, highlighting the breakdown of the dipole approximation and deriving relevant matrix elements.

## Key findings

- Dipole approximation often inadequate for interseries transitions.
- Breakdown of dipole approximation occurs even at moderate quantum numbers (~10).
- Derived transition matrix elements from band parameters.

## Abstract

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.