Interaction between Rydberg Excitons in Cuprous Oxide Revealed through Second Harmonic Generation

TL;DR

This study investigates the interactions of Rydberg excitons in cuprous oxide using second harmonic generation, revealing intensity-dependent spectral shifts, broadening, and discrepancies with atomic-like interaction theories, indicating fundamental differences.

Contribution

It provides the first combined experimental and theoretical analysis of Rydberg exciton interactions in Cu$_2$O using SHG, highlighting unexpected deviations from atomic models.

Findings

Spectral shifts and broadening with increasing laser intensity.

Quantitative exciton density measurements and interaction values.

Discrepancies with atomic-like van der Waals interaction predictions.

Abstract

We report experimental and theoretical investigations of interacting excitons of the yellow series in cuprous oxide (Cu$_2$O) with principal quantum numbers up to $n=7$ by means of second harmonic generation (SHG). Using picosecond pulsed laser excitation up to 10 GW/cm$^2$ peak intensity we observe a pronounced change of the spectra with increasing pump laser intensity: an energetic shift to lower absolute energies and a spectral broadening, while the absolute intensity for low powers scales with the square of the pump power, but saturates at higher powers. Concomitant with SHG we determined the density of the excitons excited by the ps pulse by measuring the two-photon absorption directly. This allows to derive quantitative values for the exciton-exciton interaction. Surprisingly, the results disagree both in magnitude and scaling with principal quantum number with those calculated by state-of-the art atomic-like van der Waals interaction theory. As a possible screening by an electron-hole plasma created by three-photon absorption into blue and violet band states could be ruled out, our results point toward fundamental differences between excitons and atoms.