Self-Kerr effect across the yellow $\mathrm{Cu_2O}$ Rydberg series

TL;DR

This paper studies the nonlinear optical Kerr effect caused by Rydberg excitons in Cu2O, revealing large nonlinear indices, saturation behavior, and the influence of Rydberg blockade on phase shifts at low laser intensities.

Contribution

It provides the first detailed measurement of the Kerr nonlinearity spectrum across the Rydberg series in Cu2O and explains the saturation behavior via Rydberg blockade effects.

Findings

Large nonlinear index n2 of order 10^{-3} mm^2/mW observed.

Rapid saturation of Kerr nonlinearity with decreasing saturation intensity as n^{-7}.

Maximum phase shift of 0.5 rad limited by Rydberg interactions.

Abstract

We investigate the nonlinear refraction induced by Rydberg excitons in $\mathrm{Cu_2O}$. Using a high-precision interferometry imaging technique that spatially resolves the nonlinear phase shift, we observe significant shifts at extremely low laser intensity near each exciton resonance. From this, we derive the nonlinear index $\mathrm{n_2}$, present the $\mathrm{n_2}$ spectrum for $n\geq 5$ and report large $\mathrm{n_2}$ values of order $10^{-3}$ mm$^2$/mW. Moreover, we observe a rapid saturation of the Kerr nonlinearity and find that the saturation intensity $\mathrm{I_{sat}}$ decreases as $n^{-7}$. We explain this with the Rydberg blockade mechanism, whereby giant Rydberg interactions limit the exciton density, resulting in a maximum phase shift of 0.5 rad in our setup.