Self-Kerr Effect across the Yellow Rydberg Series of Excitons in <math display="inline"><mrow><msub><mrow><mi>Cu</mi></mrow><mrow><mn>2</mn></mrow></msub><mi mathvariant="normal">O</mi></mrow></math>

International audienceWe investigate the nonlinear refraction induced by Rydberg excitons in Cu2O. 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 n2, present the n2 spectrum for principal quantum numbers n≥5, and report large n2 values of order 10-3 mm2/mW. Moreover, we observe a rapid saturation of the Kerr nonlinearity and find that the saturation intensity Isat 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