Giant Rydberg excitons in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>Cu</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:mi mathvariant="normal">O</mml:mi></mml:mrow></mml:math> probed by photoluminescence excitation spectroscopy

Rydberg excitons are, with their ultrastrong mutual interactions, giant optical nonlinearities, and very high sensitivity to external fields, promising for applications in quantum sensing nonlinear optics at the single-photon level. To design it is necessary know how other excited states relax lower-lying exciton states. Here, we present photoluminescence excitation spectroscopy as a method probe transition probabilities from various excitonic cuprous oxide, show $T=38$ mK principal numbers up $n=30$, corresponding calculated diameter of 3 $\mu$m.