High-resolution nanosecond spectroscopy of even-parity Rydberg excitons in Cu2O

We present a study of even-parity Rydberg exciton states in cuprous oxide using second harmonic generation (SHG) spectroscopy. Excitonic states with principal quantum number $n=5\ensuremath{-}12$ were excited by nanosecond pulses around $1143\phantom{\rule{0.16em}{0ex}}\mathrm{n}\mathrm{m}$. Using time-resolved single-photon counting, the coherently generated second harmonic was isolated both temporally and spectroscopically from inelastic emission due to lower-lying free and bound excitonic states, which included narrow resonances at $1.993\phantom{\rule{0.16em}{0ex}}\mathrm{e}\mathrm{V}$ associated with a long lifetime of $641\ifmmode\pm\else\textpm\fi{}7\phantom{\rule{0.16em}{0ex}}\mathrm{\ensuremath{\mu}}\mathrm{s}$. The near transform-limited excitation bandwidth enabled high-resolution measurements of the exciton lineshape and position, from which we obtained values for the quantum defects of the S and D excitonic states associated with the appropriate crystal symmetries. Odd-parity P and F excitonic states were also observed, in accordance with predicted quadrupole-allowed two-photon excitation processes. We compared our measurements to conventional one-photon spectroscopy in the same sample, and find that the SHG spectrum is cut off at a lower principal quantum number ($n=12$ vs $n=15$). We attribute this effect to a combination of spatial inhomogeneities and local heating, and discuss the prospects for observing higher principal quantum number even-parity states in future experiments.