Observation of Rydberg Excitons in Epitaxially Grown Cu₂O Film on MgO Substrates

Recently, there has been a massive surge in interest amongst the scientific community regarding Cu₂O as a solid-state host for Rydberg excitons after it was observed that a naturally grown Cu₂O single crystal can host Rydberg excitons up to quantum numbers as high as n=25 [1]. Though the existence of Rydberg states in Cu₂O has been long known [2], it is this discovery of higher-level Rydberg excitons that made the field of solid-state Rydberg physics increasingly more interesting with the prospect of realizing similar kind of exciting quantum phenomenon as observed in their atomic counterparts [3], but now in a more technologically accessible solid-state environment. However, most of the available reports on Rydberg excitons of Cu₂O have focused on single crystals, either grown naturally or synthetically [1,4]. But for device fabrication, thin films and possible heterostructures are needed. Thin-film Cu₂O samples with thicknesses less than the blockade radius are especially appealing because they make the Rydberg blockade utilizable for nonlinear behavior in semiconductors [5].<br/><br/>In this work, we have grown epitaxial Cu₂O films on MgO (110) and (100) substrates using metallic copper (purity: 99.99%) as a target using a pulsed laser deposition (PLD) technique and observed full width half maxima (FWHM) of the XRD rocking curve as low as 0.3° for the films. Here we will present the low temperature (4 K) high-resolution absorption spectroscopy data for Cu₂O films elucidating the nature of higher Rydberg excitons. We will also demonstrate the effect of thickness, the degree of crystallinity (ranging from single crystalline to polycrystalline) of Cu₂O on the optical properties of the Rydberg excitons. In this manner, we demonstrate an oxide semiconductor as a CMOS-compatible and integrable platform, which can lead to quantum optical computing in the Rydberg regime [6].<br/><br/>1.Kazimierczuk, T., Fröhlich, D., Scheel, S. et al. Giant Rydberg excitons in the copper oxide Cu₂O. Nature <b>514</b>, 343–347 (2014).<br/>2.Gross, E. F. Optical spectrum of excitons in the crystal lattice. Nuovo Cimento Suppl. <b>3</b>, 672–701 (1956).<br/>3.C S Adams <i>et. al</i> Rydberg atom quantum technologies. J. Phys. B: At. Mol. Opt. Phys. <b>53 </b>012002 (2020).<br/>4.Lynch, Stephen A., et al. "Rydberg excitons in synthetic cuprous oxide Cu 2 O." Physical Review Materials 5.8 (2021): 084602.<br/>5.DeLange, J., Barua, K., Paul, A.S. et al. Highly-excited Rydberg excitons in synthetic thin-film cuprous oxide. Sci Rep 13, 16881 (2023).<br/>6.Wu, X. L. et al. A concise review of Rydberg atom-based quantum computation and quantum simulation. <i>Chin. Phys. B</i> <b>30</b>, 020305 (2021).