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Copper(I) oxide, $\mathrm{C}{\mathrm{u}}_2\mathrm{O}$, is a copper oxide semiconductor in which copper has oxidation state $+1$. The name distinguishes it from copper(II) oxide, $\mathrm{CuO}$; the chemical formula matters when identifying the material in an optical experiment.

Its importance here is the resolved Exciton Rydberg series observed in the exciton absorption experiment in copper(I) oxide. An optical absorption resonance can occur below the unbound electron–hole threshold because an electron and an electron hole bind as an exciton. Resolving many resonances approaching that threshold tests the internal bound-state model rather than directly imaging electron–hole orbits. Sample temperature, strain, illumination and line resolution affect how far the series can be followed; the linked experiment describes those controls.