Rydberg Series of Dark Excitons in Cu2O

We demonstrate the Rydberg series of dark excitons, known as paraexcitons, up to the principal quantum number n = 6 for the yellow exciton series in Cu2O, using second harmonic generation. Each of these states is optically inactive to all orders, but their observation becomes possible by application of a magnetic field which leads to mixing with the quadrupole-allowed bright excitons, called orthoexcitons, of the same n. The dark parastates are generally located below the bright orthostates, whose energies are increased by the electron-hole exchange interaction, except for n = 2, where this order is reversed. This inversion occurs due to band mixing, namely, of the 2S(y, o) orthoexciton of the yellow series with the 1S(g, o) orthoexciton of the green exciton series.