High resolution spectroscopy of yellow 1S excitons in Cu2O

The 1S ortho- and para-exciton of the yellow series in Cu2O are studied by high resolution spectroscopy. We observe that the 3-fold ortho-exciton (Γ5+-symmetry) is not degenerate. For a general k-direction there are three resonances with line width down to 1 μeV. The splitting is quantitatively explained by taking into account the long range and short range exchange interaction. The group velocity of the quadrupole polariton is directly determined with use of 20 ns pulses with still high spectral resolution (ΔE≈0.1 μeV). Group velocities down to 4×104 m/s are quantitatively described by a one-oscillator quadrupole polariton model. The para-exciton (Γ2+-symmetry) is seen in absorption in magnetic fields down to 1 T. In selected crystals it shows a line width of 0.2 μeV.