Dynamic band gap shifts and magneto-absorption of Cu2o

We use an experimental technique which allows to push the spectral resolution for the spectroscopy of solids in the sub-μeV regime. The core element of the experimental apparatus is a frequency-stabilized ring-dye laser, with a resolution of a few neV. By means of this technique, we study dynamic band gap shifts of ∼0.1 μeV in Cu2O. A model is proposed, which explains the experimental findings in terms of a photothermal bistability. Furthermore, the magnetic field dependence of the ortho- and paraexciton resonances is studied. This yields a precise determination of the exchange splitting between the para- and the orthoexciton levels.