An experimental study of the yellow exciton series in ${\text{Cu}}_2\text{O}$ in strong magnetic fields up to 32 T shows the optical activation of direct and phonon-assisted paraexciton luminescence due to mixing with the quadruple allowed orthoexciton state. The observed phonon-assisted luminescence yields information on the statistical distribution of occupied states. Additional time-resolved experiments provide a unique opportunity to directly determine the time evolution of the thermodynamical properties of the paraexciton gas. Because the lifetime of paraexciton is hardly affected by the optical activation in a strong magnetic field, this opens new possibilities for studies aiming at Bose-Einstein condensation of excitons in bulk semiconductors.

• Received 28 February 2009

DOI:https://doi.org/10.1103/PhysRevB.80.045208