Bound excitons in Cu2O:: Efficient internal free exciton detector

We report the spectroscopic observation of bound excitons in natural Cu2O samples, which are generated by impurity capture of free excitons. Due to the broken symmetry of a bound exciton, its radiative recombination rate increases, causing a greatly enhanced luminescence intensity compared with that for a free exciton. Moreover, the bound exciton luminescence serves as an internal monitor of the free ortho- and free para-exciton densities, whereas the free para-exciton luminescence is allowed only via weak Gamma(-)(25) phonon emission. At high excitation levels, the bound exciton luminescence shows a density-dependent loss of excitons that is well explained by a recently proposed biexciton formation model. The temperature dependence of the bound ortho- and bound para-exciton luminescence indicates a T-1-dependence for the exciton-impurity capture rate.