Optical pumping in nitrogen-doped GaP

We report on the results of optical-pumping experiments in GaP doped with the isoelectronic impurity nitrogen. In the first experiment we have measured the saturation of absorption at the $A$ bound exciton as a function of pump intensity, using as the pump a pulsed dye laser tuned to the $A$ line. The observed saturation behavior is in quantitative accord with a simple three-level system involving the $A$ and $B$ bound-exciton level and the N ground state. The second experiment involves measuring the absorption at the $A$ line with a weak probe (a tunable dye laser) while the crystal is being pumped with an intense pump dye laser tuned above the indirect gap. In this case also the absorption saturates with increasing pump power and we can infer approximately 10% efficiency for the capture of excitons at N impurities. In this second experiment, a simple model predicts an amplification by a factor of 8.5 but none is observed. We conclude from these measurements that, when pumped above the indirect gap, the dominant excited state of N impurities in our crystal is a single electron trapped at N. We believe that this is caused by the presence of other impurities in the crystal and is not an intrinsic limitation of the system.