Measuring radiative current in optically pumped gain media

In Figure 2(a) ASE spectra are plotted as function of optical pump stripe length at a fixed pump power density. The modal gain, G, and un-calibrated spontaneous emission, CI^bspon, are extracted from this data as fitting parameters to a one dimensional amplifier model. The spontaneous emission is measured in arbitrary units at this point as C, the calibration factor that accounts for the fraction of the ASE generated within the device that is measured at the spectrograph, is not known. However, because we are able to measure gain in real units and because the fraction of gain to spontaneous emission is governed by the level of inversion, C can be determined with a known value of the population inversion factor, Fig. 3(a). This allows measured spontaneous emission spectra to be calibrated in real units, Fig. 3(b), and by adding the spectrally integrated emission rates for the TE and TM polarizations and multiplying by the electronic charge the total radiative recombination current, J_spon can be found. For the InP dot material optically pumped to a quasi-Fermi level separation of 1.81 eV we calculate that J_spon = (209 ± 49) A cm^-2. We validate the experiment by comparisonwith electrically pumped measurements on same device.