Co-existence of strongly and weakly localized random laser modes

Recent theoretical work on random lasing predicts that the occurrence of narrow lasing spikes can be caused by both localized and extended modes of light(1-3). Typical random lasing spikes have been observed in powders of zinc oxide nanoparticles(4), but identifying the possible degrees of localization of such modes has until now been an open issue(5). In this Letter we present an experimental procedure that directly extracts the area of localization of the modes. This method relies on the investigation of micro-structured fields of zinc oxide powder, which also allows direct correlation to the local structural properties of the ensemble of subwavelength particles by scanning electron microscopy. We find that lasing from both localized and extended modes can be observed simultaneously. Our observation also corroborates the prediction that localized modes have a lower loss rate than that of extended modes(5).