Recently, laser irradiation (LI) of colloidal nanoparticles (NPs) using a non-focused laser beam at moderate fluence attracts much attention as a novel and simple technique to obtain submicron-sized spherical particles. In the present study, we applied this technique to prepare gold SMPs. It was revealed that agglomeration of the source nanoparticles prior to laser irradiation is necessary to produce SMPs. However, when the agglomeration occurred in too much extent, significant amount of the source particles remained as the sediment after LI, leading to the lowering of the formation efficiency of SMPs. Therefore, the control of the agglomeration conditions of the source NPs is necessary to obtain SMPs efficiently. In the present study, we tried to adjust the agglomeration conditions of the source NPs by adjusting the concentration of citrate that was used as the stabilizing reagent of the source NPs. It was revealed that SMPs were obtained efficiently while the sedimentation of the source NPs were suppressed when the concentration of citrate was adjusted around 0.01-0.005 mM. In addition, observation of the temporal change in the shape of the colloidal particles during LI revealed that there is an induction period in which no formation of SMPs is brought about by LI. This finding suggested that LI removes the citrate ligands from the source NPs and induces the agglomeration of the source NPs, i.e. the agglomeration condition of the source NPs is also controlled by LI.
We experimentally examined our proposed structure for realizing the control of resonant and lasing properties even in random structures, which was composed of size-mono-dispersive scatterers and intentionally introduced defect regions. In the experiments, by intentionally introducing polymer nanoparticles as point defects into a mono-dispersive zinc oxide nanoparticle film, we succeeded that lasing properties at the defect region were drastically modified, comparing with those of typical random lasers; suppression of the number of lasing modes, decrease in the thresholds, and limiting the lasing position at the defect. These results suggest the possibility that we can realize single-mode random lasers with well-controlled modal properties even in random structures.