Black TiO₂ Nanoparticles as Plasmonically Active Scatterers for Random Lasing

Surface engineering of conventional semiconducting nanomaterials enables a range of fascinating optical features that are absent in the native materials. In particular, black TiO2 (BT) nanoparticles are known to display a highly increased free carrier concentration on their hydrogenated surface shells, endowing them with a metal-like plasmonic behavior that are absent in native “white” TiO2 (WT). Presently, although BT particles are subjected to intense research, their possible applications in photonics have been neglected so far. Here, we have used a nanosecond pulsed laser ablation technique to synthesize BT nanoparticles, and these are further employed for generation of random laser (RL) emission in the gain medium of Rhodamine B (RhB). Prior to applications, rigorous experiments and theoretical calculations have been performed to probe the existent surface defects in BT particles, which are responsible for the excess free charge carrier density (∼1021 cm–3) and hence the visible plasmonic oscillation. The reasonably larger scattering efficiency of BT along with its plasmonic behavior brings about a ∼40% reduction in the RL threshold as compared to that obtained with traditional WT nanoscatterers. The optimized geometry of RL in the form of a capillary tube appears to provide large internal reflections that ensure the reinjection of amplified photons from the liquid–capillary interface. Thus, a further 50% reduction in the lasing threshold takes place, resulting in a value of only 1.60 mJ pulse–1. The present work will pave the way for a systematic growth of black TiO2 nanoscatterer particles and hence for the development of an efficient RL illumination source with potential applications in photonics, imaging, and sensing.