Black TiO₂ Nanoparticles as PlasmonicallyActive 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.