We report our results on random lasing from rhodamine 6G based colloidal gain medium consisting of urchin-like TiO<sub>2</sub> structures. Multimode behaviour is observed even at low pump laser powers. Emission linewidth narrowing and lasing threshold are investigated. Coherent back scattering is used to obtain the disorder degree of the sample. This urchin based system is demonstrated to possess lower lasing thresholds and enhanced efficiency with multimode behaviour compared to a TiO<sub>2</sub> spherical particle system with same disorder degree. This work opens up a new avenue for low threshold, high efficiency lasing.
Dielectric-plasmonic composite media are interesting systems as far as light trapping is considered owing to their enhanced interaction with light. Core-shell particles are of particular interest due to their strong scattering and geometry-dependent plasmon resonances which are tunable from visible to IR region. Design of random media with such highly scattering particles increases the probability of light getting trapped inside the medium due to the enhanced multiple scattering. Here we investigate the light scattering scenario in ZnS-Au core-shell random medium experimentally and also provide theoretical basis using Finite Difference Time Domain (FDTD) simulations. Back scattering and transmission simulations are carried out at standard wavelengths 405, 445, 532 and 632 nm. Effect of geometry on light scattering is also investigated by varying the aspect ratios of the particles. Scattering and absorption efficiency is calculated for particles in this size range using Mie theory. Strength and tunability of plasmon resonances are explained in terms of plasmon hybridization model. Core shell particles with core radius 100 nm and shell thickness of 6 nm are found to be optimal for light trapping over the entire visible region. Dielectric-plasmonic random media consisting of ZnS-Au core shell particles with optimized aspect ratio appear to be efficient candidate materials for light harvesting, sensing and optoelectronics.