Gold nano-urchins for plasmonic enhancement of random lasing in a dye-doped polymer

We report our results on a plasmonic random laser with three-dimensional (3D) gold nano-urchins as scatterers distributed in rhodamine 6G dye-doped polymer film. The performance of anisotropic urchin scatterers is first studied using electromagnetic simulations for absorption/scattering cross-section and local field enhancement. This is compared to gold nanospheres of similar size. The simulation results indicate a two-fold local field enhancement, a higher scattering cross-section, and a low absorption cross-section in the 400 nm-570 nm spectral region of interest for nano-urchins. The effective scattering mean free path for urchins is calculated to be 90 µm less than nanoparticles. This suggests nano-urchins are efficient scatterers over conventional nanospheres for random lasing. A random laser is then experimentally demonstrated using gold nano-urchin scatterers, and incoherent lasing emission is observed for very low urchin number density of order ∼108 cm-3. A three-fold increase in scatterer concentration is shown to reduce the threshold energy from 0.8 mJ to 0.28 mJ per pulse. This is accompanied by a linewidth decrease from the rhodamine 6G emission bandwidth of 58 nm to up to 3 nm. Along with particle scattering, the waveguiding mechanism is identified to provide additional feedback for the lasing. This has been validated by the angular measurement of emission and by using spot pumping scheme. With low gold nano-urchin concentration, being incoherent and in film form, this plasmonic random laser could be an economical solution for speckle-free imaging applications.