Excitonic Lasing in Solution-Processed Subwavelength Nanosphere Assemblies.*

Lasing in solution-processed nanomaterials has gained significant interest because of the potential for low-cost integrated photonic devices. Still, a key challenge is designing low-threshold lasing devices based on a comprehensive understanding of the system's spectral and temporal dynamics. Here we show low-threshold random lasing in sub-wavelength thin films of coupled, highly crystalline zinc oxide nanospheres, with an overall thickness on the order of $\lambda $/4. The cavity-free geometry consists of 35nm zinc oxide nanospheres that collectively localize the in-plane emissive light fields while minimizing scattering losses, resulting in excitonic lasing with fluence thresholds at least an order of magnitude lower than previous UV-blue random and quantum-dot lasers. Fluence-dependent effects, as quantified by sub-picosecond transient spectroscopy, highlight the role of phonon-mediated processes in excitonic lasing. Sub-picosecond evolution of distinct lasing modes, together with 3D electromagnetic simulations, indicate a random lasing process - in violation of the commonly cited criteria of strong scattering from individual nanostructures. These results show that coupled nanostructures with high crystallinity can function as building blocks for high-performance optoelectronics.