title: Excitonic Lasing in Solution-Processed Subwavelength Nanosphere Assemblies.
abstract: Lasing in solution-processed nanomaterials has gained significant interest because of the potential for low-cost integrated photonic devices. Still, a key challenge is to utilize a comprehensive knowledge of the system's spectral and temporal dynamics to design low-threshold lasing devices. Here, we demonstrate intrinsic lasing (without external cavity) at low-threshold in an ultrathin film of coupled, highly crystalline nanospheres with overall thickness on the order of ∼λ/4. The cavity-free geometry consists of ∼35 nm 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 (<75 μJ/cm(2)). Fluence-dependent effects, as quantified by subpicosecond transient spectroscopy, highlight the role of phonon-mediated processes in excitonic lasing. Subpicosecond evolution of distinct lasing modes, together with three-dimensional electromagnetic simulations, indicate a random lasing process, which is in violation of the commonly cited criteria of strong scattering from individual nanostructures and an optically thick sample. Subsequently, an electron-hole plasma mechanism is observed with increased fluence. These results suggest that coupled nanostructures with high crystallinity, fabricated by low-cost solution-processing methods, can function as viable building blocks for high-performance optoelectronics devices.
authors: Center for Functional Nanomaterials, Brookhaven National Laboratory , Upton, New York 11973, United States.
authors: Kannatassen
authors: K
authors: Appavoo
authors: Department of Physics, City College of New York , New York, New York 10031, United States.
authors: Xiaoze
authors: X
authors: Liu
authors: Department of Physics, City College of New York , New York, New York 10031, United States.
authors: Vinod
authors: V
authors: Menon
authors: Center for Functional Nanomaterials, Brookhaven National Laboratory , Upton, New York 11973, United States.
authors: Matthew Y
authors: MY
authors: Sfeir
mesh_terms: D004583:Electrons; D007834:Lasers; D008027:Light; D054118:Nanospheres / Q000737:chemistry* / Q000648:ultrastructure; D060739:Phonons; D013696:Temperature; D015034:Zinc Oxide / Q000737:chemistry*
chemical_list: D015034:Zinc Oxide
keywords: Cavity-free; electron−phonon coupling; near-field enhancement; room-temperature random lasing; solution-processed film; ultrafast dynamics