Laser action in nanowires: Observation of the transition from amplified spontaneous emission to laser oscillation

Direct evidence of the transition from amplified spontaneous emission to laser action in optically pumped zinc oxide (ZnO) nanowires, at room temperature, is presented. The optical power evolves from a superlinear to a linear regime as the pump power exceeds threshold, concomitant with a transition to directional emission along the nanowire and the emergence of well defined cavity Fabry-Pérot modes around a wavelength of ≈ 385 nm , the intensity of which exceeds the spontaneous emission background by orders of magnitude. The laser oscillation threshold is found to be strongly dependent on nanowire diameter, with no laser oscillation observed for diameters smaller than ∼ 150 nm . Finally, we use an alternative "head on" detection geometry to measure the output power of a single nanowire laser.