Ultrafast upconversion probing of lasing dynamics in single ZnO nanowire lasers

The ultrafast lasing dynamics of single zinc oxide nanowires are studied by time-resolved upconversion of the lasing emission as a function of the ultraviolet excitation intensity. Induction times for stimulated emission in individual nanowires are observed to be 1-5 ps or 3-15 roundtrips for nominal 20 mu m long nanowires, depending on the 267 nm excitation intensity. In separate experiments, transient absorption profiles are also obtained for time-delayed 800 and 400 nm pulses to elucidate the carrier dynamics, such as rapid decay (2-3 ps) of the electron-hole plasma (EHP) states at high carrier densities during lasing in the nanowire and the slower exciton decay component (15-60 ps) at lower excitation densities. The dynamics of the lasing wavelength dependence on carrier density is also studied and related to the band gap renormalization in the EHP regime.