Title: Experimental realization of purely excitonic lasing in ZnO microcrystals at room temperature: transition from exciton-exciton to exciton-electron scattering,
Abstract: Since the seminal observation of room-temperature laser emission from ZnO
thin films and nanowires, numerous attempts have been carried out for detailed
understanding of the lasing mechanism in ZnO. In spite of the extensive efforts
performed over the last decades, the origin of optical gain at room temperature
is still a matter of considerable discussion,. We show that ZnO microcrystals
with a size of a few micrometers exhibit purely excitonic lasing at room
temperature without showing any symptoms of electron-hole plasma emission. We
then present the distinct experimental evidence that the room-temperature
excitonic lasing is achieved not by exciton-exciton scattering, as has been
generally believed, but by exciton-electron scattering. As the temperature is
lowered below ~150 K, the lasing mechanism is shifted from the exciton-electron
scattering to the exciton-exciton scattering. We also argue that the ease of
carrier diffusion plays a significant role in showing room-temperature
excitonic lasing.