Stimulated emission and lasing in polyhedral ZnO microcrystals

The fabrication of efficient miniature ultraviolet laser sources, as well as the study of fundamental aspects of stimulated emission in such microlasers, is an actual aim of modern optoelectronics. In this work, features of photoluminescence and lasing on whispering gallery modes in polyhedral ZnO microcrystals synthesized through the vapor–liquid–solid mechanism have been studied. It has been shown that near-band-edge emission has different characters in microcrystals of different sizes: low-threshold lasing occurs in crystals 10–15 $\mu$m in size, whereas amplified spontaneous emission in a wide range of excitation power densities is observed in larger crystals with a size up to 80 $\mu$m. A low density of polyhedral microcrystals on a substrate and a large dispersion of their sizes allow the separate study of both types of emission. This circumstance makes these crystals promising for the investigation of stimulated emission accompanying lasing on whispering gallery modes. Analysis makes it possible to suggest that stimulated emission in the crystals under study is due to the scattering of electron–hole pairs by free carriers, which ensures a low threshold of lasing.