The paper reports room-temperature ultraviolet (UV) laser emission from self-assembled ZnO microcrystallite thin films grown on sapphire substrates using laser molecular beam epitaxy. The hexagonal microcrystallites form a parallel array, creating natural Fabry-Pérot lasing cavities. The optical gain for UV stimulated emission is excitonic in nature and is an order of magnitude larger than that of bulk ZnO crystals. The study demonstrates the potential of these nano-sized ZnO crystals for developing nanometer photoelectronics, marking a significant step in the field of short-wavelength semiconductor diode lasers. The research also provides insights into the microscopic mechanisms responsible for the optical gain and stimulated emission, which are crucial for practical applications.The paper reports room-temperature ultraviolet (UV) laser emission from self-assembled ZnO microcrystallite thin films grown on sapphire substrates using laser molecular beam epitaxy. The hexagonal microcrystallites form a parallel array, creating natural Fabry-Pérot lasing cavities. The optical gain for UV stimulated emission is excitonic in nature and is an order of magnitude larger than that of bulk ZnO crystals. The study demonstrates the potential of these nano-sized ZnO crystals for developing nanometer photoelectronics, marking a significant step in the field of short-wavelength semiconductor diode lasers. The research also provides insights into the microscopic mechanisms responsible for the optical gain and stimulated emission, which are crucial for practical applications.