Room-temperature ultraviolet (UV) laser emission from self-assembled ZnO microcrystallite thin films is reported. The hexagonal ZnO microcrystallites are grown by laser molecular beam epitaxy and self-assembled on sapphire substrates. The facets of the hexagons form natural Fabry–Pérot lasing cavities. The optical gain for the room-temperature UV stimulated emission is of an excitonic nature and has a peak value an order of magnitude larger than that of bulk ZnO crystal. The observation of room-temperature UV lasing from the ordered, nano-sized ZnO crystals represents an important step towards the development of nanometer photoelectronics. ZnO is a wide-gap semiconductor with a large exciton binding energy of 60 meV, which should allow efficient excitonic lasing mechanisms to operate at room temperature. However, UV stimulated emission and lasing have been extensively studied in bulk ZnO crystals at cryogenic temperatures, but observation of lasing at RT was mentioned only briefly in the works of Klingshirn and in very recent reports. The ZnO thin films were grown on sapphire (0001) substrates. The films consist of an epitaxially ordered array of hexagonal microcrystallites. The facets of all hexagons are parallel to those of the others, forming natural Fabry–Pérot lasing cavities. Under moderate excitation, the optical gain responsible for the RT UV stimulated emission is of an excitonic nature. For a 55-nm-thick film, an excitonic gain of 320 cm⁻¹ is measured at a fluence of 3.0 μJ/cm², an order of magnitude larger than the gain reported for bulk ZnO crystals measured at much higher fluence. The growth of ZnO thin films on sapphire substrates and their linear optical spectra were reported many years ago. In this work, self-assembled ZnO microcrystallites were grown on sapphire substrates by the laser molecular beam epitaxy technique. A pure ceramic ZnO target (99.999%) was ablated in an ultrahigh vacuum chamber using a KrF excimer laser. X-ray diffraction measurement revealed that the ZnO microcrystallites have high crystallinity with c-axis orientation. Figure 1 shows the atomic force microscope (AFM) topography of a ZnO thin film with a thickness of 200 nm. The thin film consists of close-packed hexagons. The hexagonal column structure is also confirmed by transmission electron microscope TEM observation. The facets of the hexagons correspond to the {1100} plane and they are strictly parallel to the {1120} plane of the sapphire substrate. The lower trace in Fig. 2 shows theRoom-temperature ultraviolet (UV) laser emission from self-assembled ZnO microcrystallite thin films is reported. The hexagonal ZnO microcrystallites are grown by laser molecular beam epitaxy and self-assembled on sapphire substrates. The facets of the hexagons form natural Fabry–Pérot lasing cavities. The optical gain for the room-temperature UV stimulated emission is of an excitonic nature and has a peak value an order of magnitude larger than that of bulk ZnO crystal. The observation of room-temperature UV lasing from the ordered, nano-sized ZnO crystals represents an important step towards the development of nanometer photoelectronics. ZnO is a wide-gap semiconductor with a large exciton binding energy of 60 meV, which should allow efficient excitonic lasing mechanisms to operate at room temperature. However, UV stimulated emission and lasing have been extensively studied in bulk ZnO crystals at cryogenic temperatures, but observation of lasing at RT was mentioned only briefly in the works of Klingshirn and in very recent reports. The ZnO thin films were grown on sapphire (0001) substrates. The films consist of an epitaxially ordered array of hexagonal microcrystallites. The facets of all hexagons are parallel to those of the others, forming natural Fabry–Pérot lasing cavities. Under moderate excitation, the optical gain responsible for the RT UV stimulated emission is of an excitonic nature. For a 55-nm-thick film, an excitonic gain of 320 cm⁻¹ is measured at a fluence of 3.0 μJ/cm², an order of magnitude larger than the gain reported for bulk ZnO crystals measured at much higher fluence. The growth of ZnO thin films on sapphire substrates and their linear optical spectra were reported many years ago. In this work, self-assembled ZnO microcrystallites were grown on sapphire substrates by the laser molecular beam epitaxy technique. A pure ceramic ZnO target (99.999%) was ablated in an ultrahigh vacuum chamber using a KrF excimer laser. X-ray diffraction measurement revealed that the ZnO microcrystallites have high crystallinity with c-axis orientation. Figure 1 shows the atomic force microscope (AFM) topography of a ZnO thin film with a thickness of 200 nm. The thin film consists of close-packed hexagons. The hexagonal column structure is also confirmed by transmission electron microscope TEM observation. The facets of the hexagons correspond to the {1100} plane and they are strictly parallel to the {1120} plane of the sapphire substrate. The lower trace in Fig. 2 shows the