Cover Story：Kalapala A R K, Liu D, Cho S J, Park J P, Zhao D Y et al. Optically pumped room temperature low threshold deep UV lasers grown on native AlN substrates. Opto-Electron Adv 3, 190025 (2020). 

The cutting edge in semiconductor light sources has moved into the ultraviolet. Developers are pushing the wide-bandgap III-V gallium nitride compounds used in blue and violet lasers to shorter wavelengths by adding aluminum to increase the bandgap. In theory, that family can emit at wavelengths as short as 205 nm if the active layer is aluminum nitride. However, while high performance practical light sources are commercially available for visible and longer wavelength UV lasers, it is still a technical challenge to realize high performance UV lasers at wavelengths shorter than 280 nm (UV-C) due to the material challenges. Prof. Weidong Zhou and Dr. Akhil Raj Kumar Kalapala from University of Texas at Arlington, United States and coworkers reported an optically pumped room temperature low threshold lasers based on 21 multiple quantum wells (MQW) grown on native AlN substrate. The growth was carried out in a high-temperature reactor by low pressure organometallic vapor phase epitaxy (LP-OMVPE) process. High quality pseudomorphic growth was achieved, which resulted in high quality AlGaN based heterostructures for low threshold lasing. The team introduced a concept with large number of quantum wells for increasing optical mode confinement factor. Such a confinement factor increase can relax the gain threshold condition for the active region which results in lower threshold current per quantum wells. While further improvement is needed to reduce cavity loss and increase the optical power, the work demonstrated here has shown promising future for AlGaN MQW structures grown on native AlN substrate in realizing practical deep UV-C lasers for practical applications.