Since their introduction in the 1960s, laser diodes have been developed and commercialized for a number of applications with wavelengths ranging from infrared to blue-violet. Examples of this technology include optical communications devices with infrared laser diodes and Blu-ray Discs using blue-violet laser diodes. Still, despite the efforts of research groups around the world, deep ultraviolet (DUV) laser diodes remained elusive. Researchers from Nagoya University and Asahi Kasei reported continuous-wave lasing of deep-ultraviolet laser diode at room temperature, which is a world first, according to the team. Applications for the semiconductor lasing advancement range from those in biophotonics to those in particle measurement and gas analysis. Courtesy of Issey Takahashi. Now, researchers from Nagoya University, in collaboration with Asahi Kasei Corp., have reported room temperature continuous-wave (CW) lasing of a DUV laser diode, which the group claimed is a world first. The work is a milestone in the practical application of semiconductor lasers in all wavelength ranges and will allow UV-C laser diodes to be applied to health care, virus detection, particulate measurement, gas analysis, and high-definition laser processing. The group, led by 2014 Nobel laureate Hiroshi Amano of Nagoya University’s Institute of Materials and Systems for Sustainability, overcame serious design challenges to develop a laser diode capable of such a feat. A key breakthrough supporting the recent advancement occurred after 2007 with the emergence of technology to fabricate aluminum nitride substrates, an ideal material for growing aluminum gallium nitride (AlGaN) film for UV light-emitting devices. Starting in 2017, Amano’s research group, in cooperation with Asahi Kasei — the company that provided 2-in. aluminum nitride (AlN) substrates — began developing a deep-ultraviolet laser diode. At first, sufficient injection of current into the device was too difficult, preventing further development of UV-C laser diodes. The diode technology supports medical applications, specifically in the sterilization techniques used to establish conditions needed in operating rooms. Courtesy of Asahi Kasei Corp. and Nagoya University. In 2019, the group solved this problem using a polarization-induced doping technique. For the first time, the group produced a short-wavelength ultraviolet-visible (UV-C) laser diode that operated with short pulses of current. However, the input power required for these current pulses was 5.2 W. This was too high for CW lasing because the power would cause the diode to quickly heat up and stop lasing. To overcome this in the recent research, the team reshaped the structure of the device itself, reducing the drive power needed for the laser to operate at only 1.1 W at room temperature. Earlier devices were found to require high levels of operating power due to the inability of effective current paths owing to crystal defects that occur at the laser stripe. But in this study, the researchers found that the strong crystal strain creates these defects. With clever tailoring of the side walls of the laser stripe, they suppressed the defects, achieving current flow to the active region of the laser diode and reducing the operating power. “Its application to sterilization technology could be groundbreaking,” said Zhang Ziyi of Asahi Kasei. “Unlike the current LED sterilization methods, which are time-inefficient, lasers can disinfect large areas in a short time and over long distances.” The technology could especially benefit surgeons and nurses who need sterilized operating rooms and tap water. The research was published in two papers in Applied Physics Letters: (www.doi.org/10.1063/5.0124512) and (www.doi.org/10.1063/5.0124480).