A research group led by 2014 Nobel laureate Hiroshi Amano from the Institute of Materials and Systems for Sustainability (IMaSS) at Nagoya University in central Japan, in collaboration with Asahi Kasei Corporation, conducted successfully the first continuous wave laser at room temperature of a deep – ultraviolet laser diode (wavelengths up to the UV-C region).
These results, published in Applied Physics Lettersrepresent a step towards widespread use of a technology with potential for a wide range of applications, including sterilization and medicine.
Since their introduction in the 1960s, and after decades of research and development, the successful commercialization of laser diodes (LDs) has finally been achieved for a number of applications with wavelengths ranging from infrared to blue -purple. Examples of this technology include optical communication devices with infrared LDs and Blu-ray discs using blue-violet LDs.
However, despite the efforts of research groups around the world, no one has been able to develop deep ultraviolet LDs. A key breakthrough came only after 2007 with the emergence of aluminum nitride (AlN) substrate fabrication technology, an ideal material for growing aluminum gallium nitride film (AlGaN) for devices emitting UV light.
From 2017, Professor Amano’s research group, in cooperation with Asahi Kasei, the company that provided 2-inch AlN substrates, started to develop deep ultraviolet LD. At first, sufficient current injection into the device was too difficult, preventing further development of UV-C laser diodes.
But in 2019, the research group successfully solved this problem using a polarization-induced doping technique. For the first time, they produced short-wavelength ultraviolet-visible (UV-C) LD that works with short current pulses. However, the input power required for these current pulses was 5.2 W. This was too high for the continuous wave laser because the power would cause the diode to heat up quickly and shut down the laser.
But now researchers from Nagoya University and Asahi Kasei have redesigned the structure of the device itself, reducing the drive power needed to operate the laser to just 1.1W at room temperature. Earlier devices were found to require high levels of operating power due to the inability of efficient current paths due to crystal defects that occur at the laser band. But in this study, the researchers found that the strong crystal deformation creates these defects.
By cleverly adapting the sidewalls of the laser strip, they removed defects, achieving efficient current flow to the active region of the laser diode and reducing operating power.
Nagoya University’s industry-university cooperation platform, called Center for Integrated Research on Future Electronics, Transformative Electronics Facilities (C-TEF), has made possible the development of new UV laser technology . Under C-TEFs, researchers from partners such as Asahi Kasei share access to state-of-the-art facilities on the Nagoya University campus, providing them with the people and tools needed to build reproducible devices from high quality.
Zhang Ziyi, a research team representative, was in his second year at Asahi Kasei when he got involved in creating the project. “I wanted to do something new,” he said in an interview. “At the time, everyone assumed that the deep ultraviolet laser diode was an impossibility, but Professor Amano told me: ‘We have reached the blue laser, the time has come for ultraviolet’.”
This research is an important step in the practical application and development of semiconductor lasers in all wavelength ranges. In the future, UV-C LDs could be applied to healthcare, virus detection, particle measurement, gas analysis and high-definition laser processing.
“Its application to sterilization technology could be revolutionary,” Zhang said. “Unlike current LED sterilization methods, which are inefficient in terms of time, lasers can disinfect large areas in a short time and over long distances.” This technology could particularly benefit surgeons and nurses who need sterilized operating rooms and tap water.
The good results have been reported in two articles Applied Physics Letters.
Hiroshi Amano et al, Local stress control to suppress dislocation generation for pseudomorphically grown AlGaN UV-C laser diodes, Applied Physics Letters (2022). DOI: 10.1063/5.0124512
Hiroshi Amano et al, Key Temperature Dependent Characteristics of AlGaN Based UV-C Laser Diode and Demonstration of Continuous Wave Laser at Room Temperature, Applied Physics Letters (2022). DOI: 10.1063/5.0124480
Provided by Nagoya University
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