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Study Demonstrates Vertical-Cavity Surface-Emitting Laser of Shortest Wavelength

Scientists from the Chalmers University of Technology, together with collaborators at Technische Universität Berlin, have developed and exhibited the shortest wavelength ever reported for a vertical-cavity surface-emitting laser (VCSEL).

An artistic illustration of an optically pumped ultraviolet B (UVB) vertical-cavity surface-emitting laser (VCSEL).
An artistic illustration of an optically pumped ultraviolet B (UVB) vertical-cavity surface-emitting laser (VCSEL). Image Credit: Krantz NanoArt.

The study could lay the basis for future applications in, for instance, disinfection and medical treatment. The study findings were published recently in the scientific journal ACS Photonics.

Although there is still much work to be done, especially to enable electrically driven devices, this demonstration provides an important building block for the realization of practical VCSELs covering the major part of the UV spectral range.

Filip Hjort, Study First Author and PhD Student, Photonics Laboratory, Department of Microtechnology and Nanoscience (MC2), Chalmers University of Technology

A VCSEL is a small semiconductor laser that has found extensive use in, for instance, optical communication in data centers and facial recognition in smartphones.

Until now, these lasers are available on the market only with red and infrared wavelengths, but other visible-emitting VCSELs that could find applications in adaptive headlamps for cars or projection displays will be commercialized shortly.

If the wavelength range could be pushed further, into the ultraviolet (UV), VCSELs could find any even broader use. UV light can be used for disinfection, material curing, fluorescence excitation, and medical treatment, and UV-emitting VCSEL could, for example, be used in compact water, air and surface disinfection systems as well as for treatment of skin diseases.

Filip Hjort, Study First Author and PhD Student, Photonics Laboratory, Department of Microtechnology and Nanoscience (MC2), Chalmers University of Technology

The laser medium should be made of AlGaN to achieve UV emission wavelengths in the ultraviolet B (280–320 nm) and ultraviolet C (200–280 nm), which is required for a majority of these applications.

The research team of Åsa Haglund, Professor at the Photonics Laboratory at MC2, together with their collaborators at Technische Universität Berlin, has earlier illustrated an electrochemical etching technique that can be employed to selectively etch certain AlGaN layers. In their present study, the two research teams applied this technique to create the world’s first optically pumped UVB-emitting VCSEL.

By using the electrochemical etching technique to remove the substrate and create smooth AlGaN membranes, we solved a long-standing problem for UV VCSELs. VCSELs need two mirrors with over 99% reflectivity and these can either be made using epitaxial growth or dielectric materials.

Filip Hjort, Study First Author and PhD Student, Photonics Laboratory, Department of Microtechnology and Nanoscience (MC2), Chalmers University of Technology

However, reflectivities that high in the UVB or UVC have not been achieved using epitaxial growth, and the typical substrate removal methods used to enable the deposition of the second dielectric mirror in blue-emitting VCSELs are not suitable for AlGaN,” added Hjort.

Hjort continued, “By employing electrochemical etching, we were able to create AlGaN membranes that we could sandwich between two highly reflective dielectric mirrors. In this way, we formed a vertical cavity that lases under optical pumping.”

The latest illustration is the shortest wavelength VCSEL ever reported and the electrochemical etch method can also be extended to UVC wavelengths required for sterilization applications to, for instance, fight future pandemics and offer clean drinking water.

Journal Reference:

Hjort, F., et al. (2020) A 310 nm Optically Pumped AlGaN Vertical-Cavity Surface-Emitting Laser. ACS Photonics. doi.org/10.1021/acsphotonics.0c01382.

Source: https://www.chalmers.se/en/Pages/default.aspx

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