Far-UVC micro-LEDs for enhanced fiber coupling
Fig. 1: Flip-chip mounted far-UVC-LED coupled to a multi-mode optical fiber.
Fig. 2: Scanning electron microscopy image of a micro-LED array during the production process. Inset: Flip-chip mounted LED chip.
Fig. 3: Voltage and transmitted optical power vs. applied current of a 235 nm far UVC micro-LED through a 910 µm optical fiber.
The Ferdinand-Braun-Institut has a long history of developing efficient AlGaN-based light emitting diodes (LEDs) and laser diodes. Recently, our research activities have included a focus on LEDs emitting in the so-called far-UVC spectral range with wavelengths below 240 nm. These can be used for the disinfection of surfaces and human skin, since the radiation does not penetrate deeply into the skin while effectively inactivating pathogens such as multi-drug-resistant bacteria and viruses.
The efficiency of these LEDs is still low compared to their counterparts in the visible range. One major reason is the low light extraction efficiency (LEE), which is defined as the ratio of photons extracted from the LED chip to the number of photons generated inside the semiconductor chip. It could be shown that this value, and thus the external quantum efficiency (EQE), can be increased by a factor of two to three if the semiconductor surface is structured into micrometer-sized pixels with reflective, 45° inclined sidewalls. These so-called micro-LEDs allow photons traveling laterally inside the semiconductor to be reflected and redirected towards the backside of the transparent sapphire carrier wafer, from which they can subsequently be extracted. Using 10,000 to 100,000 micro-LEDs with diameters down to 1 µm (Fig. 2), record EQEs of 3.0 % and wall plug efficiencies (WPE) up to 2.0 % were achieved [1,2].
Another challenge is coupling the light generated by LEDs into optical fibers. This would allow to not only disinfect surfaces, but also body cavities such as the throat, where many germs have their habitat. An endoscope equipped with a UV-transparent optical fiber could effectively eliminate these bacteria and viruses. However, coupling light from LEDs into fibers is usually inefficient, since the emission angle of the LEDs is large while the half-acceptance angle of UV-transparent fibers is limited to ~ 12°.
Recently, we used the approach of micro-LED arrays to reduce the full far-field emission angle of 235 nm LEDs from 140° in the standard design to 90° for 1.5 µm micro-LEDs. This, in turn, led to a fivefold increase in the transmitted power through a 910 µm multimode fiber. The improvement is caused by the enhanced LEE (factor 2.5) and the higher coupling efficiency (factor 2), which was increased from 4 to 8 % [3].
By further improving the technology, it was possible to integrate 125,000 closely packed circular micro-LEDs, each with a diameter of less than 1.5 µm, into a single 1 mm x 1 mm LED chip. At a drive current of 200 mA, more than 1 mW optical power at a wavelength of 235 nm was transmitted through a 1 m long fiber with a core diameter of 910 µm (Fig. 3). Partners at Charité Berlin predict that this power level should be sufficient to achieve disinfection of the human throat in less than 5 minutes, paving a new path for the eradication of multi-drug-resistant germs.
Publications
[1] J. Rass, “UV LEDs go micro”, Compound Semiconductor, vol. 30, no. 3 (2024).
[2] J. Rass, S. Hagedorn, H. K. Cho, T. Kolbe, M. Guttmann, S. Breuer, J. Ruschel, S. Einfeldt, “Influence of the AlN-sapphire template on the optical polarization and efficiency of AlGaN-based far-UVC micro LED arrays”, Semicond. Sci. Technol., vol. 40, no. 1 (2025).
[3] Jens Rass et al., “Far-UVC micro-LED arrays for efficient light extraction and fiber coupling”, Appl. Phys. Lett. 127 (2025).