Scientists Develop Bacterium-Sized Microlaser

An international team of researchers, including scientists from HSE University–St Petersburg, has developed microlasers that emit deep-ultraviolet light at a wavelength of 255 nanometres. The devices operate at room temperature, and the smallest of them measures just two micrometres in diameter—roughly the size of a bacterium. These microlasers could be used in sensors, spectroscopic systems, photonic chips, and communication devices. The paper has been published in Optics & Laser Technology.

Deep ultraviolet (DUV) is the portion of the ultraviolet spectrum with wavelengths shorter than 300 nanometres. This light is invisible to the human eye and does not illuminate its surroundings in the way visible light does. However, because its photons carry high energy, deep-ultraviolet radiation is readily absorbed by matter and can initiate photochemical reactions. As a result, it is widely used in applications such as gas analysis, the detection of biologically active substances, disinfection, and short-range data transmission.

Conventional sources of DUV, such as mercury lamps and gas lasers, contain toxic substances and are bulky. This limits their use in many applications: the smaller the light source, the easier it is to integrate it into a chip, sensor, or other compact device. However, designing miniature lasers is far more challenging, as material defects, radiation losses, and even slight imperfections in the device's geometry have a much greater impact on their performance.

An international team of scientists from HSE University–St Petersburg, the Ioffe Institute of the Russian Academy of Sciences, the B.I. Stepanov Institute of Physics of the National Academy of Sciences of Belarus, and Qilu University of Technology (China) has developed miniature short-wavelength lasers on sapphire substrates.

'Sapphire is already widely used in manufacturing, making it more affordable and cost-effective than many alternative materials. At the same time, it can be processed using standard microelectronics techniques, such as layer deposition, patterning, and etching of device structures. This paves the way for the development of compact ultraviolet photonic chips for spectroscopy, biosensing, and communication systems,' says Eduard Moiseev, co-author of the study and Senior Research Fellow at the HSE International Laboratory of Quantum Optoelectronics.

The researchers grew thin semiconductor layers on sapphire and then used microfabrication technology to form microdisks with diameters of about two micrometres. In these tiny structures, light is confined by the whispering gallery effect and amplified in an active region containing three quantum wells. Just as sound can travel along a curved wall in a gallery, light in a microdisk laser circulates along the edge of the disk, repeatedly reflecting from its perimeter. This allows the radiation to remain confined within a very small resonator without the need for a complex system of mirrors.

The resulting lasers operate at room temperature and emit light at a wavelength of about 255 nanometres. According to the authors, this represents one of the shortest-wavelength implementations of microdisk whispering gallery mode lasers on sapphire. For the smallest device, with a diameter of 2 micrometres, the threshold power density is about 280 kW/cm², which corresponds to some of the best reported values for such short wavelengths.

'These devices are currently optically pumped using an external laser, but the next step is to switch to electrical pumping. In practical terms, this would be far more convenient, as it would enable the use of microlasers in portable devices without the need for bulky external light sources. To achieve this, it is necessary to reduce the electrical resistance of the layers, ensure efficient injection of charge carriers into the active region where lasing occurs, and at the same time preserve the high crystal quality,' says Eduard Moiseev.

The study demonstrates that a DUV laser can be miniaturised to the size of a bacterium while still operating at room temperature. In the future, such microlasers could be used in spectroscopic systems, biochemical and gas sensors, UV-C communication devices, and photonic chips—anywhere a compact source of DUV light is required.