Russian Team Develops Microscopic Sapphire-Based Ultraviolet Laser for Photonic Chips

A research collaboration involving scientists from Russia, Belarus, and China has successfully developed microscopic lasers built on sapphire substrates that emit ultraviolet rays at room temperature.

These laser devices are extremely small, measuring approximately two micrometers, a size comparable to that of bacteria.

The press service of the Higher School of Economics in Moscow stated that these lasers have potential applications in sensors, photonic chips, and communication devices.

Edward Moiseev, the lead researcher at the International Laboratory of Quantum Optical Electronics at the Higher School of Economics, explained that sapphire is widely used in various industries and can be processed using established microelectronics techniques such as layer growth, patterning, and device element etching. This facilitates the production of integrated photonic chips for spectral analysis, biosensors, and ultraviolet communication systems.

The scientific team has been working for several years on developing ultra-small lasers that can be integrated into microcircuits and miniature devices comparable in size to red blood cells. Achieving this is highly complex because reducing the laser size rapidly increases the challenges of light confinement within the resonator, which is the structure where light reflects and amplifies multiple times.

Recently, the researchers discovered that this issue can be addressed by employing the so-called "whispering gallery" effect alongside a special insulating layer composed of a gradually varying mixture of aluminum nitride and gallium aluminum nitride. This intermediate layer compensates for mechanical stresses between the sapphire substrate and the gallium indium nitride layers, while also reducing radiation leakage, allowing the laser to operate stably even at very small sizes.

Subsequent experiments demonstrated that using sapphire-based substrates enables the fabrication of compact lasers operating in the deep ultraviolet range. Their performance matches that of the best existing integrated laser models, while the emitting element itself measures only two micrometers in diameter, comparable to bacterial size.

The term "whispering gallery" originates from a well-known acoustic phenomenon observed in the dome of St. Paul's Cathedral in London, where a whisper spoken on one side of the dome can be clearly heard by someone standing on the opposite side, tens of meters away.

This occurs because sound waves repeatedly reflect off the curved dome walls and travel along the surface without losing energy, as if "walking" along the wall.

In physics, when applied to light or lasers, the whispering gallery effect relies on total internal reflection. When a light beam enters a very small circular object, such as a glass sphere or disc, the light reflects off the internal walls at very large angles, circulating around the inner perimeter without escaping. This traps the light in a very small circular orbit.

For microscopic lasers, the whispering gallery effect provides a solution to the miniaturization challenge. Unlike conventional lasers that require opposing mirrors to confine light, in a micro-laser shaped as a disc or sphere, the curved surface itself acts as a "circular mirror." Internally reflected light travels along this circular path, remaining confined long enough to amplify and produce laser emission without needing large space or external mirrors.

This principle enabled the scientists to create a laser just two micrometers in size—smaller than a red blood cell—while maintaining its efficiency.