A team of researchers has developed what is reportedly the first chip-scale titanium-doped sapphire laser — an innovation that could lead to new applications ranging from atomic clocks to quantum computing and spectroscopic sensors. The titanium-doped sapphire laser was a major advance in the field of lasers when it was introduced in the 1980s. Key to its success was the material used as its gain medium. Sapphire doped with titanium ions proved particularly powerful, providing a much wider laser emission bandwidth than traditional semiconductor lasers. The tabletop titanium-sapphire laser is a staple in many academic and industrial labs, although the large bandwidth comes at a high cost in terms of energy and floor space, making its use largely limited to laboratory research. Without overcoming this limitation, said Yubo Wang, graduate student at Yale and lead author of the study, titanium-sapphire lasers would remain limited to niche customers. The performance of titanium-sapphire lasers combined with the small size of a chip could drive applications that are limited by how much power or space they can consume, such as atomic clocks, portable sensors, visible light communication devices, and even quantum computing chips. To that end, researchers in the lab of Hong Tang, Yale’s Llewellyn West Jones Jr. Professor of Electrical Engineering, Applied Physics, and Physics, demonstrated the world’s first titanium-doped sapphire laser integrated with a chip-scale photonic circuit, providing the widest gain spectrum yet seen on a chip, according to the researchers. The laser’s low threshold is critical. While conventional titanium-doped sapphire lasers have a threshold of more than 100 mW, the Tang lab’s system had a threshold of about 6.5 mW. With further refinement, the team believes it can further reduce it to 1 mW. The system is also compatible with the family of gallium-nitride optoelectronics, which are widely used in blue LEDs and lasers. The research was published in Nature Photonics (www.doi.org/10.1038/s41566-022-01144-2).