Supported by the National Natural Science Foundation of China (Grant No. 52025021, 92163207, 51890863), Haohai Yu and Huaijin Zhang’s group (Shandong University), and Yanfeng Chen’s group (Nanjing University), have achieved progress in laser physics for realizing direct lasing far beyond the fluorescence spectrum of the gain medium. The research result was published in Nature Physics on September 22, 2022, entitled "Multiphonon-assisted lasing beyond the fluorescence spectrum". (article link: http://doi.org/10.1038/s41567-022-01748-z). In addition, Prof. Alessandra Toncelli from Pisa University, Italy, wrote a perspective article entitled “Light in the darkness” for this work. (article link: http://doi.org/10.1038/s41567-022-01756-z).

Laser is an artificial coherent light with high brightness and high intensity. Its invention and rapid development have profoundly changed our industrial production and daily life, thus providing a powerful tool for understanding and reforming our world. The theoretical basis of laser emission was laid by Albert Einstein in 1917, who introduced the concept of stimulated emission. For a long time, stimulated emission is inherently linked to spontaneous emission, and one would therefore expect the lasing emission to be limited to the spectral range of the fluorescence spectrum of the active material. Therefore, the fluorescence spectrum range has been considered as a crucial parameter for evaluating the performances of laser materials. At present, laser wavelengths beyond the fluorescence spectrum can only be achieved with indirect conversion methods based on high-order nonlinear optics, but suffer from complicated configurations, direction sensitivity, and high costs. Accordingly, the discovery of new laser generation mechanism and direct laser availability beyond the fluorescence spectrum have always been a hotspot in the laser crystal and laser physics field.

The research team proposed a novel multiphonon-electron coupling mechanism to access this aim. The laser emission far beyond the spontaneous emission was realized by coupling electron transitions with the mechanical vibrations of the atoms in the laser material. A physical model of multiphonon coupling was built and a crucial functional unit of “free-oxygen” for strengthening the multiphonon-electron coupling was discovered in the Yb-doped YCa₄O(BO₃)₃ crystal. Then, a segmentally tunable laser emission involving a various number of phonons was obtained from 1,110-1,465 nm, far beyond the intrinsic fluorescence spectrum of more than 400 nm. As expected, the efficiency of the system decreases with the number of phonons involved, from a maximum of 41% with three phonons to 0.3% with a maximum of seven phonons involved at 1,436 nm. Moreover, a similar laser performance was realized in another active material (a Yb-doped La₂CaB₁₀O₁₉ crystal), demonstrating that the same principle can be applied in other laser materials to extend the laser spectrum.

This multiphonon-electron coupled laser technology would be a new route to extend the laser emission, besides traditional frequency-doubling, stimulated Raman effect, optical parametric oscillation, etc. The possibility of extending the tunable laser also opens up new perspectives for the discovery of new active medium, integrated on-chip nonlinear optics, and chirped-pulse amplification of ultrashort pulse lasers and frequency-comb generation.

Figure. Multiphonon-assisted lasing beyond the fluorescence spectrum. (a) Laser generation at 1,254 nm and SHG red laser at 627 nm (top), 1,144 nm and SHG yellow laser at 572 nm (middle) and 1,020 nm and SHG green laser at 510 nm (bottom) in Yb:YCOB crystal. (b) Segmentally tunable laser emission at 1,110–1,180 nm, 1,160–1,240 nm, 1,236–1,288 nm, 1,335–1,381 nm and 1,421–1,465 nm. Insert: coated Yb:YCOB crystals (size: 3 × 3 × 6 mm³). (c) Mechanism of multiphonon–electron coupled laser.