Recently, a research team led by Prof. Yu Haohai from the Institute of Crystal Materials achieved significant progress in the fields of phonon-coupled laser crystals and yellow laser technology. The research entitled “Phonon engineering in Nd³⁺-doped garnet lasers: filling the yellow gap and boosting fluorochrome excitation,” has been published online in the international journal Advanced Photonics on May 5, 2026.

The first author of the paper is a PhD student Hao Hong at the State Key Laboratory of Crystal Materials, Shandong University. The corresponding authors are Prof. Liang Fei, Prof. Yu Haohai, and Prof. Zhang Huaijin. Shandong University is the sole corresponding institution for this work.

Yellow lasers at 570–590 nm spectral range are highly valuable for applications in biomedicine, precision metrology, and astronomical applications. Their wavelengths align well with peak human eye sensitivity and match key transitions in atomic systems. At present, based on the “laser emission - nonlinear frequency doubling” scheme, green lasers have already satisfied practical demands and achieved industrialization. However, limited by the intrinsic electronic energy levels of Nd³⁺- and Yb³⁺-doped crystals, commercial laser crystals lack efficient electronic transitions in the 1140–1180 nm spectral range, thus making frequency-doubled yellow laser unattainable. This limitation has led to a long-standing ‘yellow gap’ over sixty years in solid-state laser technology.

To address this issue, several indirect approaches have been developed, such as nonlinear sum-frequency generation and stimulated Raman scattering techniques, which require multiple crystals with different functions to operate cooperatively. As a consequence, laser systems become highly complicated. Therefore, exploring a simple laser-emission mechanism to obtain yellow laser is of great importance for practical applications.

Figure 1.Yellow laser generation in a frequency-doubled Nd:YAG laser system.

This work proposed a phonon-coupled laser design strategy, in which new-wavelength lasers are achieved through regulation of the coupling effect between electronic transitions and lattice vibrations. Taking Nd:YAG crystals as an example, Nd³⁺ ions occupy Y³⁺ sites with D₂ symmetry, theoretically allowing electronic transitions to couple with all-symmetry phonon modes. Via designing the laser cavity, conventional strong transitions at 1.06 μm are suppressed, while weak phonon-coupled transitions at 1.15-1.16 μm are selectively amplified. As a result, the emitted laser wavelength is shifted from the typical 1064 nm to longer wavelengths at 1151 nm and 1166 nm. By incorporating a coated frequency-doubled LBO crystal, CW yellow laser emission at 575.5–583 nm was realized in Nd:YAG crystals for the first time. Benefiting from the thermally-activated lattice vibrations, this yellow laser exhibited‌ anomalous‌ thermal dependence, in which the output power increases at elevated temperatures. The maximum yellow laser power reached 75 mW, which can satisfy the requirements for flow cytometry detection. Experimental results further demonstrated the 583 nm yellow laser enables higher excitation efficiency for fluorescent dyes than conventional green lasers. This phonon-coupled yellow laser requires only one laser crystal and one frequency-doubling crystal. This design simplifies the resonator configuration and provides technical support for the development of highly integrated yellow laser sources.

This work was supported by the National Natural Science Foundation of China, the National Key Research and Development Program of China, the Natural Science Foundation of Shandong Province, the Future Plans of Young Scholars at Shandong University, and the Institute of Crystal Materials.