Efficient and wavelength-tunable second-harmonic generation toward the green gap

Achieving compact and efficient visible laser sources is crucial for a wide range of applications. However, traditional semiconductor laser technology faces difficulties in producing high-brightness green light, leaving a "green gap" in wavelength coverage. Second-harmonic generation (SHG) offers a promising alternative by converting near-infrared sources to visible wavelengths with high efficiency and spectral purity. Here, we demonstrate efficient and tunable SHG within the green spectrum using a high-Q Si₃N₄ microresonator. On-chip green power as high as 5.3 milliwatts is generated with a conversion efficiency of 141% per watt (absolute 7.9%). A space-charge grating induced by the photogalvanic effect realizes reconfigurable grating numbers and flexible wavelength tuning over a range of 2.6 terahertz. In addition, grating formation dynamics and competition are observed. These findings underscore the potential of Si₃N₄ as a robust, integrative platform for on-chip, tunable green light sources.