Broadband and continuous wave pumped second-harmonic generation from microfiber coated with layered GaSe crystal

Second-order optical nonlinear effects have received widespread attention from researchers since its first discovery in the 1960s. Based on second-harmonic and sum-frequency effects, the lasers ranging from ultraviolet to far infrared bands have been generated, greatly promoting the development of laser techniques, optical information processing, high-resolution microscopic imaging. According to nonlinear optics and polarization theory, even-order nonlinear optical effects are strongly determined by the crystals' centrosymmetry, and the second-order nonlinear susceptibility is nonzero only in non-centrosymmetric medium. However, as the most fundamental second-order nonlinear effect, the centrosymmetry of the silica fiber hinders the generation and utilization of second-harmonics. The widely employed poling methods (such as optical poling, thermal poling, electric field poling) can artificially break the centrosymmetry of the fiber, effectively improving the second-order nonlinearity of the fiber. However, complex processes and harsh fabrication conditions are required, and quasi-phase-matching conditions can only be satisfied at discrete wavelengths, which limits the operation bandwidth. The optical fiber ring based on whispering gallery mode also greatly limits the wide spectral excitation of second-harmonics.