Comb-Like Frequency Conversions in Few Layer GaSe-Integrated Microfiber Resonators

While advancements in optical fiber-based second harmonic generation (SHG) have leveraged novel waveguide designs and two-dimensional nonlinear material integration, conventional fiber schemes remain constrained by limited multi-frequency control and restricted efficiency growth. We demonstrate an all-fiber resonant SHG strategy achieving periodic enhancement and polarization modulation by integrating a few-layer GaSe crystal with a microfiber knot resonator (MKR). Benefitting from MKR-enabled resonant interaction with GaSe, which possesses ultrahigh second-order nonlinear susceptibility, the maximum excitation of comb-like SHG at resonant wavelengths achieves a 32-fold enhancement compared to GaSe-microfiber integration under continuous-wave laser pumping. Meanwhile, the twisted MKR structure facilitates dynamic polarization-dependent control of the split SHG wavelengths through pump polarization modulation. Experimental results further reveal distinct intensity patterns of SHG intensity in both under-coupling and over-coupling regimes. Our findings offer insights into accessing broad, efficient, polarization-controllable SHG combs for multifunctional nonlinear photonic devices with on-demand spectral tailoring capabilities.