关键词: third-order optical nonlinearities, franckeite, mid-infrared, $Q$-switching

Abstract: The study of nonlinear optical responses in the mid-infrared (mid-IR) regime is essential for advancing ultrafast mid-IR laser applications. However, nonlinear optical effects under mid-IR excitation are rarely reported due to the lack of suitable nonlinear optical materials. The natural van der Waals heterostructure franckeite, known for its narrow bandgap and stability in air, shows great potential for developing mid-IR nonlinear optical devices. We have experimentally demonstrated that layered franckeite exhibits a broadband wavelength-dependent nonlinear optical response in the mid-IR spectral region. Franckeite nanosheets were prepared using a liquid-phase exfoliation method, and their nonlinear optical response was characterized in the spectral range of 3000 nm to 5000 nm. The franckeite nanosheets exhibit broadband wavelength-dependent third-order nonlinearities, with nonlinear absorption and refraction coefficients estimated to be about 10$^{-7}$ cm/W and 10$^{-11}$ cm$^{2}$/W, respectively. Additionally, a passively $Q$-switched fluoride fiber laser operating around a wavelength of 2800 nm was achieved, delivering nanosecond pulses with a signal-to-noise ratio of 43.6 dB, based on the nonlinear response of franckeite. These findings indicate that layered franckeite possesses broadband nonlinear optical characteristics in the mid-IR region, potentially enabling new possibilities for mid-IR photonic devices.

Key words: third-order optical nonlinearities, franckeite, mid-infrared, $Q$-switching