Crystalline FeOCl as a novel saturable absorber for broadband ultrafast photonic applications

Abstract

Due to their tunable nonlinear coefficients, minimal energy loss, and high carrier densities, two-dimensional van der Waals materials have found extensive use as saturable absorbers for generating ultra-short pulses. The layered FeOCl material, a typical van der Waals material, demonstrates controllable optical properties through physical or chemical methods. However, its nonlinear optical characteristics have received limited attention. To explore the nonlinear optical properties of FeOCl in the near-infrared region, we synthesized crystalline FeOCl via a partial pyrolysis method and evaluated its broadband nonlinear absorption at 1.5-μm and 1-μm bands using an in-line balanced twin detector system. The results showed a modulation depth of 2.67 % and a saturation intensity of 4.62 MW cm⁻² at the 1.5-μm band, while at the 1-μm band, the modulation depth was 2.81 % and the saturation intensity was 4.04 MW cm⁻². Furthermore, FeOCl saturable absorbers were connected to Er-doped and Yb-doped fiber lasers, resulting in distinct mode-locking behaviors. Conventional soliton mode-locking was achieved in the Er-doped fiber laser, yielding a central wavelength of 1564.8 nm and a pulse duration of 1.11 ps. Noise-like pulse mode-locking was observed in the Yb-doped fiber laser with a central wavelength of 1034.1 nm and a pulse duration of 572 fs. These findings highlight the superior nonlinear optical properties of crystalline FeOCl material and its significant potential for near-infrared applications, paving the way for its future use in advanced ultrafast photonics.