Nano-Cs_xWO₃: Ultra-broadband nonlinear optical modulator for near-infrared and mid-infrared ultrafast fiber lasers generation

Saturable absorbers (SAs) covering wavelengths from near-infrared (NIR) to mid-infrared (MIR) band are required for mode-locking and Q-switching lasers in muti-band wavelengths. Here, broadband nonlinear optical property was discovered in Cs_xWO₃ nanorods (NRs), which as a novel non-stoichiometric SA for realizing ultrafast fiber lasers is first demonstrated. The Cs_xWO₃ NRs based SA exhibited good mode-locking ability in three key wavelengths from NIR to MIR region, which is a key advantage over the most reported broadband SAs. The given Cs_xWO₃ NRs showed a broadband optical absorption from 800 to 3,200 nm, and excellent SA properties at 1-μm, 1.5-μm, and 2-μm optical bands. Employing such SA, the ultrashort pulse lasers with a pulse duration/repetition rate of 530 fs/37.42 MHz at 1,567 nm and 5.6 ps/41.50 MHz at 1,965 nm from Er- and Tm-doped fiber lasers (TDFL) were realized separately. In addition, a stable mode locked operation at 1,030 nm with a repetition rate of 48.80 MHz was also achieved from Yb-doped fiber laser (YDFL). This work not only offers a new and reliable broadband mode locker for ultrafast laser generation, but also broadens the application of Cs_xWO₃ materials in the field of nonlinear fiber optics.