Broadband ultrafast fiber lasers enabled by the defect regulation in Ti4-TiN saturable absorbers

Metallic-rich titanium‑titanium nitride (Ti4-TiN) is an interesting novel transition metal compound with a broadband absorption response, high-temperature sensitivity, and excellent air stability, making them potential for the realization of optical modulators. We explore picosecond fiber lasers at 1.06 μm and 2.0 μm with Ti4-TiN nanosheets as a saturable absorber (SA). The produced defect regulation in Ti4-TiN nanosheets, the mode-locked fiber lasers exhibit the highest average output powers of 7.9 mW and 133.5 mW at 1.06 μm and 2.0 μm. At the same time, the corresponding pulse durations are about 260 ps and 2.59 ps, respectively. Additionally, the broadband effect of the prepared Ti4-TiN-SA device enables the realization of a stable passively Q-switched (PQS) free space laser with a minimum pulse duration of 564 ns and a maximum frequency of 87.0 kHz at 2.8 μm. Moderate ultrasonic probe strategies are applied to precisely regulate the C and O defect states in Ti4-TiN nanosheets. The defect regulation engineering effectively accelerates the saturation fluence and modulation depth characteristics. The saturation fluence of the optimized defect regulation in the prepared device can be 1.80 MW∙cm−2 and 21.8 MW∙cm−2 at the wavelengths of 1.0 μm and 2.0 μm, respectively. The enhanced saturation fluence and saturable absorption endow the optimal mode-locked fiber and PQS laser with both narrow pulse width and high output power.