Band structure tuning of g-CN via sulfur doping for broadband near-infrared ultrafast photonic applications.

Graphitic carbon nitride (g-CN) featuring a stable heptazine ring structure and high polymerization degree, was indexed as a high thermochemical stability material, attracting rising research enthusiasm for diverse applications. However, the poor near-infrared (NIR) optical absorption and resulting limited NIR applications were pronounced for g-CN due to its large bandgap of 2.7 eV. In the present work, sulfur-doping was manifested by first-principles calculations to introduce impurity level and result in anisotropic spin splitting in g-CN for enhancing broadband nonlinear optical characteristics in NIR regime. The modified sulfur-doped g-CN (S-CN) exhibited the maximum effective nonlinear absorption coefficient to be -0.82 cm/GW. Pulse duration within hundred nanoseconds was realized with high modulation stability employing S-CN as saturable absorber in Q-switching operations. Moreover, broadband ultrafast photonics properties were successfully demonstrated in constructed ytterbium-doped and erbium-doped fiber lasers, generating highly stable dissipative soliton and traditional soliton mode-locking pulses. The presented S-CN nanomaterial with remarkable nonlinear optical performances might explicitly boost the development and application of g-CN materials in advanced optoelectronic and ultrafast photonic devices.