Enhancing mid-infrared saturable absorption of reduced graphene oxide via a defect-engineered Ti:Sb₂Te₃/rGO heterostructure for narrow-pulse mid-IR laser generation

Mid-infrared (Mid-IR) pulsed lasers are vital for diverse applications, yet their development is hindered by the scarcity of high-performance saturable absorbers (SAs) for Q-switching and mode-locking. While graphene and its analogs have emerged as promising candidates, their low modulation depth in the mid-IR regime hinders its practical implementation. To overcome this, we engineer a Ti:Sb₂Te₃/reduced graphene oxide (rGO) heterostructure via hydrothermal synthesis, leveraging synergistic Ti⁴⁺ doping and interfacial charge control. Ti⁴⁺ introduces tailored trap states in Sb₂Te₃ nanosheetsselected for their narrow bandgap and high mobility-suppressing non-radiative recombination and extending carrier lifetimes. This defect-mediated design achieves a significantly enhanced modulation depth of 1.8% (2.4× higher than graphene) and a nonlinear absorption coefficient of 6.84 cm•GW⁻¹ at 2.8 μm. Transient absorption spectroscopy confirms trap-assisted carrier dynamics and accelerated charge transfer. Integrating the heterostructure as an SA mirror into an Er:YAP laser generates stable 2.8 μm pulses with a 253 ns width, 163 kHz repetition rate, and 230 mW peak power at 5.5 W pump power. This work demonstrates defect-engineered Ti:Sb₂Te₃/rGO as a promising high-performance mid-IR SA for narrow-pulse lasers.