Observation of large nonlinear responses in a graphene-Bi₂Te₃ heterostructure at a telecommunication wavelength

We report the large nonlinear response and ultrafast carrier relaxation dynamics of a graphene-Bi₂Te₃ heterostructure produced by two-step chemical vapour deposition. The nonlinear refractive index reaches n₂ = 0.2 × 10^-7 cm²/W at the telecommunication wavelength of 1550 nm, which is almost seven orders of magnitude larger than that of the bulk Si material. Additionally, a pump-probe experiment is performed to investigate the ultrafast dynamic process (intraband relaxation time τ₁ = 270 ± 20 fs; interband relaxation time τ₂ = 3.6 ± 0.2 ps) of the graphene-Bi₂Te₃ heterostructure. Then, based on the donor-acceptor structure model, we propose a theoretical model to explain the dynamic relaxation process. Our results show that the graphene-Bi₂Te₃ heterostructure is a promising saturable absorber for ultrafast pulse laser applications at telecommunication wavelengths.