Influence of methane concentration on the optical indices of Titan's aerosols analogues

► Real and imaginary refractive indices of Titan aerosols analogues were determined. ► We study the influence of methane concentration on optical indices. ► High absorption is observed for tholins produced at low methane concentration. ► Mid-IR spectra are reported at different methane concentration. ► Our results are compared to other studies and Titan's observations.