Titan is the largest satellite of Saturn. It has a dense atmosphere of 1.5 bar at the surface mainly composed of nitrogen and methane. One of the most important results delivered by the ongoing Cassini space mission was to affirm that Titan's aerosols synthesis begins in the upper atmosphere. However, the Cassini mission has also highlighted the incompleteness of our knowledge on Titan's ionopheric chemistry, showing numerous unexplained species and a fortiori unknown processes coupling nitrogen and hydrocarbon chemistry, and involving neutrals, and positive and negative ions. The complex mechanisms leading to the production of the organic aerosols surrounding Titan remain thus mostly unknown. One way to study this reactivity is to reproduce in the laboratory the whole chain of reactions occurring in Titan's atmosphere. Several experimental setups have been developed in order to reproduce Titan's atmospheric chemistry in the laboratory. Among them the RF-CCP device PAMPRE provided significant clues on the understanding of the polymeric chemical structure of tholins, analogs of the atmospheric aerosols. The influence of the methane initial concentration on the aerosol production efficiency was studied, highlighting a surprising decrease of the aerosol production yield with the methane concentration. In order to find some clues on the volatile products controlling the aerosol production, we performed an extensive study on the gas phase composition correlated with the aerosol production. Moreover, in Titan's atmosphere, the CH4 concentration may have varied through time during the evolution of this atmosphere. So we studied the sensitivity of the optical constants of tholins towards different CH4 concentration.