Time resolved spectroscopy of two dimensional systems : from the conventional method to a novel cavity enhanced solution The work presented in this dissertation is dedicated to the characterization of the excited state dynamics of thin films through time resolved spectroscopy with emphasis on developing a methodology that is able to resolve weak transient absorption signals from optically thin films. With this aim the conventional transient absorption spectroscopy method is first utilized to characterize semiconducting monolayers and organic nanosheet semiconductors. Although these are physically thin they present relatively strong transient absorption signals of a few mOD (units of optical density) which allows to characterize their excited state dynamics with the conventional machinery not needing further signal enhancements or complex noise minimizing techniques. Nonetheless the former does not represent the reality of detecting the transient photoexcited dynamics of few layered systems. For this reason the last chapter of this thesis introduces a new approach for the sensitive detection of two dimensional samples with marginal molecular extinction coefficients: A novel methodology that multiplies the interaction length of the light with the sample designated cavity ring down transient absorption spectroscopy (CRD TAS). Being at the present time in the midst of its development the prospect efficiency and working capabilities of the novel CRD TAS technique are hereby evaluated and the strategies for further improvements are discussed.