Étude de semi-conducteurs par spectroscopie d'excitation cohérente multidimensionnelle

Multidimensional spectroscopy is an ultrafast optics technique which allows to reveal various electronic and vibrational coherent mechanisms. Using a controlled sequence of laser pulses, one can extract the correlations between different energy levels of a material or a compound and follow their evolution on a femtosecond timescale. The experimental system corresponds to an excitation technique, where a sequence of four phase modulated colinear pulses generates the desired nonlinear signals. These signals are isolated through the sample's total photoluminescence with a synchronous detection method and allow to construct a 2D spectrum.    Multidimensional excitation spectroscopy is applied for the first time to study diverse semiconducting materials. In P3HT, a semiconducting polymer being a model material in organic electronics, this technique can circumvent the effects of disorder and extracts the homogeneous line width. The measurement confirms the nature of P3HT photoexcitations, an exciton delocalized in two dimensions with a wave function expending along polymer backbone as well as between adjacent chains. Since the homogeneous line width does not vary with temperature, it can be deduced that static disorder limits the exciton coherence length even up to room temperature.  The experimental response of a perovskite photovoltaic cell illustrates a case where the excitation technique is not applicable in 2D spectroscopy. In that instance, a population mixing, identified by a sublinear dependence of the emission with excitation density, causes an {\it incoherent} signal that hide the multidimensional coherent signal. A theoretical model reveals the similarities between those two signals, which can be misleading since the incoherent signal simply contains information about the sample linear response. It can not show the correlations between energy levels. The modeling of population mixing and simulations of the 2D coherent response allow to determine which types of materials are adapted for the excitation technique and what are the evidences of a contamination of the measurement by the incoherent signal.   However, the population mixing can still enlighten us on the dynamic of optical excitations, as shown by our study of an GaAs/AlGaAs polariton microcavity. A polariton is an hybrid light-matter quasiparticle caused by the strong coupling between an exciton and a photon trapped in an optical microcavity. Above a given critical density, a polariton gaz undergoes a phase transition to a Bose-Einstein condensate, provoking the emission of coherent light with a strong nonlinear behavior with respect to polariton density. Thereby, the polariton stimulated relaxation prevents the acquisition of a 2D spectrum with the excitation technique, the population mixing being too important around the condensation threshold. Still, the population mixing signal reveals the condensate formation time and the signature of various thermalisation mechanisms.

Keyphrases: Polariton, Multidimensional spectroscopy, Ultrafast optics, Semiconductor, Phase modulation, Semiconducting polymer, Polariton, Optical microcavity, Population mixing