Time-resolved two-photon absorption in quantum dots and polaritons

We used two-photon absorption to study semiconductor samples. The energy of the photons used to excite the particle is half the energy of the electronic transition. In one set of experiments we have performed time-resolved measurements of the time scale for conversion of excitons in dark states to bright (light-emitting) states in GaAs quantum dots. The dark states are pumped using two-photon absorption, while the bright state emission is observed in single-photon emission. This conversion time is connected to the spin flip time for carriers in the quantum dots. The time scale is found to be of the order of several hundred picoseconds.\ud \ud In a second set of experiments, we performed time-resolved measurements of two-photon excitation directly into microcavity polariton states. Although this process is forbidden by symmetry for light at normal incidence, we observed that it is allowed at non-zero angle of incidence due to state mixing. Fermi's golden rule for bosons states that absorption into a final state is enhanced by the occupation of that state. This thesis reports experiments aimed at using a laser beam to create a condensate and modulate the two-photon absorption of a second beam