Terahertz-induced optical modulation in quantum-well microcavity

Strong exciton-photon coupling in a high-Q microcavity leads to the formation of two new eigenstates, called excitonpolaritons. We present the quantum dynamics of exciton-polaritons driven by strong few-cycle THz pulses. Our study focuses on an intriguing question of how THz radiation interacts with the strongly coupled light-matter system. We performed THz-pump and optical-probe experiments to answer the question: we observed the time-resolved optical reflectivity of the lower and higher exciton-polariton (LEP and HEP) modes in a QW microcavity in the presence of strong few-cycle THz pulses. In a previous study with a bare QW, a strong THz field tuned to the 1s-to-2p intraexciton transition induced an excitonic Rabi splitting. Since THz radiation interacts only with the excitonic components of exciton-polaritons and has no impact on cavity modes, it is an interesting question how THz radiation drives the excitonpolariton states to higher energy states in the microcavity system. Our study shows that THz radiation resonantly drives the exciton-polariton polarizations giving rise to LEP-to-2p or HEP-to-2p transitions. LEP-to-HEP transition is forbidden because they have the same symmetry. Our experimental and theoretical investigations demonstrate that LEP, HEP, and 2p-exciton states form a three-level Λ system in an optically excited QW microcavity.