Transient magneto-photoluminescence quenching by intra-excitonic THz absorption

Optical properties of III-V semiconductor heterostructures are strongly influenced by excitons. With increasing carrier confinement, as in quantum wells and quantum dots, the exciton binding energy increases. This results in distinct features in absorption and emission spectra arising from the excitonic transitions and recombinations. The temporal behavior of the optical properties of these systems is also influenced by the exciton dynamics. Recently, there has been an increasing interest on the study of intraexcitonic transitions. The energy separations of the excitonic states (1s, 2s, 2p, and so on) fall in the terahertz (THz) frequency range. Intense and tunable THz sources like free electron lasers have been used to probe these transitions. However, most experiments like optically detected magnetic resonance measurements, were done in the time integrated mode and the intraexcitonic carrier dynamics could not be probed.
Here we present our work on time resolved measurements of photoluminescence (PL) quenching in presence of a varying external magnetic field. We performed measurements on GaAs/AlGaAs multiple quantum well sample with a narrow PL line width (2 meV) associated with the 1s heavy-hole exciton. THz pulses from a free electron laser were used to induce intraexcitonic transitions resulting in the quenching of the PL. The quench manifested itself as a dip in the PL transient during the incidence of the THz pulse and the depth of the dip was proportional to the THz absorption. The intraexcitonic transition energies were tuned using an external magnetic field. Excitonic 1s-2p and cyclotron resonances appeared as maxima in the plot of the dip depth vs. magnetic field. Time resolved measurement enabled us to investigate the relaxation dynamics of the 2p state which does not undergo radiative recombination and thus cannot be observed directly in the PL spectra. The carriers excited to the 2p state, by the THz pulse, eventually relaxed back to the 1s-state resulting in the gradual recovery of the 1s PL intensity after the quench. Some of the carriers in the 2p state however got scattered to the 2s state resulting in the enhancement of the 2s emission during the incidence of the THz pulse. This is a direct consequence of intraexcitonic carrier transfer between the 2p and 2s states. From magnetic field dependence of the THz induced 2s emission intensity we showed the possibility of externally controlling intraexcitonic transitions.