arXiv subjects
Publications and source records attributed to P. G. Lagoudakis.
We demonstrate the role of LO-phonon assisted polariton relaxation in reducing the stimulation threshold in strongly coupled microcavities. When the energy of the relaxation bottleneck is one LO-phonon above the ground polariton state, we observe a ten-fold improvement of the polariton relaxation rate in the linear regime, and a two-fold reduction of the threshold to the non-linear polariton lasing regime.
We fabricate a highly efficient colour conversion light emitting diode consisting of surface-patterned blue emitters and semiconductor colloidal nanocrystal quantum dots (NQDs). Electrically injected carriers in the blue emitter (donor) are efficiently transferred to the NQDs (acceptor) via nonradiative energy transfer in addition to conventional radiative energy transfer. The existence of nonradiative energy transfer is verified by the simultaneous observation of increased donor emission decay rate, the transient transfer of carriers at the acceptor and a 2-fold enhancement of the NQD electroluminescence.
We observe ground-state polariton condensation in a two dimensional GaAs/AlAs semiconductor microcavity under non resonant pulsed optical excitation. We resolve the formation of a polariton condensate by studying the spatial, angular, coherence, energy and transient dynamics of polariton photoluminescence. For high excitation densities we also observe a transition from the weak- to the strong-coupling regime in the time-domain and resolve the build-up of a coherent polariton state.
We observe spontaneously driven non-ground state polariton condensation in GaAs pillar microcavities under non-resonant optical excitation. We identify a regime where the interplay of exciton-exciton and pair polariton scattering can lead to mode switching from non-ground state to ground state polariton condensation. A simple kinematic model satisfactorily describes the observed mode switching as each of the above scattering mechanisms becomes prevalent at different carrier densities.
To an electron moving in free space an electric field appears as a magnetic field which interacts with and can reorient the electron spin. In semiconductor quantum wells this spin-orbit interaction seems to offer the possibility of gate-voltage control in spintronic devices but, as the electrons are subject to both ion-core and macroscopic structural potentials, this over-simple picture has lead to intense debate. For example, an externally applied field acting on the envelope of the electron wavefunction determined by the macroscopic potential, underestimates the experimentally observed spin-orbit field by many orders of magnitude while the Ehrenfest theorem suggests that it should actually be zero. Here we challenge, both experimentally and theoretically, the widely held belief that any inversion asymmetry of the macroscopic potential, not only electric field, will produce a significant spin-orbit field for electrons. This conclusion has far-reaching consequences for the design of spintronic devices while illuminating important fundamental physics.
Non-radiative optical energy transfer from a GaAs quantum well to a thin overlayer of an infrared organic semiconductor dye is unambiguously demonstrated. The dynamics of exciton transfer are studied in the time-domain using pump-probe spectroscopy at the donor site and fluorescence spectroscopy at the acceptor site. The effect is observed as simultaneous increase of the population decay rate at the donor and of the rise time of optical emission at the acceptor sites. The hybrid configuration under investigation provides an alternative non-radiative, non-contact pumping route to electrical carrier injection that overcomes the losses imposed by the associated low carrier mobility of organic emitters.
We perform an all-optical spin-dynamic measurement of the Rashba spin-orbit interaction in (110)-oriented GaAs/AlGaAs quantum wells. The crystallographic direction of quantum confinement allows us to disentangle the contributions to spin-orbit coupling from the structural inversion asymmetry (Rashba term) and the bulk inversion asymmetry. We observe an unexpected temperature dependence of the Rashba spin-orbit interaction strength that signifies the importance of the usually neglected higher-order terms of the Rashba coupling.