We investigate the transport of dipolar indirect excitons along the growth plane of polar (Al,Ga)N/GaN quantum well structures by means of spatially and time-resolved photoluminescence spectroscopy. ...The transport in these strongly disordered quantum wells is activated by dipole-dipole repulsion. The latter induces an emission blue shift that increases linearly with exciton density, whereas the radiative recombination rate increases exponentially. Under continuous, localized excitation, we observe continuously decreasing emission energy, as excitons propagate away from the excitation spot. This corresponds to a steady-state gradient of exciton density, measured over several tens of micrometers. Time-resolved microphotoluminescence experiments provide information on the dynamics of recombination and transport of dipolar excitons. We account for the ensemble of experimental results by solving the nonlinear drift-diffusion equation. Quantitative analysis suggests that in such structures, exciton propagation on the scale of 10 to 20 mu m is mainly driven by diffusion, rather than by drift, due to the strong disorder and the presence of nonradiative defects. Secondary exciton creation, most probably by the intense higher-energy luminescence, guided along the sample plane, is shown to contribute to the exciton emission pattern on the scale up to 100 mu m. The exciton propagation length is strongly temperature dependent, the emission being quenched beyond a critical distance governed by nonradiative recombination.
Spin-charge interconversion (SCI) phenomena have attracted a growing interest in the field of spintronics as a means to detect spin currents or manipulate the magnetization of ferromagnets. The key ...ingredients to exploit these assets are a large conversion efficiency, the scalability down to the nanometer scale, and the integrability with optoelectronic and spintronic devices. Here, we show that, when an ultrathin Bi film is epitaxially grown on a Ge(111) substrate, quantum size effects arising in nanometric Bi islands drastically boost the SCI efficiency, even at room temperature. Using x-ray diffraction, scanning tunneling microscopy, and spin- and angle-resolved photoemission, we obtain a clear picture of the film morphology, crystal, and electronic structures. We then directly probe SCI with three different techniques: magneto-optical Kerr effect to detect the charge-to-spin conversion generated by the Rashba-Edelstein effect (REE), optical spin orientation, and spin pumping to generate spin currents and measure the spin-to-charge conversion generated by the inverse Rashba-Edelstein effect (IREE). The three techniques show a sizable SCI only for 1–3-nm-thick Bi films corresponding to the presence of bismuth nanocrystals at the surface of germanium. Due to three-dimensional quantum confinement, those nanocrystals exhibit a highly resistive volume separating metallic surfaces where SCI takes place by (I)REE. As the film size increases, the Bi film becomes continuous and semimetallic leading to the cancellation of SCIs occurring at opposite surfaces, resulting in an average SCI that progressively decreases and disappears. These results pave the way for the exploitation of quantum size effects in spintronics.
Continuous-wave, time-integrated, and time-resolved photoluminescence experiments are used to study the excitonic optical recombinations in wurtzite ZnO/Zn0.78Mg0.22O quantum wells of varying widths. ...By comparing experimental results with a variational calculation of excitonic energies and oscillator strengths, we determine the magnitude (0.9MV∕cm) of the longitudinal electric field that is induced by both spontaneous and piezoelectric polarizations. The quantum-confined Stark effect counteracts quantum confinement effects for well widths larger than 3nm, leading to emission energies that can lie 0.5eV below the ZnO excitonic gap and to radiative lifetimes that can be larger than milliseconds.
The formation and propagation of a polariton condensate under tightly focused excitation is investigated in a ZnO microcavity both experimentally and theoretically. 2D near-field and far-field images ...of the condensate are measured under quasi-continuous non-resonant excitation. The corresponding spatial profiles are compared to a model based on the Gross-Pitaevskii equation under cylindrical geometry. This work allows to connect the experiments performed with a small excitation laser spot and the previous kinetic models of condensation in a 2D infinite microcavity, and to determine the relevant parameters of both the interaction and the relaxation between the reservoir and the condensate. Two main parameters are identified: the exciton-photon detuning through the polariton effective mass and the temperature, which determines the efficiency of the relaxation from the reservoir to the condensate.
We report on spatially- and time-resolved emission measurements and observation of transport of indirect excitons in ZnO/MgZnO wide single quantum wells.
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