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DOI: 10.1063/1.3232228
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33 Citations
Résumé:

The strong coupling regime in a ZnO microcavity is investigated through room temperature photoluminescence and reflectivity experiments. The simultaneous strong coupling of excitons to the cavity mode and the first Bragg mode is demonstrated at room temperature. The polariton relaxation is followed as a function of the excitation density. A relaxation bottleneck is evidenced in the Bragg-mode polariton branch. It is partly broken under strong excitation density, so that the emission from this branch dominates the one from cavity-mode polaritons.

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Les boîtes quantiques GaN présentent des propriétés de confinement très attrayantes, jusqu'à 300K. De nombreux développements récents ont montré qu'elles peuvent se comparer favorablement aux boîtes quantiques antérieures et mieux maîtrisées, InAs et CdTe en particulier. Mais l'étude de la structure fine des complexes excitoniques dans les boîtes GaN n'a jusqu'à présent pas été abordée. Nous présentons les propriétés de polarisation, à basse température, de boîtes isolées GaN/AlN. La photoluminescence est fortement polarisée linéairement, jusqu'à 90%, suivant des directions non corrélées aux axes cristallins. Nous n'observons pas de dédoublement du doublet radiatif. Nous interprétons ces résultats dans le cadre d'un modèle matriciel, incluant toutes les symétries pertinentes, qui permet une comparaison entre les systèmes GaN, InAs et CdTe. Nous montrons que la forte polarisation est liée à la proximité des bandes de valence A et B de GaN, et nous proposons que l'absence de doublet soit la signature d'un fort splitting de structure fine, ou bien de l'émission d'excitons chargés.

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We present numerical simulations of angle-resolved reflectivity of bulk ZnO microcavities with SiN/SiO2 distributed Bragg reflectors. The goal of these studies is to determine the conditions for observation of strong exciton-photon coupling at room temperature in such systems, which has been reported very recently. Indeed ZnO microcavities present a very specific hierarchy of interactions compared to GaAs or II-VI microcavities : the awaited Rabi splitting is larger than the Rydberg energy for excitons, which is larger than the splitting between the A,B,C valence bands. In our model the coupling of both fundamental and excited states of the optically active free excitons (XA, XB, XC) with the cavity mode is taken into account. The specific values of oscillator strength and excitonic binding energy in ZnO require that we include the above band gap absorption continuum. We do so by using Elliott's model of the dielectric function. We neglect disorder and inhomogeneous broadening in our calculations. The reflectivity spectra of the microcavity is calculated by a transfer matrix model, where either the ZnO thickness or the angle is varied in order to calculate the dispersion of the polaritonic branches. Those are compared to the dispersions obtained from a quasi-particle model which includes the excitonic continuum. The expansion coefficients of the polaritonic modes upon the photon-exciton basis are determined, as well as their coherence time. We show that the lower polariton branch is a well-defined and well-mixed exciton-photon state for low angles and/or negative detunings. The energy splitting induced by the strong oscillator strength in ZnO can be larger than the exciton binding energy, thus pushing the upper polariton branch into the absorption continuum and leading to a large intrinsic broadening. Therefore experimental evidence of strong coupling regime in bulk ZnO microcavities and its interpretation are difficult. Finally, as a criterion of strength of the strong coupling, we compute the ratio of the splitting between polariton branches to the polaritonic mode homogeneous broadening, for different detunings.

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We have investigated a series of samples embedding ZnO/(Zn,Mg)O quantum wells of different sizes, in wurtzite phase, by using timeresolved photoluminescence. The samples were grown by molecular beam epitaxy on ZnO templates, themselves deposited on sapphire substrates. The presence of large internal electric fields in these quantum wells manifests itself not only through the energies of the optical recombinations, but also through the size dependence of the recombination times. An envelope-function model that includes the variational calculation of the exciton binding energy allows us to determine a value of 0.9 MV/cm for the internal electric field.