Ref HAL: hal-00805877_v1
DOI: 10.1063/1.2885018
WoS: 000254297300031
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7 Citations
Résumé:

We study the emission properties of confined polariton states in shallow zero-dimensional traps under non resonant excitation. We evidence several relaxation regimes. For slightly negative photon-exciton detuning, we observe a nonlinear increase of the emission intensity, characteristic of carrier-carrier scattering assisted relaxation under strong-coupling regime. This demonstrates the efficient relaxation toward a confined state of the system. For slightly positive detuning, we observe the transition from strong to weak coupling regime and then to single-mode lasing.

Ref HAL: hal-00388717_v1
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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.

Commentaires: Oral

Ref HAL: hal-00797187_v1
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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.

Ref HAL: hal-00327759_v1
Ref Arxiv: 0810.1811
DOI: 10.1103/PhysRevB.78.235323
WoS: 000262245400086
Ref. & Cit.: NASA ADS
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49 Citations
Résumé:

Wide bandgap semiconductors are attractive candidates for polariton-based devices operating at room temperature. We present numerical simulations of reflectivity, transmission and absorption spectra of bulk GaAs, GaN and ZnO microcavities, in order to compare the particularities of the strong coupling regime in each system. Indeed the intrinsic properties of the excitons in these materials result in a different hierarchy of energies between the valence-band splitting, the effective Rydberg and the Rabi energy, defining the characteristics of the exciton-polariton states independently of the quality factor of the cavity. The knowledge of the composition of the polariton eigenstates is central to optimize such systems. We demonstrate that, in ZnO bulk microcavities, only the lower polaritons are good eigenstates and all other resonances are damped, whereas upper polaritons can be properly defined in GaAs and GaN microcavities.

Ref HAL: hal-00348125_v1
Ref Arxiv: 0812.3383
DOI: 10.1103/PhysRevB.79.125302
WoS: 000264769300053
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54 Citations
Résumé:

We present experimental observation of the strong light-matter coupling regime in ZnO bulk microcavities grown on silicon. Angle resolved reflectivity measurements, corroborated by transfer-matrix simulations, show that Rabi splittings in the order of 70 meV are achieved even for low finesse cavities. The impact of the large excitonic absorption, which enables a ZnO bulk-like behavior to be observed even in the strong coupling regime, is illustrated both experimentally and theoretically by considering cavities with increasing thickness.

Ref HAL: hal-00261181_v3
Ref Arxiv: 0803.0899
DOI: 10.1103/PhysRevB.77.235315
WoS: 000257289500082
Ref. & Cit.: NASA ADS
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63 Citations
Résumé:

We report micro-photoluminescence studies of single GaN/AlN quantum dots grown along the (0001) crystal axis by molecular beam epitaxy on Si(111) substrates. The emission lines exhibit a linear polarization along the growth plane, but with varying magnitudes of the polarization degree and with principal polarization axes that do not necessarily correspond to crystallographic directions. Moreover, we could not observe any splitting of polarized emission lines, at least within the spectral resolution of our setup (1 meV). We propose a model based on the joint effects of electron-hole exchange interaction and in-plane anisotropy of strain and/or quantum dot shape, in order to explain the quantitative differences between our observations and those previously reported on, e.g. CdTe- or InAs-based quantum dots.

Ref HAL: hal-00541635_v1
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We report a comparison of continuous-wave photo luminescence spectra with spatially-resolved micro-photoluminescence data collected at low temperature on as-grown stacking faults in a 4H-SiC epitaxial layer. We find that the defects have a large triangular shape (50 mu m x 50 mu m x 50 mu m) and that the maximum signal wavelength shifts when scanning across one triangular defect. These results show that the built-in electric field in the stacking fault well can be screened, more or less depending on the incoming light intensity.