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Strong coupling of multiple branches polariton in a ZnO microcavity.

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DOI: 10.1063/1.3232228
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33 Citations
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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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DOI: 10.1103/PhysRevB.77.125201
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21 Citations
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By performing nondegenerate pump- probe experiments, we study the relaxation dynamics of spin- polarized A and B excitons in wurtzite epitaxial GaN. By analyzing the differential reflectivity spectra (Delta R/R) of the two circularly polarized probe components, we are able to identify each spin relaxation channel ( electron, light- and heavy- hole spin flips ) separately and to extract characteristic times of the different spin relaxation processes. In addition, spectral oscillatory features are observed for negative delays. They show a rapid rise determined by the fast dephasing time T-2 of the excitonic transitions. We show that the high density of dislocations increases the spin relaxation of electrons and holes through the Elliot - Yafet mechanism and makes the exciton dephasing time very short. The measured heavy- hole relaxation time, which is not extremely short compared to the electron relaxation time, can be related to the band structure, in which the degeneracy between different spin valence bands is partially lifted.

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DOI: 10.1103/PhysRevB.77.155212
WoS: 000255457400068
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The full point group symmetry of a crystal can be broken due to internal or external effective fields. In the study of excitons, such symmetry breaking can lead to a coupling of different exciton states and if a system is prepared in an exciton state with a defined total angular momentum (pseudospin), spin beating is obtained. Looking at the fluctuations of these fields, we use the invariant expansion of an effective Hamiltonian to investigate exciton-spin-relaxation dynamics in a model two-band bulk semiconductor and discuss the respective importance of the different spin-flip processes. We find that interaction terms leading to an electron or hole spin flip give rise to a pure transverse dephasing. Terms where the electron and hole spins are simultaneously reversed lead to transitions between the spin states, which are characterized by the longitudinal relaxation time. Similar to motional narrowing in the case of free carriers, the latter process can lead to an increase in the exciton-spin-relaxation times if extrinsic electric, magnetic, or strain fields rapidly fluctuate in the sample. This effect is shown to be due to the electron-hole exchange interaction.

Ref HAL: hal-00545175_v1
DOI: 10.1002/pssc.200778564
WoS: 000256695700270
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By performing non-degenerate pump-probe experiments, we study the relaxation dynamics of spin-polarized A and B excitons in wurtzite epitaxial GaN. By analyzing the differential reflection of the two circularly polarized probe components, we extract characteristic times of the different spin relaxation processes. We show that the spin relaxation of the exciton as a whole is negligible in regard to the spin relaxation of the individual carriers. We determined T-e = 15 ps for the electron in the conduction band, T-hh = 5 ps, and T-lh = 1.5 ps for the heavy hole and for the light hole, respectively. The quite long HH relaxation time can be related to the band structure in which the degeneracy between different spin valence bands is lifted.

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DOI: 10.1063/1.3021358
WoS: 000262364000065
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57 Citations
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The present study focuses on the structural and optical properties of ZnO thin films fabricated by sol-gel process and spin coated onto Si (100) and quartz substrates. The ZnO films have a hexagonal wurtzite structure with a grain size of about 50 nm. The x-ray photoelectron spectroscopy measurements reveal the presence of Zn2+ and of zinc hydroxyl groups at the film. Optical properties were studied by photoluminescence (PL) and absorption spectroscopy at low and room temperatures. The absorption spectrum is dominated by a sharp excitonic peak at room and low temperatures. At room temperature, PL observations show two transitions: one near the absorption edge in the ultraviolet (UV) region and the second centered at 640 nm, characteristic of the deep electronic levels in the bandgap. The spectrum at 6 K is dominated by donor bound exciton lines and donor-acceptor pair transitions. LO-phonon replica and two-electron satellite transitions are also observed. These optical characteristics are a signature of good structural quality of the films. c 2008 American Institute of Physics. [DOI: 10.1063/1.3021358]

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DOI: 10.1103/PhysRevB.77.195311
WoS: 000256971600090
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4 Citations
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We study the relaxation of excitons in CdZnTe quantum wells by emission of a LO-phonon cascade, ending with a trapping-in quantum dots. The state filling of the dots is measured in two-color pump-probe experiments. The observed optical-phonon emission time is found to be smaller (130 fs) than the phonon oscillation period (165 fs), showing that the interaction occurs in a quantum kinetic regime. This is evidenced by measuring the buildup of the phonon replica of an initially photocreated electron-hole pair distribution on a subpicosecond time scale.