450 research outputs found

    Photonic Crystal Architecture for Room Temperature Equilibrium Bose-Einstein Condensation of Exciton-Polaritons

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    We describe photonic crystal microcavities with very strong light-matter interaction to realize room-temperature, equilibrium, exciton-polariton Bose-Einstein condensation (BEC). This is achieved through a careful balance between strong light-trapping in a photonic band gap (PBG) and large exciton density enabled by a multiple quantum-well (QW) structure with moderate dielectric constant. This enables the formation of long-lived, dense 10~μ\mum - 1~cm scale cloud of exciton-polaritons with vacuum Rabi splitting (VRS) that is roughly 7\% of the bare exciton recombination energy. We introduce a woodpile photonic crystal made of Cd0.6_{0.6}Mg0.4_{0.4}Te with a 3D PBG of 9.2\% (gap to central frequency ratio) that strongly focuses a planar guided optical field on CdTe QWs in the cavity. For 3~nm QWs with 5~nm barrier width the exciton-photon coupling can be as large as \hbar\Ome=55~meV (i.e., vacuum Rabi splitting 2\hbar\Ome=110~meV). The exciton recombination energy of 1.65~eV corresponds to an optical wavelength of 750~nm. For N=N=106 QWs embedded in the cavity the collective exciton-photon coupling per QW, \hbar\Ome/\sqrt{N}=5.4~meV, is much larger than state-of-the-art value of 3.3~meV, for CdTe Fabry-P\'erot microcavity. The maximum BEC temperature is limited by the depth of the dispersion minimum for the lower polariton branch, over which the polariton has a small effective mass ∼10−5m0\sim 10^{-5}m_0 where m0m_0 is the electron mass in vacuum. By detuning the bare exciton recombination energy above the planar guided optical mode, a larger dispersion depth is achieved, enabling room-temperature BEC

    High-occupancy effects and stimulation phenomena in semiconductor microcavities

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    This paper describes recent work on high-occupancy effects in semiconductor microcavities, with emphasis on the variety of new physics and the potential for applications that has been demonstrated recently. It is shown that the ability to manipulate both exciton and photon properties, and how they interact together to form strongly coupled exciton-photon coupled modes, exciton polaritons, leads to a number of very interesting phenomena, which are either difficult or impossible to achieve in bulk semiconductors or quantum wells. The very low polariton density of states enables state occupancies greater than one to be easily achieved, and hence stimulation phenomena to be realized under conditions of resonant excitation. The particular form of the lower polariton dispersion curve in microcavities allows energy and momentum conserving polariton-polariton scattering under resonant excitation. Stimulated scattering of the bosonic quasi-particles occurs to the emitting state at the center of the Brillouin zone, and to a companion state at high wave vector. The stimulation phenomena lead to the formation of highly occupied states with macroscopic coherence in two specific regions of k space. The results are contrasted with phenomena that occur under conditions of nonresonant excitation. Prospects to achieve "polariton lasing" under nonresonant excitation, and high-gain, room-temperature ultrafast amplifiers and low-threshold optical parametric oscillator under resonant excitation conditions are discussed

    Collective coherence in planar semiconductor microcavities

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    Semiconductor microcavities, in which strong coupling of excitons to confined photon modes leads to the formation of exciton-polariton modes, have increasingly become a focus for the study of spontaneous coherence, lasing, and condensation in solid state systems. This review discusses the significant experimental progress to date, the phenomena associated with coherence which have been observed, and also discusses in some detail the different theoretical models that have been used to study such systems. We consider both the case of non-resonant pumping, in which coherence may spontaneously arise, and the related topics of resonant pumping, and the optical parametric oscillator.Comment: 46 pages, 12 figure

    Polariton condensates at room temperature

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    We review the recent developments of the polariton physics in microcavities featuring the exciton-photon strong coupling at room-temperature, and leading to the achievement of room-temperature polariton condensates. Such cavities embed active layers with robust excitons that present a large binding energy and a large oscillator strength, i.e. wide bandgap inorganic or organic semiconductors, or organic molecules. These various systems are compared, in terms of figures of merit and of common features related to their strong oscillator strength. The various demonstrations of polariton laser are compared, as well as their condensation phase diagrams. The room-temperature operation indeed allows a detailed investigation of the thermodynamic and out-of-equilibrium regimes of the condensation process. The crucial role of the spatial dynamics of the condensate formation is discussed, as well as the debated issue of the mechanism of stimulated relaxation from the reservoir to the condensate under non-resonant excitation. Finally the prospects of polariton devices are presented.Comment: 22 pages, 3 figures, 1 tabl

    Propagating Polaritons in III-Nitride Slab Waveguides

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    We report on III-nitride waveguides with c-plane GaN/AlGaN quantum wells in the strong light-matter coupling regime supporting propagating polaritons. They feature a normal mode splitting as large as 60 meV at low temperatures thanks to the large overlap between the optical mode and the active region, a polariton decay length up to 100 μ\mum for photon-like polaritons and lifetime of 1-2 ps; with the latter values being essentially limited by residual absorption occurring in the waveguide. The fully lattice-matched nature of the structure allows for very low disorder and high in-plane homogeneity; an important asset for the realization of polaritonic integrated circuits that could support nonlinear polariton wavepackets up to room temperature thanks to the large exciton binding energy of 40 meV

    Exciton-photon hybridisation in ZnSe based microcavities

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    This thesis presents the design, fabrication and experimental analysis of ZnSe based microcavities. Semiconductor microcavities are micro-structures in which the exciton ground state of a semiconductor is coupled to a photonic mode of an optical cavity. The strong light matter coupling mixes the character of excitons and photons, giving rise to the lower and upper cavity polaritons, quasiparticles with an unusual dispersion due to the extreme mass contrast between the composite exciton and photon. In particular, the dispersion of the lower polariton forms a dip around the lowest energy state with zero in-plane momentum. In this dip, which can be seen as a trap in momentum space, the polaritons are efficiently isolated from dephasing mechanisms involving phonons. The features of these quasiparticles promise a variety of applications, for instance lasing without inversion and micro-optical parametric amplifiers, and an environment to study fundamental physics, such as Bose-Einstein condensation in the solid state. By overcoming the longstanding fabrication problems of ZnSe-based microcavities, the enlarged exciton binding energy in combination with the use of highly reflective dielectric mirrors makes this material system ideally suited to the realisation of polariton-based devices operating at room temperature. An epitaxial liftoff technology is developed that relies on the high etch selectively between the ZnSe heterostructure and a novel II-VI release layer, MgS. Three hybrid microcavities are fabricated with the liftoff technique and spectroscopically characterised. Angle resolved transmission experiments reveal strong hybridization of the ZnSe/Zn0:9Cd0:1Se quantum well excitons and cavity photons in a fixed microcavity. A completely length tunable microcavity is presented and shown to exhibit similar dispersion as for the fixed microcavity, with the addition of evidencing the cavity polariton bottleneck effect. The nonlinear optical features are discussed. Photoluminescence data is presented that evidences the first observation of the build up of cavity polaritons at the edge of the momentum space trap in the lower polariton branch, the bottleneck effect, in a ZnSe based microcavity. Finally, lasing at room temperature in the blue spectral region is presented for a metal/dielectric hybrid microcavity
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