(c) Dr Paul Kinsler. [Acknowledgements & Feedback]

Motional Narrowing of Polaritons in Semiconductor Quantum Microcavities

LOCATION: IQEC, Sydney, Australia, 1996: oral presentation.
WORK DONE AT: Department of Physics, University of Sheffield, UK.
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AUTHORS: Paul Kinsler, David M. Whittaker.

ABSTRACT: Unexpected line narrowing of polaritons has been observed in semiconductor microcavities. We introduce a theory that accounts for the hybrid photon - exciton nature of the polariton, and predicts 'motional narrowing' of its spectrum

SUMMARY: Semiconductor quantum microcavities are structures in which quantum wells are grown inside an optical cavity. The light field is confined by two Bragg mirrors, and the 2D quantum wells create confined exciton states. In an appropriately designed sample the photons in the cavity and the quantum well exciton can couple together creating a quasi particle called a cavity polariton. Fisher et al [1] have observed unexpected narrowing of the polarition spectral lines near cavity mode exciton resonance.
We have extended an earlier theory of dispersive particles in random gaussian potentials [2] to describe two dimensional cavity polaritons. The exciton is scattered by imperfections and disorder in the microstructure leading to an inhomogeneously broadened spectrum. This disorder can be modelled by a random potential. The resulting 'disorder linewidth' is combined properly with the contribution from the cavity linewidth [3]. The complete theory predicts significant linewidth reduction near resonance.
Briefly, the coherent superposition of the cavity mode and exciton create a polariton, and this has properties which are a combination of its two components. In particular, the dispersion of the polariton is a weighted average of the dispersions of the photon and exciton. The cavity mode has a large dispersion and is delocalised. In contrast, the nearly dispersionless exciton is very localised and sees the maximum potential fluctuations. This means it is strongly broadened compared to its homogeneous linewidth.
The partly photonic nature of the polariton has an interesting consequence it gives the polariton a significant dispersion. This delocalises the polariton so that it sees more of the disorder potential, and averages over more potential fluctuations. As a result it undergoes less broadening than a bare exciton. This spatial averaging is more commonly known as 'motional narrowing', and needs to be considered when describing the interaction of polaritons with well disorder.
[1] TA Fisher et al, "Quantum Optics in wavelength scale structures", eds J Rarity and C Weisbuch, NATO ASI series, (in press).
[2] BI Halperin, Phys. Rev. 139, 104 (1965).
[3] P Kinsler, DM Whittaker, "Line width narrowing of polaritons", submitted to Phys. Rev. B.

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Date=20020417 19990211 19980423 Author=P.Kinsler