V. Lashkaryov Institute of Semiconductor Physics, NAS of Ukraine,
41, prosp. Nauky, 03680 Kyiv, Ukraine

Abstract. The concept of exciton implies a collective excited state able to travel in a particle-like fashion. Its size is determined by the radius of excited electron-hole pair and, although it may vary by two orders of magnitude, it is always spatially restricted, while its delocalization length owing to the exciton wavefunction spatial dynamics may provide even a larger scale of changes. In this work, the limitations of exciton sizes are discussed by analysis where the exciton concept is still applicable. It is shown that the exciton size can be as small as few angstroms, but even smaller sizes can be, probably, justified. At the same time, coupling of exciton to polariton mode can enlarge the exciton-polariton coherence length to values as high as 20 µm, thus extending the scale of possible exciton sizes up to five orders of magnitude.

Keywords: Wannier–Mott exciton, Frenkel exciton, delocalization, self-trapping, low-dimensional media, Rashba effect.

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