Optical response of excitonic polaritons in photonic crystals

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Abstract
Nonlocal investigations are presented for exciton-photon coupling in photonic crystals consisting of two kinds of alternating slabs (CuCl/NaCl), for which excitons exist only in one (CuCl) of the two slabs. Studies are carried out for several typical combinations of period and slab thickness. The lower branch of the excitonic polariton for this system is found to split into many small bands separated by small band gaps (polariton gaps). This phenomenon is explained as the band splitting caused by the coherent interference of polaritonic waves in periodic systems. At the same time, the group velocity of light is greatly reduced in the presence of the excitons. The present nonlocal study demonstrates a double exciton-photon coupling, in which the upper branch of the polariton couples again with the size-quantized exciton states. A long-wavelength approximation is also presented along with a discussion of its validity for simplifying the nonlocal theory. The absorbance and reflectance spectra computed using the transfer matrices exactly reproduce the above small bands for the same systems. An examination of the coupling scheme among excitons, photons, and the structural periodicity indicates that the former two couple with each other more strongly than the other combinations of them. The exciton component of polariton, which is localized in each slab in darkness, could be construed as being delocalized with the assistance of the photon.