Optical response of excitonic polaritons in photonic crystals

Nonlocal investigations are presented for exciton-photon coupling in photon ic crystals consisting of two kinds of alternating slabs (CuCl/NaCl), for w hich, excitons exist only in one (CuCl) of the two slabs. Studies are carri ed out for several typical combinations of period and slab thickness. The l ower branch of the excitonic polariton for this system is found to split in to many small bands separated by small band gaps (polariton gaps). This phe nomenon is explained as the band splitting caused by the coherent interfere nce of polaritonic waves in periodic systems. At the same time, the group v elocity of light is greatly reduced in the presence of the excitons. The pr esent nonlocal study demonstrates a double exciton-photon coupling, in whic h the upper branch of the polariton couples again with the size-quantized e xciton states. A long-wavelength approximation is also presented along with a discussion of its validity for simplifying the nonlocal theory. The abso rbance and reflectance spectra computed using the transfer matrices exactly reproduce the above small bands for the same systems. An examination of th e coupling scheme among excitons, photons, and the structural periodicity i ndicates that the former two couple with each other more strongly than the other combinations of them. The exciton component of polariton, which is lo calized in each slab in darkness,could be construed as being delocalized wi th the assistance of the photon. [S0163-1829(99)05208-X].