Two-photon absorption spectra and optical nonlinearity in an MX-chain compound

Two-photon absorption spectra of an MX-chain compound [Pt(en)2][Pt(en)2I2](SO4)2·6H2O have been measured. The two-photon absorption coefficient at the peak energy of the two-photon absorption band has been found to be 1.3×103 cm/GW. This value is two orders of magnitude larger than that predicted by the gap-dependent scaling law derived for conventional semiconductors and larger than the value that has been reported in polydiacetylenes. Electro-reflectance spectra have been measured and analyzed by a three-level model. The large two-photon absorption coefficient can be explained in the framework of the model.