Increase of refractive index induced by absorbingcentres: homogeneous and inhomogeneous line-broadeningcases

. Besides the extinction coe_cient, the refractive index of a dielectricmedium is modi_ed by the presence of resonant absorbing centres, too. Fully an-alytical expressions of the complex refractive index dispersions are derived for thecases of homogeneous and inhomogeneous absorption-line broadening. In both cases,we demonstrate that the modi_cation is such that the refractive index results asincreased at photon energies lower than the resonance absorption energy. If theabsorbing centres also emit Stokes-shifted radiation, the increase of refractive indexcan be exploited for passive and active waveguiding applications. An example isreported regarding active waveguides based on colour centres in a lithium uoridecrystal.Keywords: Absorbing centres, active waveguiding, colour centres, Gaussian line-shape, inhomogeneous line broadening, refractive index.