title: Higher-order perturbation theory for the diffusion equation in heterogeneous media: application to layered and slab geometries.
abstract: We apply a previously proposed perturbation theory of the diffusion equation for studying light propagation through heterogeneous media in the presence of absorbing defects. The theory is based on the knowledge of (a) the geometric characteristics of a focal inclusion, (b) the mean optical path length inside the inclusion, and (c) the optical properties of the inclusion. The potential of this method is shown in the layered and slab geometries, where calculations are carried out up to the fourth order. The relative changes of intensity with respect to the unperturbed (heterogeneous) medium are predicted by the theory to within 10% for a wide range of contrasts dDeltamu(a) (up to dDeltamu(a) approximately 0.4-0.8), where d is the effective diameter of the defect and Deltamu(a) the absorption contrast between defect and local background. We also show how the method of Padé approximants can be used to extend the validity of the theory for a larger range of absorption contrasts. Finally, we study the possibility of using the proposed method for calculating the effect of a colocalized scattering and absorbing perturbation.
authors.affiliation: Department of Biomedical Engineering, Tufts University, 4 Colby Street, Medford, Massachusetts, USA. angelo.sassaroli@tufts.edu
authors.forename: Angelo
authors.initials: A
authors.lastname: Sassaroli
authors.forename: Fabrizio
authors.initials: F
authors.lastname: Martelli
authors.forename: Sergio
authors.initials: S
authors.lastname: Fantini