We present a study of the optical mode structure in metal-dielectric multilayer structures that represent amorphous silicon thin film solar cells with metallic back contact. Knowledge of the modal structure represents a first step toward describing absorption enhancement by the interface texture in solar cells. We present a method for determining experimentally the dispersion relations of multilayer films by coupling polarized light in a spectral reflection measurement to eigenmodes, using a one-dimensional sinusoidal grating. Because the used grating represents only a minor perturbation that establishes the coupling, the experimental data is well explained by the modal structure of a geometry with flat interfaces. On the basis of the measured mode structure, we propose an explanation for the beneficial effect of a low index buffer layer between the silicon absorber and the metallic back reflector. © 2009 American Institute of Physics.