Estimating the Absorption and Waveguiding in Porous Slabs from Multi-modal Measurements

Knowledge of the radiation transfer through porous materials is important for a variety of engineering systems. In our experiments with layers of packed microparticles on metal, the fraction of light that is directed from normal incidence into waveguided modes is non-negligible; we seek to estimate the absorption in this porous layer with a few, simple experiments. We employ a ray transfer-function approach and provide analytic relations for the reflected, waveguided, and transmitted light. With these relations and multi-modal experimental data taken at different laser wavelengths, the interplay between reflection, waveguiding, and transmission can be estimated. We find that waveguiding is significant when the single-pass absorption of a porous slab is more than a few percent.