Coherent reflectance of light from confined nanocolloid films: Modeling, experiment and applications

We analyze simple theoretical tools to calculate the optical reflectivity of a confined colloidal film. We extend a previously reported multiple-scattering model for the coherent reflectance of a colloidal half-space for the... [ view full abstract ]

We analyze simple theoretical tools to calculate the optical reflectivity of a confined colloidal film. We extend a previously reported multiple-scattering model for the coherent reflectance of a colloidal half-space for the reflectance of a confined colloidal film. We refer to this model as the Coherent Scattering Model (CSM). We compare its predictions with experiments and effective medium models (EMM); namely the Maxwell Garnett and the so-called van de Hulst EMMs. We find an excellent agreement of the CSM with experimental measurements with gold, latex and metal-oxide nanocolloidal films. We show that the reflectivity of a confined colloidal film provides more information than the reflectivity of a semi-infinite space of the same colloid. Also, working with confined films, usually require only very small amounts of the sample. In this work, to test the theoretical models, we use an internal reflection configuration to measure the coherent reflectance of light from a colloidal thin film. The experimental setup consists of an optical cylindrical prism in contact with a nanocolloid layer of the sample and backed with a cover slide. The amount of sample used in each experiments is in the order of microliters. In this configuration there are two critical angles for light incident from the prism side, one between the prim’s glass and the air outside, and another one between the prism and the nanocolloid sample. The theoretical models and the experiments show a high sensitivity of the reflectance to the optical properties of the nanocolloid between the two critical angles and specially just before the second critical angle. The method used in this work is suitable for developing a variety of compact sensors for characterizing and monitoring processes in nanocolloids. We will illustrate some sensing capabilities of the coherent reflectance of light at specific angles of incidence to monitor processes in thin colloidal films.