Backside Absorbing Layer Microscopy: a Universal Relationship Between Physical Thickness and Reflectivity

The Backside Absorbing Layer Microscopy (BALM) is a recently introduced surface imaging technique in reflected light with an unprecedented combination of sensitivity and lateral resolution, hence very promising for the development of imaging sensors. This requires to turn BALM images into quantative analyte measurements. The usual way to analyze reflectivity is to compare the optical signal and a numerical model with many adjustable parameters. Here we demonstrate a universal relationship between the sample reflectivity and the physical thickness of the sample, ruled by three measurable quantities. Mapping the physical sample thickness becomes possible whatever the instrument setting and the sample refractive index. Application to kinetic measurements is discussed.