Rapid and simple technique for the determination of the refractive index of ultra-thin organic films on planar transparent substrates using forward light scatter Original Research Article

A simple technique that we refer to as angularly dependent light scattering (ADLS) has been developed for the rapid determination of the refractive index of monolayer organic films that are covalently coated on optically transparent substrates. ADLS involves monitoring the response of a photodetector positioned directly below the point of reflection of a collimated beam of light at the interface between two optically transparent media of differing refractive index as a function of incident angle. A local maximum in the detector response is observed as the angle of incidence approaches the critical angle for total internal reflection (TIR) due to light scattering caused by surface roughness and imperfections. It has been empirically determined that the critical angle for TIR may be calculated from the angle at which the scatter intensity returns to baseline values. This experimentally determined critical angle may be used in conjunction with Snells’ law to calculate the refractive index of a thin film to ±0.05 if the refractive index of the other medium in the system is known. The accuracy of the technique has been confirmed by investigation of monolayer films of hydrocarbon (C18 silane) and hybridization reactions of monolayer films of single-stranded polythymidilic acid (dT20) with complementary oligomer (dA20).