How to determine the spectral properties of a 3-nm thin film?

Abstract

The optical characterization of ultra-thin films, such as molecular monolayers, is simultaneously a research tool for fundamental studies in molecule-interface interactions and an attractive configuration for a sensor platform. Molecular monolayers, which exhibit thickness of only a few nanometers, are attractive candidates for transducer layers in highly species-selective bio and chemical sensors. The planar waveguide is a configuration that has seen increasing use for research in thin film structure and surface characterization. The single mode planar waveguide, a substrate-supported dielectric layer, is an inherently sensitive geometry for probing ultra-thin films. At visible wavelengths, a single-mode planar waveguide supports up to several thousand reflections per cm of beam propagation using a ray optics model. This reflection density is about 4 orders of magnitude greater than using bulk optical elements in conventional attenuated total reflection (ATR) techniques, and yields a concomitantly much higher sensitivity. Nonetheless almost all-previous waveguide based studies have utilized monochromatic sources. Only a few works have reported spectroscopic detection either by using discrete laser lines1 or a monochromator2,3 to sequentially select a frequency to perform absorbance measurements.