There is a need for sensitive methods to analyze thin films of polymers, biological cells, and self-assembled monolayers. Herein, we present a newly-developed instrument for collecting directional Raman scattering (DRS) with monolayer sensitivity and its application for measuring time-dependent molecular interactions. DRS is performed by optically coupling a sample to a gold-coated Weierstrass-type prism in the Kretschmann configuration and scanning the angle of the incident laser. At an appropriate incident angle, propagating surface plasmons are generated in the metal film and produce a cone of scattered radiation through the prism caused by a momentum conserving process called optical coupling. The DRS instrument can acquire two-angle dependent parameters: a digital image of the surface-plasmon-polariton cone and the directional Raman signal. Polymer films were used to quantify the signal and show that the parameters are well modeled by reflectivity and electric field calculations. As the polymer film thickness increases from 10 to 100-nm there is an increase in the incident angles that produce the cone (i.e., surface plasmons) and Raman signal as well as an increase in the cone diameter. For polymer thicknesses greater than 300-nm, the sample behaves as a waveguide and multiple concentric cones are observed in the reverse-Kretschmann configuration (illumination from the sample side). DRS can be applied to study a variety of thin films where the chemical composition, orientation, and morphology are important to their function, including photovoltaic and energy harvesting devices.

Directional surface-plasmon-coupled Raman spectroscopy: A new laboratory instrument for combined adsorption and sensitive Raman detection of thin films on smooth planar surfaces

2019 Spring National Meeting

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