Date of Award
Doctor of Philosophy
Emily A Smith
There is a need for sensitive methods to analyze thin films of polymers, biological cells, dielectric waveguides, and self-assembled monolayers. In this dissertation, we discuss a newly-developed instrument with combined benefits of surface plasmon resonance, plasmon waveguide resonance, and Raman spectroscopy for collecting the chemical information of adsorbates with monolayer sensitivity. Additionally, the instrument is applicable for measuring angle-dependent molecular interactions. Directional-surface-plasmon-coupled Raman spectroscopy (i.e., directional Raman scattering) is a viable non-destructive method equivalent to total internal reflection Raman spectroscopy using a smooth metal film. The excitation of surface plasmons produces directional Raman scattering in the plane of the metal film (in-coupling) and the emission of the scattered light through a Weierstrass prism (out-coupling). A hollow cone of directional scattering at a sharply defined angle results in the surface-plasmon-polariton cone radiating from the Weierstrass prism. The directionality of the signal, as well as the enhanced electric field, produces relatively large Raman signals at a smooth metal interface, without the use of surface-enhanced Raman substrates. The electric field intensity is amplified by 20-fold due to the directional emission of the scattered light and the collection of the entire surface-plasmon-polariton cone.
The directional Raman spectrometer has the capability of measuring the full surface-plasmon-polariton cone image, cone intensity, and directional Raman scattering radiating from the cone as a function of the incident angle. On the same instrument, the Kretschmann and reverse-Kretschmann configurations can provide multimodal spectral data (e.g., thickness and refractive indices) collection. The directional Raman spectrometer utilizes translational stages (as opposed to rotational stages, commonly used in surface plasmon resonance sensing). The instrument design provides faster acquisition times and precise control of the light incident on the prism interface with 0.06° angle resolution.
We can quantify the surface-plasmon-polariton cone properties and intensity from the digitized surface-plasmon-polariton cone image by extracting the cone diameters from the cone angles. The calculated cone parameters are obtained using three-dimensional finite-difference time-domain simulations of the far-field angular radiation pattern in combination with Fresnel reflectivity calculations. The approach has equivalent sensitivity to alternative methods used to collect surface plasmon resonance and plasmon waveguide resonance data. Further, we can simultaneously measure the adsorption and chemical identification of thin films, waveguides, and self-assembled monolayers. The sensitivity of all the waveguide-coupled surface-plasmon-polariton cone modes is between 0.009 and 0.02° nm-1. The incident angles that produce the surface-plasmon-polariton cones and the surface-plasmon-polariton cone angles are linearly dependent; therefore, it is straightforward to determine the optimum incident angle for collecting directional Raman scattering. According, the acquisition time is reduced for collecting plasmon waveguide resonance data. The thickness and chemical composition for thin films, as well as the structure and orientation of guided modes in waveguides, can be obtained in our multi-detection directional Raman scattering instrument.
Directional Raman spectroscopy can be applied to study photovoltaic thin films, polymer brushes, energy harvesting devices, optoelectronics, and sensor readout devices where the chemical composition, orientation, and morphology are essential to their function. This spectroscopic technique will propel new and emerging technologies in which functionalization of a surface is required.
Charles Kofi Adarkwa Nyamekye
Nyamekye, Charles Kofi Adarkwa, "Directional Raman spectroscopy: A surface-sensitive tool for measuring the chemical, optical, and physical properties of adsorbates on metal surfaces" (2020). Graduate Theses and Dissertations. 17945.