Surface Plasmon Resonance
Thin Silver Film Thickness Dependance of Surface Plasmon Resonance
Sara Moore and Klara Northrup
Advisor: Jacob Ritz
University of Minnesota
Spring 2021 MXPII
Introduction
In this experiment we explored the relation between the thickness of a sample of silver and the angle and intensity at which SPR occurred. We reflected a photo-diode laser off silver samples of various thicknesses, used a camera to capture the diffraction pattern, and performed image analysis to determine the intensity and angle where plasmon resonance occurs. We concluded that 50 nm is the optimal thickness for resonance to occur with the most consistent, repeatable, and intense resonance.
Contents
A plasmon is a collective oscillation of the free electrons [1], and surface plasmon resonance (SPR) refers to a coherent oscillation of electrons that takes place at the surface of a metal [4]. Interest in SPR began in 1902 with Wood’s discovery of anomalies in the diffraction pattern of light by diffraction gratings that manifested in rapid variations in the intensity of the diffracted spectral light, known as Wood’s anomalies [5]. Plasmon waves were first observed in 1968 by Otto, and his experimental setup was further improved by Kretschmann through the use of thin metal films [4]. The study of the properties of SPR and the application of those properties to developments in instrumentation and experimental techniques has grown into the field of plasmonics, which has applications that range from bio-sensing to solar energy capture [4] [1]. One such application is in the detection of certain molecules (such as immunoglobulin) in a biological system. The real time detection of molecules, the accuracy of SPR, and the affordability of such methods could make SPR critical in point of care tests. Point of care tests are essential in both healthcare and environmental monitoring [6].
Goal: Explore the relationship between the thickness of a Silver sample and angle and intensity at which SPR occurs, and decipher the optimal thickness for SPR.
Acknowledgements
We would like to thank both Kevin Booth and Jacob Ritz for their time and guidance throughout this project. We also appreciate the access to Dr. Zimmermann’s vacuum evaporation deposition machine.
References:
[1] Bethel University: Nathan C Lindquist PhD, Plasmonics and Surface-Enhanced Spectroscopy, 2012.
[2] Erik J. S ́anchez, Prof.: Portland State University, Surface Plasmon Resonance (SPR) Lab, 2010.
[3] Nagata, Kazuhiro., et al. Real-Time Analysis of Biomolecular Interactions : Applications of BIACORE.
Springer, 2000.
[4] Olivier Pluchery, Romain Vayron, Kha-Man Van, Laboratory Experiments for Exploring the Surface
Plasmon Resonance, 2011.
[5] Springer, Plasmonics: From Basic to Advanced Topics, Springer-Verlag Berlin Heidelberg, 2012.
[6] Yun Liu, Shimeng Chen, Qiang Liu, Jean-Fran ̧cois Masson, and Wei Peng, Compact multi-channel surface
plasmon resonance sensor for real-time multi-analyte biosensing, Opt. Express 23, 20540-20548, 2015.
The factors that contribute to optimizing the SPR can all be varied in order to model the ideal conditions for SPR to occur. The best model that explains the contribution of these variables, especially thickness of the metal, are the Fresnel Equations which describe the behavior of reflectance between the surface of the metal and the prism [2]. This is the model that we will attempt to recreate and that we will analyze our results based upon.
When sending light at a thin sample of silver only certain wavelengths of light can resonate in the electrons at the surface of the gas. When photons at the appropriate wavelength hit the surface it will be absorbed and form a surface plasmon, leaving a dark band in the reflection where the absorbed photons would’ve landed [3]. This is how we will observe the SPR that occurs.
