SURFACE PLASMON RESONANCE AT A SILVER-AIR INTERFACE

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Surface Plasmon Resonance at a Silver-Air Interface

Molly Andersen & Jesse Grindstaff

Adviser: Kevin Booth

Surface plasmons are mechanical waves of electrons oscillating at the interface between an insulator and a conductor. Coupling this charge density wave with an oscillating electric field is known as surface plasmon resonance.

In the bulk of a metal, electrons oscillate at eigenfrequencies determined by the electromagnetic restoring force provided by the Coulombic attraction between the electrons and the positively charged nuclei. These oscillations have energies on the order of 10 eV.[2] However near the surface of the metal, electrons are more loosely bound and therefore oscillate with a lower energy. Surface plasmon resonance coordinates these oscillating electrons to form a mechanical charge density wave by coupling the surface plasmon with an electromagnetic wave. In the case of certain metals such as gold and silver, this coupling occurs in the visible region [1], which is why silver was chosen as the subject of this study.

No Resonance- random oscillation of charge density

[click image for animation]

The coupling of the electric field to electron oscillation and the unique interfacial activity give rise to interesting optical properties [2,3]. Surface plasmon resonance is highly dependent on the dielectric functions of the two bounding materials which create the interface along which the wave propagates; it is corresponingly highly sensitive to changes in surface characteristics of this interface. Biophysics makes use of this sensitivity to measure molecular binding interactions that would otherwise be difficult to quantify due to unwanted system alteration by current labeling techniques [4]. A simple apparatus for determining the conditions of surface plasmon resonance at a silver-air interface was constructed. Specifically, the constraints on the wavevector of the incident light for a variety of frequencies was measured and mapped into a dispersion curve. The data obtained was compared to literature values [5] for the dielectric function of silver. The simplicity of the apparatus and the wide range of potential applications of the subject matter, allow this work to be the basis of future experimentation in optical, solid-state, and bio-physics.

References:

[1] Pluchery O., Vayron R., and Van K., 2011 “Laboratory experiments for exploring the surface plasmon resonance.” Eur. J. Phys. 32 585–99.

[2] Haes A.J. and Van Duyne R.P., 2002 “A Nanoscale Optical Biosensor: Sensitivity and Selectivity of an Approach Based on the Localized Surface Plasmon Resonance Spectroscopy of Triangular Silver Nanoparticles.” J. ACS 124 (35), 10596-10604.

[3] Barnes W.L., Dereux A. and Ebbesen T.W., 2003 “Surface plasmon subwavelength optics.” Nature 424 824-830.

[4] Drescher, D. G., Ramarkrishnan, N. A., and Drescher, M. J., 2009 “Surface plasmon resonance (SPR) analysis of binding interactions of proteins in inner-ear sensory epithelia.” Meth. Mol. Biol. (493) 323-343.

[5] Yang, H.U., D’Archangel, J., Sundheimer, M.L., Tucker, E., Boreman, G.D., Raschke, M.B., 2015 “Optical dielectric function of silver” Physical Review B 91 234137.

[6] Griffiths, D.J. “Introduction to Electrodynamics.” (Pearson Education, Inc.) 4^th ed. 2013.

[7] Novotny, L., and Hecht, B. “Principles of nano-optics.” (Cambridge University Press, New York, New York). 2006.

Resonance - coherent oscillation of charge density