Abstract
We analyze the response of surface plasmon (SP) sensors using a transmission line model. We illustrate this analysis with particular reference to a layered structure in which plasmon hybridization occurs. By applying the appropriate resonant condition to the system, we derive a circuit model which predicts the responsivity of different modes. This gives new physical insight into the sensing process. We discuss how the change in the sample region may be modeled as a change in the reactance in the equivalent circuit and from this, it follows that a single parameter can determine the change in resonance position with reactance. This approach is used to predict the response of a generic sensor to binding of an analyte and the bulk change of refractive index. This parameter arises naturally from the circuit representation in a way not readily accessible with the transfer matrix approach. The parameters can be expressed in terms of the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${Q}$ </tex-math></inline-formula> of a resonant circuit and confirms the intuition that a high <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${Q}$ </tex-math></inline-formula> is associated with poor responsivity, however, we demonstrate that there is another circuit parameter, the resistance at resonance, that can mitigate this effect, providing a route for optimization of the sensor properties.
Highlights
S URFACE plasmon resonance (SPR) is a powerful and well-established technique for measuring variations in refractive index
The present paper has given a new perspective on the performance parameters associated with sensors
The system is analyzed with a transmission line model and its associated equivalent circuit
Summary
S URFACE plasmon resonance (SPR) is a powerful and well-established technique for measuring variations in refractive index. Figure. compares the movement of the resonance position between the KO structure (n0 = 1.52, ε1 = ε1 = εau = −12.33 + 1.21i, n2 = 1.4, n3 = 1, d3 = 576nm) (blue color) and 50nm gold film based Kretschmann structure (red color) at 633nm wavelength with layer deposition and change of bulk refractive index, respectively. It shows that compared to the SP mode in the Kretschmann structure, the change of resonance position of the 1st order FP mode at 576nm gap separation in response to the layer and bulk moves by a factor of 1.8 times and 10 times, respectively, compared to the SP modes, at the cost of wider dip transition. This is discussed for different detection methods in [3]
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