A Highly Sensitive Sensing Technique via Surface Plasmons With Tunable Prism Refractive Index

Abstract

A new perspective using the Kretschmann configuration is presented to excite surface plasmon polaritons (SPP) at liquid crystal (LC)/metal interface. The excitation of SPP takes place through the electrical tuning of the prism refractive index instead of the commonly used angular interrogation. The results show that the proposed methodology is 30% superior to the traditional angular interrogation. The 5CB (4-pentyl- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$4^{\prime} $ </tex-math></inline-formula> -cyanobiphenyl) is one of the best-suited nematic LC materials for the proposed detection framework. Its refractive index is linearly tunable in the range 1.55–1.78 (at wavelength <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.55 \mu \text{m}$ </tex-math></inline-formula> ) via an externally applied electric field. In that range of refractive index variation, the SPP modes are efficiently excited; hence, any minute variation in the analyte refractive index can be detected reliably and accurately through a simple tuning of the refractive index of the prism LC material. The fundamental characteristics of the sensor are assessed through the evaluation of the linearity, sensitivity ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\textit {S}{)}$ </tex-math></inline-formula> , the full-width at half-maximum (FWHM) of the reflectivity curve, and the figure of merit (FOM). The variation of these characteristics over a whole range of analyte refractive index (1.3–1.4) is presented as well. The electrical tuning provides a well-matched design with micro-scale devices.