On columnar thin films as platforms for surface-plasmonic-polaritonic optical sensing: higher-order considerations

TL;DR

This paper investigates how the size and distribution of particles in columnar thin films affect surface-plasmon-polariton waves, revealing key optical property changes relevant for enhanced sensing applications.

Contribution

It introduces a higher-order homogenization model that accounts for particle size and distribution, providing new insights into SPP wave behavior in CTF-based sensors.

Findings

Increased particle size decreases SPP phase speed and increases attenuation.

Larger correlation length reduces SPP penetration depth.

Sensitivity to refractive index changes decreases with larger particles.

Abstract

The ability to tailor the porosity and optical properties of columnar thin films (CTFs) renders them promising platforms for optical sensing. In particular, surface-plasmon-polariton (SPP) waves, guided by the planar interface of an infiltrated CTF and a thin layer of metal, may be harnessed to detect substances that penetrate the void regions in between the columns of a CTF. This scenario was investigated theoretically using a higher-order homogenization technique, based on an extended version of the second-order strong-permittivity-fluctuation theory, which takes into account the size of the component particles which make up the infiltrated CTF and the statistical distribution of these particles. Our numerical studies revealed that as the size of the component particles increases and as the correlation length that characterizes their distribution increases: (i) the phase speed of the SPP wave decreases and the SPP wave's attenuation increases; (ii) the SPP wave's penetration into the CTF decreases; (iii) the angle of incidence required to excite the SPP wave in a modified Kretschmann configuration increases; (iv) the sharpness of the SPP trough in the graph of reflectance versus angle of incidence increases; and (v) the sensitivity to changes in refractive index of the infiltrating fluid decreases.