Figure 1. (a) 3D model of a MIM WG structure, which constitutes a thin metal film deposited on a substrate. And a narrow slit (few nm) is inscribed in the thin-film which provides a way for the SPP to propagate in a low-index medium, (b) a cross-sectional view of the E-field distribution taken at the output of the MIM WG, (c) The relationship between the real part of the effective index and the ambient refractive index.

In this mini-review, the recent advances in MIM WG-based plasmonic sensors are investigated and trying to figure out the possible causes which limit the experimental demonstration of such attractive devices. To the best of the author’s knowledge, almost all the reports are devoted to the numerical analysis of the nanoscale sensing devices without addressing the real challenges that can arise during nanofabrication, sensing mechanisms (by using microfluidic channels), and optical characterization. There are more than ~6000 reports published up till now on MIM WG devices indexed in the Scopus database. Butt et al. have also published more than 30 papers on the numerical study of the MIM WG-based plasmonic sensors. Most of the MIM WG-based sensors are based on unique resonant cavity schemes (as discussed later) where the gap between the bus WG and the cavity is ~10 nm-50 nm, furthermore, the additional geometric parameters such as the width of the bus WG, defects or baffles are also in nm scale. This raises a question on the method of fabrication, reproducibility, and light coupling methods during device characterization.