Metal-insulator-metal nanoresonators – strongly confined modes for high surface sensitivity

George Duffett,Ralph Wirth,Mathieu Rayer,Emiliano R Martins,Thomas F Krauss

doi:10.1515/nanoph-2020-0199

George Duffett, Ralph Wirth + Show 3 more

Open Access

https://doi.org/10.1515/nanoph-2020-0199

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Abstract

Abstract Photonic and plasmonic refractive index sensors are able to detect increasingly smaller refractive index changes and concentrations of clinically relevant substances. They typically exploit optical resonances and aim to maximise the field overlap with the analyte in order to achieve high sensitivity. Correspondingly, they operate on the basis of maximizing the bulk sensitivity, which favours spatially extended modes. We note that this strategy, counter-intuitively, is not necessarily suitable for detecting biomolecules and one should focus on the surface sensitivity instead. Here, we show that by confining light tightly in metal-insulator-metal (MIM) nanoresonators, the surface sensitivity is significantly increased despite a clear decrease in bulk sensitivity. In particular, we experimentally show the operation of third order MIM resonators which support both extended surface plasmon polariton (SPP) modes and localized MIM modes. We are able to demonstrate that the MIM mode has a sensitivity of 55 nm/RIU to a 10 nm layer, which is approximately twice as high as that of the SPP mode. Overall, our work emphasizes the importance of the surface sensitivity over the more commonly used bulk sensitivity and it shows a novel approach for improving it. These insights are highly relevant for the design of next generation optical biosensors.

Highlights

Biosensors have applications in detecting and measuring pollutants, monitoring food quality, early disease detection and drug discovery [1]
We experimentally show the operation of third order MIM resonators which support both extended surface plasmon polariton (SPP) modes and localized MIM modes
Our work emphasizes the importance of the surface sensitivity over the more commonly used bulk sensitivity and it shows a novel approach for improving it

Summary

Introduction

Biosensors have applications in detecting and measuring pollutants, monitoring food quality, early disease detection and drug discovery [1]. The classical surface plasmon resonance (SPR) method has a high bulk sensitivity of 2500 nm/RIU [5, 6] and has been commercialized as the Biacore system for studying DNA, proteins, cells and viruses [7]. MIM structures have been used for bulk refractive index sensing [14, 23] and as thermal emitters for optical gas sensing [24] Their remarkably high surface sensitivity remains unexplored, which is the focus of this work, and we show values of 55 nm/RIU for a mode confined to a 10 nm thick dielectric layer, which is twice the surface sensitivity of the extended surface plasmon resonance in the same system

Design

Materials and methods

Results and discussion

Conclusions