Designing Mid-Infrared Gold-Based Plasmonic Slot Waveguides for CO2-Sensing Applications.

Parviz Saeidi,Bernhard Jakoby,Andreas Tortschanoff,Gerald Stocker,Reyhaneh Jannesari,Jasmin Spettel,Gerald Pühringer,Florian Dubois,Thomas Grille

doi:10.3390/s21082669

Abstract

Plasmonic slot waveguides have attracted much attention due to the possibility of high light confinement, although they suffer from relatively high propagation loss originating from the presence of a metal. Although the tightly confined light in a small gap leads to a high confinement factor, which is crucial for sensing applications, the use of plasmonic guiding at the same time results in a low propagation length. Therefore, the consideration of a trade-off between the confinement factor and the propagation length is essential to optimize the waveguide geometries. Using silicon nitride as a platform as one of the most common material systems, we have investigated free-standing and asymmetric gold-based plasmonic slot waveguides designed for sensing applications. A new figure of merit (FOM) is introduced to optimize the waveguide geometries for a wavelength of 4.26 µm corresponding to the absorption peak of CO2, aiming at the enhancement of the confinement factor and propagation length simultaneously. For the free-standing structure, the achieved FOM is 274.6 corresponding to approximately 42% and 868 µm for confinement factor and propagation length, respectively. The FOM for the asymmetric structure shows a value of 70.1 which corresponds to 36% and 264 µm for confinement factor and propagation length, respectively.

Highlights

Published: 10 April 2021In recent years, surface plasmon polaritons (SPPs) have received much attention as they have opened up opportunities in many photonics applications
As for our sensing application, two important factors, i.e., the confinement factor (Γ) and the propagation length, play important roles, we defined a figure of merit (FOM) which is obtained by multiplying the aforementioned key quantities
Once the sensor platform is exposed to a gaseous medium which forms the cladding, the evanescent electromagnetic field (characterized by the evanescent field ratio (EFR), which describes the fraction of the guided electromagnetic field energy present in the evanescent field) begins to interact with the gas through absorption occurring in this region

Summary

Introduction

Published: 10 April 2021In recent years, surface plasmon polaritons (SPPs) have received much attention as they have opened up opportunities in many photonics applications. SPPs, which are guided electromagnetic waves, can be created when surface localized light waves along metaldielectric interfaces are coupled to free electron oscillations in the metal [1,2] Once they are excited, they allow for efficient light–matter interactions [3]. The maximum intensity of the resulting guided mode occurs at a metal/dielectric interface where the field amplitudes decay exponentially in the direction perpendicular to the interface [4] This field, which is an evanescent field, indicates the bound, non-radiative nature of SPPs, preventing power from leaking away from the surface [5]. The popularity of plasmonic waveguides is due to several features including high field intensity at the metal–dielectric interface and the capability of confining modes on the nanometer scale [6].

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