Planar Nanophotonic Structures for Intensity-Based Readout Refractive Index Sensing of Dissolved Methane
Talk, Symposium F.NM02—Advanced Linear/Nonlinear, Tunable and Quantum Materials for Metasurfaces, Metamaterials and Plasmonics (Includes Symposium S.EL03—Novel Approaches and Material Platforms for Enhanced Light–Matter Interaction, Plasmonics and Metasurfaces), Virtual
As global temperatures rise, permafrost in the Arctic is thawing, stimulating increased release of methane, a key greenhouse gas. Accurate measurements of the dissolved methane concentration in seawater and freshwater are important for finding and quantifying the release of natural seabed and Arctic methane seeps to better understand how these sources are contributing to increasing global methane levels. In surface waters, the methane concentration can be as low as 3-10 nM (atmospheric equilibrium) while concentrations as high as 800-1000 nM are found in methane saturated deep seawater and near thawing permafrost. To measure the concentration of dissolved methane, changes in the refractive index (RI) of polymers functionalized to selectively trap methane molecules can be measured via an optical readout mechanism. However, the range of the RI change is very narrow, from 1.41198 to 1.41358 for atmospheric to saturated methane concentration levels ranging from 0 nM to 300 nM, which requires the use of highly sensitive optical sensors. This paper analyzes and compares three simple and low-cost planar nanophotonic and plasmonic structures as optical transducers for measuring refractive index change of polydimethylsiloxane (PDMS) polymer films doped with cryptophane-A molecules, which selectively trap methane. These structures include (i) a Bragg reflector with a defect layer, (ii) a hybrid plasmonic-photonic multi-layer stack that supports a Tamm plasmon mode, and (iii) a hybrid plasmonic-photonic stack that acts as a magnetic mirror, and can be used in a simple intensity-based measurement scheme with low cost light sources and detectors. Through numerical simulations, we evaluate the sensitivity of the proposed structures in both the angular-resonant-mode shift readout mode and in the reflectance intensity readout mode and compare them to the standard surface-plasmon-polariton-mode Spreeta sensor as a reference. Our results show that the planar Bragg reflector with a defect layer exhibits the largest intensity response and is the most promising design for a low cost and robust photonic chip for dissolved methane sensing. A practical implementation of this chip with a simple intensity-based measurement scheme is proposed. Integration of this planar structure into a small, portable, and low-cost dissolved methane sensor offers a way to make climate monitoring more widespread and accessible to researchers. The structures evaluated here all show promise for other chemical and biological sensing applications that require monitoring very small RI changes. Significantly, each structure has been designed to support resonant modes in both s and p polarizations, which allows enhanced sensitivity by calculating the ratio of the spectral responses of orthogonal polarizations, similar to the complex reflectance ratio measured with ellipsometry. Importantly, the response in both polarizations only needs to be monitored in the reflectance intensity readout mode, avoiding the use of expensive and large ellipsometry equipment.