Owing to the advantages of natural abundance, low cost, and amenability to manufacturing processes, aluminum has recently been recognized as a highly promising plasmonic material that attracts extensive research interest. Here, we propose a cavity-enhanced ultra-thin plasmonic resonator for surface enhanced infrared absorption spectroscopy. The considered resonator consists of a patterned ultra-thin aluminum grating strips, a dielectric spacer layer and a reflective layer. In such structure, the resonance absorption is enhanced by the cavity formed between the patterned aluminum strips and the reflective layer. It is demonstrated that the spectral features of the resonator can be tuned by adjusting the structural parameters. Furthermore, in order to achieve a deep and broad spectral line shape, the spacer layer thickness should be properly designed to realize the simultaneous resonances for the electric and the magnetic excitations. The enhanced infrared absorption characteristics can be used for infrared sensing of the environment. When the resonator is covered with a molecular layer, the resonator can be used as a surface enhanced infrared absorption substrate to enhance the absorption signal of the molecules. A high enhanced factor of 1.15×105 can be achieved when the resonance wavelength of resonator is adjusted to match the desired vibrational mode of the molecules. Such a cavity-enhanced plasmonic resonator, which is easy for practical fabrication, is expected to have potential applications for infrared sensing with high-performance.
We propose an infrared biosensor for nanofluidic analysis based on graphene plasmonics, which consists of a nanochannel etching on a silicon substrate and a graphene sheet covered on the top of the channel. The change of refractive index due to the absorption of biomolecules in the nanochannel can be measured by detecting the wavelength shifts of resonant dips. To achieve the best optical performances of the biosensor, an optical model based on finite element method is built to optimize the structure parameters of the biosensor. Numerical simulation results show that a biosensor with a larger top width and a higher depth shows a better overall performance and a high sensitivity value of up to 1920nm/RIU can be achieved in an optimized structure. In addition, the biosensor can dynamically work at a wide range of infrared region by adjusting the Fermi level of graphene. Graphene is pre-coated with poly methyl methacrylate to overcome the effect that the portion of graphene over the nanochannel will be strained and the influence of the thickness of this coated layer on the performances of biosensor is very small. The designed graphene plasmonics devices will advance further applications of graphene in integrated nanofluidic analysis and infrared biosensors.
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