Near-Field Enhancement and Absorption Properties of Metal-Dielectric-Metal Microcavities in the Mid-Infrared Range

An important property of optical metamaterials is the ability to concentrate light into extremely tiny volumes, so as to enhance their interaction with quantum objects. In this work, we numerically study the near-field enhancement and absorption properties inside the cylindrical microcavities formed by a Au-GaAs-Au sandwiched structure. At normal incidence, the obtained reflection spectra show that the resonance wavelength of microcavities operates in the range of 5–5.8 μm. We also calculate the contrast C (C=1?R_min), which can be increased to 97% by optimizing the structure's geometry parameters. Moreover, we demonstrate that the multilayer structure with sub-wavelength electromagnetic confinement allows 10³–10⁴-fold enhancement of the electromagnetic energy density inside the cavities, which contains the most energy of the incident electromagnetic radiation and has a higher quality factor Q, indicating a narrower linewidth for surface enhanced molecular absorption spectroscopy and the tracking of characteristic molecular vibrational modes in the mid-infrared region. The structure is insensitive to the polarization of the incident wave due to the symmetry of the cylindrical microcavities. The unique properties of the metal-dielectric-metal metamaterials will have potential applications in new plasmonic detectors, bio-sensing and solar cells, etc.