Study On The Properties Of Metamaterial Absorber Based On Surface Plasmonic Metal - Medium - Metal Structure

Metal-dielectric-metal (MDM) structures based on surface plasmons can concentrate light into extremely small cavities. Because of perfect absorption and ultra-strong near field, MDMs have potential applications in surface enhanced molecule spectroscopy, solar cells, bio-sensing and the design of optical absorption devices. In this paper, the physical mechanism of the perfect absorption and the related applications of MDMs are analyzed. The commerical software of High Frequency Structure Simulator (HFSS) is utilized to carry out a systematic study. The main works and conclusions are described as following.1. We propose a design of "cylindrical microcavities" and study the optical properties, including the energy density inside the microcavities. Under normal incidence, the minimum (dip) of the reflection spectra is in the range of the mid-infrared. The absorption rate is close to one by optimizing the structure’s geometry parameters, about 97%. We demonstrate that the multilayer structure with sub-wavelength electromagnetic confinement allows 104 fold enhancement of the electromagnetic energy density inside the cavity, 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. Besides, the structure is insensitive to the polarization of the incident wave due to the symmetry of the cylindrical microcavities.2. We design a triple-band metamaterial absorber whose compact single unit cell consists of concentric metallic rings with a metallic disk at the center and a metallic ground plane separated by a dielectric layer. Meanwhile, we also study the near-field enhancement and absorption properties inside the double cylindrical micrcavities of the absorber. The obtained reflection spectra show that the resonance frequencies of the double microcavities are 16.65THz,20.65THz and 25.65THz, respectively. The absorption rate can reach to 95%,97% and 95% at the corresponding frequencies by optimizing the geometry parameters of the structure. And by changing the parameters of the structures, we can modulate the single or multiple resonant frequencies, greatly improving the efficiency and flexibility. Moreover, we demonstrate that the multilayer structure with sub-wavelength electromagnetic confinement allows 105 fold enhancement of the electromagnetic energy density inside the cavities. The proposed structure is insensitive to the polarization of the incident wave due to the symmetry of the double cylindrical microcavities and is a promising candidate as the substrate of surface enhanced molecular spectroscopy.3. We numerically study the multi-band absorption properties and near-field enhancement inside the cuboid cavity based on the interference theory. The compact single unit cell consists of a metallic square patch placed on the top of a metallic ground plane, separated by a dielectric layer. Under normal incidence of electromagnetic radiation, four bands with a maximum absorption of 98% are accomplished by appropriated sizes of the square patch. Furthermore, we demonstrate that these four bands, which are corresponding to the fundamental mode and higher modes of the standing wave, can be readily tuned in the mid-infrared region. Because the functional groups fingerprints spectrum mostly locate in mid-infrared region, we can tune the resonance peaks of the structure to overlap the vibrational positions of the groups, which can be used to realize the multi-channel surface enhanced infrared absorption spectroscopy...