The Covert Transmittance Enhancement Effect of Terahertz Metamaterials Coupling with Vapor

Metamaterials with Fano resonance have been developed to investigate the behavior of terahertz wave propagation, light-matter coupling mechanism, and improvement in detection efficiency. However, studies about gas molecules interacting with metamaterials in the terahertz regime were rarely reported due to the lack of structure enabling the strong coupling between two systems. In this paper, a novel design strategy based on mirror split rings and closed ring combined metamaterials is proposed. The established structure gives quadrupole mode Fano resonance, whose frequency locates around 0.56 THz. The frequency alignment with one of the well-known molecular rotation modes of water vapor leads to a strong coupling occurring on the surface of the metamaterial when illuminated by the terahertz wave. The strong coupling was experimentally observed by performing transmission THz-TDS measurements on metamaterial with several humidity environments. The humidity is controlled by nitrogen purging and oversaturated salt solutions. A transmittance window can be observed in the spectra instead of the expected water vapor absorption feature occurring around 0.56 THz. Furthermore, the conversion from attenuation to transmittance becomes more potent when the Fano resonance is excited. The physical mechanism of the transmission enhancement is interpreted and discussed in detail to show that the metamaterials' resonance could be effectively coupled with water vapor molecules' vibration mode in the atmosphere. Such novel covert effects in the transmission spectra can be exploited to reduce the loss of THz waves in the atmosphere, pointing out its remarkable potential in terahertz communication research and gas sensing.