We propose multi-band metamaterial absorbers at microwave frequencies. The design, the analysis, the fabrication, and the measurement of the absorbers working in multiple bands are presented. The numerical simulations and the experiments in the microwave anechoic chamber were performed. The metamaterial absorbers consist of an delicate arrangement of donut-shape resonators with different sizes and a metallic background plane, separated by a dielectric. The near-perfect absorptions of dual, triple and quad peaks are persistent with polarization independence, and the effect of angle of incidence for both TE and TM modes was also elucidated. It was also found that the multiple-reflection theory was not suitable for explaining the absorption mechanism of our investigated structures. The results of this study are promising for the practical applications.

Gas hydrates are becoming an attractive way of storing and transporting large quantities of natural gas, although there has been little effort to understand the preferential occupation of heavy hydrocarbon molecules in hydrate cages. In this work, we present the formation kinetics of mixed hydrate based on a gas uptake measurement during hydrate formation, and how the compositions of the hydrate phase are varied under corresponding formation conditions. We also examine the effect of silica gel pores on the physical properties of mixed hydrate, including thermodynamic equilibrium, formation kinetics, and hydrate compositions. It is expected that the enclathration of ethane and propane is faster than that of methane early stage hydrate formation, and later methane becomes the dominant component to be enclathrated due to depletion of heavy hydrocarbons in the vapor phase. The composition of the hydrate phase seems to be affected by the consumed amount of natural gas, which results in a variation of heating value of retrieved gas from mixed hydrates as a function of formation temperature. 13C NMR experiments were used to measure the distribution of hydrocarbon molecules over the cages of hydrate structure when it forms either from bulk water or water in silica gel pores. We confirm that 70% of large cages of mixed hydrate are occupied by methane molecules when it forms from bulk water; however, only 19% of large cages of mixed hydrate are occupied by methane molecules when it forms from water in silica gel pores. This result indicates that the fractionation of the hydrate phase with heavy hydrocarbon molecules is enhanced in silica gel pores. In addition when heavy hydrocarbon molecules are depleted in the vapor phase during the formation of mixed hydrate, structure I methane hydrate forms instead of structure II mixed hydrate and both structures coexist together, which is also confirmed by 13C NMR spectroscopic analysis.

Using a planar metamaterial, which consists of two silver strips, we theoretically demonstrate the plasmonic electromagnetically-induced transparency (EIT)-like spectral response at optical frequencies. The two silver strips serve as the bright modes, and are excited strongly by the incident wave. Based on the weak hybridization between the two bright modes, a highly-dispersive plasmonic EIT-like spectral response appears in our scheme. Moreover, the group index is higher than that of another scheme which utilizes the strong coupling between the bright and dark modes.

We numerically and experimentally investigated dual-band absorption of sandwich-structure metamaterials which include periodic metal coupled rings at the front separated from the metal plane at the back by a dielectric layer. The properties are demonstrated in both GHz and mid-IR regimes of electromagnetic (EM) wave. The dual-band perfect absorber with polarization independence is observed under normal incidence. In order to understand the EM properties of dual-band perfect absorber, the plasmonic excitation was clarified for both peaks. Finally, by connecting the rings, the perfect-absorption peaks can be controlled with the polarization angle of incident EM wave.

Plasmonic electromagnetically-induced transparency (EIT) can be excited by a single optical field unlike EIT in atom system, since the coupling between the bright and the dark modes is inherently induced through the near-field interaction in metamaterials. As a result, the complexity of the experimental realization can be reduced significantly, while the tunability is lost inevitably.We suggest a scheme that the plasmonic EIT is possible to be actively manipulated even by the single optical field. The bright and the dark modes are selective to be either coupled or uncoupled, depending on the angle of incidence. Even though the mechanical control has the disadvantage for high-speed applications, it paves the way for active manipulation of plasmonic EIT and benefits the clarification of its origin.

Magnetic resonance is considered to be a necessary condition for metamaterial perfect absorbers, and dual-band absorbers can be composed of a pair of metallic layers with anti-parallel surface currents. We designed and fabricated a tunable dual-band perfect absorber based on extraordinary-optical-transmission (EOT) effect and Fabry-Perot cavity resonance. The idea and the mechanism are completely different from the absorber based on the near-field interaction. The important advantage of our structure is that we can switch a single-band absorber to a dual-band absorber by changing the distance between two metallic layers and/or incident angle. The peak originating from the EOT effect becomes significantly narrower, resulting in an increase of the Q-factor from 16.88 to 49. The dual-band absorber can be optimized to be insensitive to the polarization of the incident electromagnetic wave by slightly modifying the absorber structure.

We propose a dual-band metamaterial perfect absorber at microwave frequencies. Using a planar metamaterial, which consists of periodic metallic donut-shape meta-atoms at the front separated from the metallic plane at the back by a dielectric layer, we demonstrate the multi-plasmonic high-frequency perfect absorptions induced by the third-harmonic as well as the fundamental magnetic resonances. The origin of the induced multi-plasmonic perfect absorption was elucidated. It was also found that the perfect absorptions at dual peaks are persistent with varying polarization.

Experimental data on the kinetics of methane and carbon dioxide hydrate formation obtained in silica gel pores at pressures of 0.5−4.85 MPa above equilibrium are reported. The results indicate that the rates of hydrate formation are closely related to the driving force. Rapid hydrate formation and a much smaller induction time are obtained. The hydrate formation rate is fastest when carbon dioxide and methane are used in 100 nm silica gel pores even under low driving force. Owing to the rapid rate of hydrate formation in silica gel pores and the accumulation of the released heat of hydrate formation among the void among silica gel particles, a sudden temperature increase occurs at the beginning of the measurement. Some memory effects occur depending upon the state of the samples, but the pattern is random. Compared to mechanical stirring and the ice powder methods, the hydrate formation in silica gel pores shows a more rapid rate of hydrate formation and shortened induction time.