The Design And Experimental Study On Double Fano Resonances In Plasmonic Metasurfaces

Metasurface is a kind of artificial electromagnetic composite material which has subwavelength periodic structures.In the rich electromagnetic response of metasurfaces,the Fano response based on the destructive interference between bright mode and trapped mode has always been the hot spot of the researches,and is of great importance in fields such as sensing with high sensitivity and designing slow light device.In recent years,the multiple Fano resonances have attracted extensive attentions due to its superior performance in terms of modifying resonant waveforms at multiple spectral locations simultaneously.However,there are still few reports in this area.The main contents are as follows:(1)In the terahertz region,a double Fano resonator in S-shaped plasmonic metasurface is designed.When the electric field of normal incidence is polarized along the direction of the aluminum arms which are vertically placed,two electric dipole resonances near 0.3 THz and 1.3 THz are excited in symmetric S-shaped structure.Breaking the central symmetry can achieve two trapped modes at 0.6 THz and 1.0 THz,which are mainly attributed to a magnetic dipole and an electric quadrupole,respectively.At the same time,we calculate the transmission spectrum of the metasurfaces when the polarization direction is changed to be consistent with the horizontal aluminum arms.The results show that an electric dipole resonance appears in the symmetric structure,and its frequency is consistent with the high-frequency trapped mode;while in the transmission spectrum of asymmetric structure,double Fano resonances are observed,which are caused by the optical effect between trapped modes and bright mode.Furthermore,we numerically analyze the sensing characteristics of the asymmetric S-shaped plasmonic metasurface.The results show that when the thickness of the film is increased by 10?m,the maximum shifts of the double Fano resonance frequencies are 57.8GHz and102.4GHz,respectively.For refractive index sensing,the sensitivities are 39.2GHz/RIU and 66.2 GHz/RIU,respectively.In addition,double Fano resonances in three different S-shaped plasmonic metasurfaces are studied under oblique incidence.Firstly,when the electric field of electromagnetic wave is polarized along the vertical aluminum arms,two trapped modes of different metasurfaces can be excited in both E-plane(plane containing vectors E and k)and H-plane(plane containing vectors H and k).When the polarization direction is changed to be along the horizontal arms,double Fano resonances are successfully realized in both symmetric and asymmetric structures.From the surface current distributions of these two Fano resonances,it is known that they are caused by the destructive interference between two trapped modes and bright mode.Furthermore,by comparing the resonance characteristics in E-plane and H-plane oblique incidence,it is proved that the magnetic component of H-plane provides a coupling path for bright mode and trapped modes.(2)Based on the above structures,we experimentally investigate the electromagnetic response of S-shaped plasmonic metasurface in the terahertz region.The samples are fabricated by laser direct writing and wet etching,and tested by terahertz time-domain spectroscopy system.The results show that the double trapped modes and double Fano resonances can be excited in the asymmetric metasurfaces,which are consistent with the simulated results.Furthermore,we numerically study the influence of the absorption loss and plasmonic conductivity on resonance characteristics.The results show that the larger the absorption loss is,the lower the resonance amplitude is,and the change of the conductivity will cause the change of the resonant frequencies.