Enhanced And Tunable Circular Dichroism Of Bilayer Chiral Metamaterials

Most of the biomolecules in nature have chirality,and chiral substances are closely related to the human life.Therefore,circular dichroism(CD)spectroscopy as an important method to detect and characterize chiral enantiomers has broad research prospects in life sciences,physics,chemistry and other fields.However,the CD signals of chiral structures in nature are relatively weak and response in the near-ultraviolet band,so it is difficult to use the existing equipment to detect and research.With the development of research on metamaterials,the design of chiral metamaterials with high sensitivity,convenient regulation,and strong signal has become a hotspot for researchers.In this paper,we mainly study the optical properties and circular dichroism of bilayer metamaterials.Based on the coupling between waveguide modes and surface plasmon modes in metamaterials,the CD signal in the visible and near-infrared bands could be enhanced and tuned through the change of structural parameters,which could have potential applications in chiral selective imaging,sensing and spectroscopy.The main research contents of this paper are as follows:1.Enhanced and tunable circular dichroism in double-layer asymmetric metal gratingsIn this work,we design a double-layer asymmetric metal grating structure separated by a dielectric waveguide layer,discuss the optical response characteristics and circular dichroism,and then clarify the mechanism of enhanced circular dichroism of the structure.The results show that the circular dichroism can be enhanced in the visible light range by considering the coupling of the waveguide mode and the localized surface plasmon resonance.Especially at the wavelength of 635 nm,the maximum value of the CD signal of the structure can reach 0.52,which is more than 4 times higher than the traditional double-layer grating without considering the waveguide effect.In addition,the circular dichroism signal could be tuned in the visible band by adjusting the structural parameters,such as the period of the top grating and the thickness of the waveguide layer.2.Enhanced and tunable circular dichroism in double-layer metal gammadion structureIn this work,Applied the waveguide layer into the double-layer gammadion metastructure,we discussed the optical response characteristics and circular dichroism in detail and obtained the enhancement and tuning of the circular dichroism signal.Under the illumination of different circularly polarized light,the CD signal could be enhanced in the near infrared band by considering the coupling between the waveguide mode and the surface plasmon,which could reach 0.474 at the wavelength of 882 nm.Compared with the traditional structure without considering the waveguide effect,the CD value of the proposed structure can be increased by 20 times.At the same time,it can achieve the purpose of tuning the strength and position of the CD signal by adjusting the structural parameters,such as changing the period of the gammadion structure,the thickness of the spacer layer and the thickness of the waveguide layer.