Plasmonic Near-Field Chiral Responses of Planar Chiral Graphene Metasurfaces
Objective Chirality is a geometric property of objects that cannot be superimposed on their mirror images through simple rotation or translation.Many biomolecules,such as nucleic acids,DNA,and carbohydrates,exhibit chirality.Molecules with different handedness often show different physiological activities and biological toxicities.Therefore,accurate and efficient identification,detection,and separation of chiral molecules are essential in fields like analytical chemistry and biopharmaceuticals.Research has shown that the ultra-strong chiral near-field generated by metasurfaces can significantly amplify the weak chiral response of chiral molecules,making it highly valuable for chiral sensing,molecular recognition,and separation.Various chiral,achiral,2D,and 3D metal nanostructures have been designed to produce chiral near-fields.Some of these near-fields exhibit opposite chirality in different regions,limiting the enhancement of volume-averaged optical chirality;some structures are complex to fabricate;others generate background chiral signals.In addition,the response wavelength of metal nanostructures is usually restricted to the visible and near-infrared regions.However,important drugs and biomolecules exhibit chiral signals in the mid-infrared range.Graphene nanomaterials,with advantages such as low loss and dynamic tunability,have thus gained significant attention.We theoretically design a planar chiral metasurface composed of simple graphene nanosheets to achieve a single-handed,more uniform,and stronger chiral near-field distribution.Methods A rotating planar chiral metasurface composed of three rectangular graphene nanosheets is proposed to generate a single-handed,uniform,and strong chiral near-field response.Graphene nanosheets are deposited on a silicon substrate,and the entire metasurface is placed in air.Simulations are conducted using COMSOL software based on the finite element method.Circularly polarized light(CPL)propagates along the-Z direction,with an electric field intensity of 1 V/m.The graphene metasurface is placed on the X-Y plane,with periodic boundary conditions applied to the boundaries perpendicular to this plane,while the top and bottom surfaces are set as perfect matching layers.The transmittance of the metasurface is set as T,reflectance as R,and absorbance as A=1-R-T.The physical properties of the graphene metasurface are described by its conductivity,and optical chirality(C)characterizes the chiral near-field intensity.Results and Discussions When the chiral graphene metasurface is excited by CPL along the-Z direction,a plasmonic resonance at a wavelength of 20 μm is induced.Peaks and valleys of chiral near-field enhancement are observed on both sides of the resonant peak,and opposite-handed chiral responses are observed with left-handed CPL(LCP)and right-handed CPL(RCP).The volume-averaged chiral near-field enhancement reaches up to 50 times,significantly surpassing symmetric metasurfaces(Fig.1).Near-field maps show the electric fields of the nanosheets are mainly concentrated at the ends and tips under RCP excitation.With a rotated distribution of the nanosheets,the tip of one nanosheet couples with the end of another,increasing the coupling area.Due to the rotational arrangement of the nanosheets and the incident light's effect,the magnetic field shifts diagonally across the nanosheets,increasing the chiral near-field within the region enclosed by the nanosheets(Fig.2).By adjusting the misalignment between the nanosheets,structural chirality is enhanced,with an intensified tip coupling effect,leading to an increased chiral near-field response(Fig.3).Changes in the length and width of the nanosheets affect the plasmonic resonance's intensity and peak(valley)position(Figs.4-6).In addition,the Fermi level of the graphene nanosheets allows for future adjustment of the chiral near-field response,with higher levels enhancing the response intensity and inducing a blue shift of the peak(valley)(Fig.7).When chiral molecules of opposite handedness interact with the chiral near-field,the absorption spectra are symmetrically distributed relative to achiral molecules(Fig.8).Conclusions In this paper,we propose a planar chiral metasurface composed of simple rectangular graphene nanosheets.This design achieves a stronger and more uniform one-handed chiral near-field response.Near-field maps show that the coupling between the rotated nanosheets enhances the electromagnetic response in the central region,resulting in a stronger and more uniform one-handed near-field response.As the misalignment increases,the chiral structure strengthens,and optical chirality increases.As the nanosheet width increases,chiral peak(valley)positions show a blue shift,while holding the enclosed area constant results in minimal chiral near-field response change.However,expanding the enclosed area decreases the volume-averaged optical chirality enhancement.As nanosheet length increases,chiral peak(valley)positions exhibit a redshift.The Fermi level of graphene can further tune both the optical chiral intensity and response position.When the region enclosed by the nanosheets is filled with chiral molecules of opposite chirality,the absorption spectrum is symmetrically distributed compared to non-chiral molecules.This theoretical study provides a reference for chiral sensing and the detection of small amounts or single molecules in experiments.
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