Effect of Chiral Media-Metal Core-Shell Structure and Its Shell Thickness on Optical Forces
Objective The optical response of natural chiral substances is usually very weak,necessitating the enhancement of the chiral optical response.Surface plasmon polaritons,collective oscillations of free electrons on metal surfaces interacting with incident light,possess the ability to surpass the diffraction limit and subwavelength constraints and are widely employed to miniaturize opto-electronic devices and to enhance light-matter interactions.In recent years,more and more studies have begun to focus on leveraging surface plasmon polaritons to enhance the optical response of chiral substances.While optical operation for chiral particles has garnered significant attention,the properties of optical operation for chiral media-metal core-shell structures remain unclear.On the one hand,the intricate relationship between the interaction of the core shell and the metal shell of chiral media and the shell thickness necessitates in-depth investigation.On the other hand,it is not clear how the shell thickness affects the optical response and optical force of the core-shell structure.This paper focuses on the modulation properties of the chiral medium/Ag core-shell structure and its shell thickness in terms of chiral sphere optical response and optical force.Methods Based on the plasma resonance effect of chiral media and silver,the optical response and optical force characteristics of the chiral media-Ag core-shell structure are simulated and calculated by applying Maxwell's stress tensor method using linearly polarised light.Special attention is devoted to investigating the influence of the silver shell thickness and chiral parameter on the scattering spectrum and optical force.Results and Discussions Coating silver shells outside the chiral spheres can effectively enhance the optical response(Fig.2)and optical force(Fig.3)of the chiral spheres.Moreover,the electric dipole resonance peak of the core-shell structure is blueshifted and the full width at half-maximum becomes wider as the thickness of the Ag shell increases[Fig.4(a)],which is due to the increase of Ag shell loss with its thickness.When the thickness of the silver shell is thick enough,the core-shell structure exhibits almost the same spectral response as the pure silver spheres.Accompanied by a change in the electric dipole resonance peak of the scattering spectrum,the optical force corresponding to this mode is blueshifted[Figs.4(b)and 4(c)],and the magnitude of the optical force depends on the strength of the electric field excited by the core-shell structure.The thickness of the silver shell plays an important role in the enhancement of the chiral spherical optical force,which differs from the optical torque when the chiral parameters are different for the same thickness of the silver shell(Figs.6 and 7).We also find that it is more favorable to achieve optical force enhancement of chiral spheres when the silver shell is thinner.As the thickness of the silver shell increases,the effect of the core-shell structure on the optical force enhancement of the chiral spheres diminishes,irrespective of the chiral parameters of the chiral spheres.When the silver shell thickness achieves sufficient thickness,the core-shell structure exhibits optical forces as a pure silver sphere.Conclusions We simulate and calculate the scattering spectra and optical force of silver-coated chiral sphere structures,comparing the results with those of pure chiral spheres and pure silver spheres.The results demonstrate that the silver-coated chiral sphere structure can effectively amplify and modulate the resonance peak of the scattering spectrum of the chiral sphere as well as the optical force properties at the mode corresponding to this resonance peak.This enhancement is attributed to the interaction between surface plasmon polariton within the chiral media and the silver shell.Further,the impact of the silver shell thickness on the scattering spectrum and optical force of the chiral media-silver core-shell structure is considered.It is found that the electric dipole peak of the scattering spectrum of the core-shell structure is blueshifted and the full width at half-maximum becomes significantly wider as the thickness of the silver shell increases,and the optical force corresponding to this mode undergoes the same change at the same time that the optical torque decreases rapidly.Upon reaching a sufficient thickness,the chiral media-Ag core-shell structure exhibits the same optical properties as pure silver spheres.The scattering spectra and optical force of the chiral medium/Ag core-shell structure are affected by the chiral parameters.However,when the silver shell is thin,the structure can effectively enhance the optical force.When the silver shell is thick enough,the optical properties of the core-shell structure are the same as those of Ag spheres.Our results provide a reference scheme for designing chiral spheres to enhance and modulate optical forces,thereby augmenting the interaction between chiral media and light.
shell thicknesschiral mediacore-shell structureoptical force
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