Study On Surface Plasmon-polaritons Of Metal Nanostructures And Fano Resonance Effect

Surface plasmons are electromagnetic motion modes formed by the interaction between free electrons and photons.They exist only on metal surfaces and have excellent optical properties such as local field enhancement,sensitivity to environmental changes,selective absorption and scattering of light.Surface plasmons have developed into the current international frontier research field.And metal nanostructures can support surface plasmon resonance.Its unique optical characteristics make it a subject of great concern in the field of nanoscience.Metal nanostructures are widely used in the design and improvement of wavelength-tunable optical devices and high-sensitivity sensors.In this paper,we design and simulate several different metal nanostructures.The local field enhancement of the grating structure,tunable Fano resonance,and tunable surface plasmon-induced transparency are studied.The main research contents are shown below:（1）A two-dimensional composite grating nanostructure was designed by using multi-physics field coupling software.Each structural unit consists of a short metal rod and a long metal rod,with a long square cut off on each side of the long rod to form a mortise structure.It is found that the field enhancement effect of the composite grating is up to 3.61×10⁶ in the slit under the light wave irradiation.It can be seen that the composite grating structure has stronger field local effect than the traditional double-width plasma grating with an enhancement coefficient of 4.6×10⁵.（2）Fano resonance generally originates from the mutual coupling of light and dark modes.Based on it,a metal nanostructure composed of a metal nano-square ring and two metal nanorods is designed.The light was incident perpendicularly,and the Fano resonance characteristics of the structure were studied.The full-width at half maximum height was 36nm and the contrast ratio was 87%.The contrast ratio of Fano resonance is higher than that of single rod and square ring-disk.The peak of Fano resonance of this structure is affected by several parameters.For example,when the length of inner ring₂ and the length of metal bar₀ increase,the peak redshifts.The increase in the refractive index of the surrounding environment causes the redshift of the Fano extinction spectrum with a sensitivity of 797 nm/RIU.Its excellent nature,which is sensitive to changes in the surrounding environment,may have potential applications in the field of sensors.（3）Combining metamaterials with plasmons can generate a phenomenon similar to electromagnetically induced transparency,that is,plasmon-induced transparency.We also calculated its plasmon-induced transparency characteristics in the same structure.Due to the similarities between the formation of PIT and Fano resonance,the two results also have similarities.We calculated that the maximum transmittance exceeds90%,and the tunability of surface plasmon-induced transparency is achieved.When the refractive index of the surrounding medium changes,the line shape and transmittance of the PIT curve are basically unchanged.The increase of the refractive index of the surrounding environment gradually redshifts,and the sensitivity is 795 nm/RIU.Varying several other structural parameters will also change the transmission spectrum.The structure is sensitive to changes in the refractive index of the surrounding environment and may have potential applications in the field of plasmon-induced transparency.