Study On The Fano Resonance In Nanostructure Under Ultrafast Laser Illumination

Plasmonic Fano resonance,which has the character of narrow line widths,high medium refractive index sensitivity,strong incident light confinement,and high quality factor,has been widely used in many frontier science and technology fields.Plasmonic Fano resonance results from the interference of superradiant bright mode and subradiant dark modes.When multiple dark modes are supported by the structure and coupled with the superradiant mode at the same time,multiple plasmonic Fano resonance can be excited.Multiple Fano resonances not only have all advantages of single Fano resonance,but also can modulate the spectrum at multiple frequencies simultaneously and can realize controllable spectral line shaping efficiently.However,the depth of the high-order resonance peaks in multiple plasmonic Fano resonances is generally shallow,and the adjustable spectral range of multiple plasmonic Fano resonances is limited.Moreover,the optical response of the excited multiple plasmonic Fano resonances are generally strongly dependent on the polarization direction of the incident light,which will undoubtedly limit the performance of the multiple plasmonic Fano resonances assisted solar devices and fully polarized sensors.In addition,directly near field imaging of plasmonic Fano resonance in metal nanostructure is,up to now,inadequate.These problems will undoubtedly restrict the comprehensive development and practical application of plasmonic Fano resonance.Based on the previous studies,we carried out the research on plasmonic Fano resonance in the asymmetric gold nanoring-disk,asymmetric gold nanosplit-ring-disk structure and gold solar-shape heptamer structure.Some progress has been made in controlling of high-order plasmonic Fano resonance peak depth,in designing a structure with the polarization-insensitivity of multiple plasmonic Fano resonance,and in directly imaging of the plasmonic Fano resonant distribution.Its main research contents and results are as follows:(1)A study on the multiple Fano resonances excitation of asymmetry gold ring-disk/ asymmetry split-ring-disk nanostructure by ultra-fast laser is carried out,from which we have obtained the mechanism of each Fano resonance.The resonant wavelength,depth and amounts of Fano resonances are controlled by changing the relative displacement of different structures and the variation of gap width.In particular,we have found that by adjusting the offsets between the center of disk and the center of cavity,independent control of the depth of high order Fano resonances can be achieved.(2)A study on the polarization sensitivity of asymmetry gold ring-disk/ asymmetry gold split-ring-disk nanostructure has been carried out.It is found that the multiple Fano resonance of asymmetry ring-disk nanostructure is extremely insensitive to the polarization direction of the incident light,while the asymmetry split-ring-disk nanostructure is sensitive to polarization direction.The results show that the number of multiple Fano resonant peaks can be controlled by changing the polarization direction of the incident light.(3)A directly near-field imaging of the Fano resonance in gold solar shaped heptamer nanostructure is carried out.By Photoemission Electron Microscopy(PEEM),we have obtained Fano resonance near-field distribution and mode evolution in this nanostructure.The experimental images of PEEM are consistent with the FDTD simulation results.