Focusing Shaping And Resonant Tuning Of Surface Plasmon Plaritons

Plasomonic functional devices are enjoying an intense current interest in the field of nano optics.In this paper,the control method and modulation of plasmonic field via micro/nano structures have been discussed we present the design of a plasmonic lens(PL)for coupling and focusing of surface plasmon polaritons(SPPs)into multiple focal spots on the optical axis,a plasmonic lens for focusing of linearly polarized light into a rotationally symmetric sharp focal spot.we also investigate the method of utilizing a periodic array structure to tune the(LSPR)peak of silver nano particles with a radius of 20nm.We propose the design of a plasmonic lens,which is composed of pixelated nano-grooves,for coupling and focusing of surface plasmon polaritons into multiple focal spots on the optical axi.The desired field pattern is obtained by controlling the x-position of each pixeleated nano-groove,which modulates the phase front of the SPPs equavelently.The arrangement of the pixelated grooves are optimized by simulated annealing method PLs that implement two and three on-axis foci are presented and the designed structures have been validated with FDTD simulations.We also successfully construct a long-focal-depth PL whose longitudinal FWHM of the focus reaches 25 plasmonic wavelengths,while its transverse field profile is maintained over 15 μm distance.The presented design method constitutes a new basis for plasmonic beam engineering,and the proposed particular SPP focal fields have potential applications in multiple imaging,particle manipulating,and plasmonic on-chip signal transmission.We designed a three-segmented PL,in which the central plasmonic lens is flanked by two inclined Fresnel zone plate like structures to increase the effective numerical aperture,thus reducing the full-width at half maximum(FWHM)of the focus to 1/3 wavelength of incident light.At the same time,two such lenses are placed facing each other will create a rotationally symmetric focus under a linearly polarized light,which overcome the limitation of a conventional PL that a rotationally symmetric focus can only be formed under radially polarized beam.Under the condition of 830nm incident light,the transverse and longitudinal FWHMs of the rotationally symmetric focus are 315 nm and 311 nm,respectively.Finally,we investigate the method of using the periodic array structure to change the LSPR peak of silver nano particles with a radius of 20nm.Proper periodic structure can move the shape of the LSPR spectrum peak shift to a specific location,which is difficult to achieve by changing the size or the shape with diameter less than 50nm.The range of adjustment is about 200-300nm wavelength.This is due to the broadband LSPR resonant with non-radiation surface waves generated by Wood anomalies in periodic structures formed a Fano resonance.Broadband LSPR being coupled to a narrowband Fano resonance,its width can be reduced to a few nanometer.At the peak wavelength of Fano resonance,the excited LSP on the surface of silver nanospheres was enhanced.