Plasmonicfano Resonance Of Goldrectangular Split Nano-rings And Nano-crescents

Surface plasmon polarizations (SPPs) is the coupling mode of collective freecharge oscillation and electromagnetic (EM) wave on interface of metal anddielectricthatexcited by light. Because of the bonding property of electromagneticfield energy, it could generate large enhanced electromagnetic field near the metalsurface. Because this enhanced property is closely related to media, the environmentalsensors based on SPPs have been widely investigated and used in biochemistry. Howto improve the sensitivity and enlarge the used region are the key points.Fano resonance is an asymmetric lineshape resonant scattering phenomenon thatinterference between a resonant scattering process and background. Plasmonic Fanoresonance is produced by the interference between bright mode which can bemotivated by light and a dark mode which cannot be motivated by light directly. Thelow loss at Fano resonance has been an important way to improve the sensitivity ofplasmonic sensors.Different structures have different plasmonic properties, by the finite elementmethod (FEM), therectangular split nanorings(RSNRs) and nanocrescentsof goldhavebeen investigated in this paper.Firstly, the plasmonic properties including the distribution of EM fields, surfacecharge density and current density of symmetric gold RSNRs arrays have beeninvestigated. Theelectric (E) field polarization dependence has been found on thisstructure. Based on this, the influence of split distance on absorption spectra issimulated by the incident light with horizontal E field polarization. It turns out that,with the increasing of split distance, the peaks have a blue shift, and the shiftis moreobviously when split distance is small.And the peaks are wider when there is asmaller split distance. And then, the enhancement of E field on the middle of split andmagnetic (M) field on the middle of cross arms have been investigated, the peaks alsohave a blue shift, and there isa bigger enhancement with a smaller split distance. Theenhancement reach42times when the split distance d=5nm, but the enhancement ofM field almost keeps the same because the length of cross arms are the same.Then,we fix the spilt distance d=20nm, the influence of refractive index of medium whenthe incident light with horizontal electric E field polarizationhas been studied. We findthe absorption spectra is highly sensitive influenced by therefractive index of medium, when the refractive index changed0.001, the peaks of spectra shift20nm, to comparethe sensitivity with other structures, we use the figure of merit (FoM), and get FoM≈9.0when refractive index of media n=1.4, it is higher than many nanostructures,thiswill be a evidence to study onnear infrared ultra-sensitive sensor.Then, we fixthe total length of cross arms and trailing arms, and keepthe spiltdistance d=20nm, changed the position of spilt, making anasymmetric gold RSNRs.Firstly, the plasmonic properties and transmission spectra of right part have beeninvestigated, we find that, with the increasing of the cross arms’length, both the peaksof bright mode3and high order bondingmode1have a red shift, but the peaks of darkmode2keep the same because the thickness of RSNRs is constant. When3mode or1modeoverlaps with dark mode2,there will be a Fano lineshape.Then throughthesurface charge density of RSNRs, the mechanism of Fano resonance of RSNRs hasbeen found bythe hybridization theory, with the movement of split, there has a Fanoresonance when Dmode overlaps with Hmode. To investigate the sensitivity ofasymmetric RSNRs, we study on the absorption spectra of d=20nm with differentrefractive index. And we estimate the FoM of D mode and Hmode, we find the FoMof H is higher than D, and the peaks are in visible region, D is in infrared, it providesthe basis to make the LSPR sensor in visible using the H mode.At last, we improve the RSNRs to nanocrescents. We begin with the comparisonof transmission spectra of nanocrescents from experiment and simulation, using thehybridization theory and surface charge density of nanocrescents and transmissionspectra under different waist width of nanocrescents, the Fano resonance ofnanocrescents has been investigated. The bright mode and dark mode will be got bythe coupling from nanodisks and nanoholes. And there will be a Fano resonance whenthe mode motivated by quadrupoles of nanopores coupled with the dark mode ofnanocrescents. And when the peak of bright mode or dark mode overlaps with Fanoresonant peak, there will be an obvious Fano lineshape. And the Fano lineshapebecome weaker and weaker with the increasing of d, at the same time, theFoM=2.2~3.1have been estimated, it will provide a design guideline formakingplasmonic devices.