Research On Periodically Nanostructured Magneto-Plasmonic Sensing Devices

Designing magneto-plasmonic systems with prominent magnetic-optical（MO）activity by means of combination of magnetic materials and plasmonic metals have become an active topic of research in recent years.By an appropriate internal architecture composed of these two materials,the MO activity in these systems can be largely increased due to the surface plasmon resonance（SPR）.Meanwhile,the plasmonic properties of nanosystems can be controlled effectively by an applied magnetic field due to their magnetic functionality.Therefore,the magneto-plasmonic systems find application in,for instance,gas and biosensing areas,and in integrated photonic devices for telecommunications.To date,most of MO SPR sensors adopt the conventional architectures of continuous film which need the prism-coupling mechanism and the excitation of monochromatic light,which cannot realize the miniaturization,and also increases fabrication cost.In this paper,we devise and investigate the nanostructured magneto-plasmonic sensing systems by utilizing the finite element method（FEM）,and demonstrate the enhanced MO effects and sensing performance in these systems.Compared with the general film system,our proposed nanostructures not only achieve the device miniaturization,but can also obtain a higher MO signal.With advancement of fabrication technologies,our works are to provide a guidance in theory for the investigation of complicated magneto-plasmonic nanostructures in the future.The works in this paper includes specifically three aspects as follows:Firstly,in order to have a contrast of sensing performance between the plasmonic and magneto-plasmonic sensors,we propose a nanostructure composed of nanoring arrays on a gold substrate.By studying the relationship between the structural symmetric and optical response,we demonstrate that the high-order modes can be excited in the nanostructure with symmetric breaking at normal incidence.The generation mechanism behind each mode is revealed through the near field distributions of the structure at resonance wavelengths,where the high-order plasmon modes feature a narrower spectral line width due to their sub-radiant properties.By an investigation of the reflection spectrum in the system for different dielectric environment,calculated sensing figure of merit（Fo M）for the quadrupolar mode reaches up to 137.4RIU^-1.Secondly,we propose a simple magneto-plasmonic crystals that consists of gold nanowire arrays standing on a metallic substrate.We demonstrate theoretically that the structure can obtain an enhanced transverse MO Kerr effect（TMOKE）which features a Fano-like line shape,due to plasmon.The TMOKE response of the system can be tuned by changing the nanowire size,and we demonstrate the correlation of the magnitude of MO signal and the contrast ratio of reflection spectrum,that is,the high contrast ratio is favorable to obtaining the MO signal with narrow line width and high amplitude.Based on the measurement of TMOKE,the system as a sensor obtains a sensitivity of 693 nm/RIU and a Fo M reaching up to 4192 RIU^-1 in a wide refractive index range of 1.00–1.40.Secondly,on the basis of the grating system,we devised a magneto-plasmonic structure based on nanodisk array,which is composed of Au-Co bilayer nanodisk arrays on an Au substrate.A surface lattice resonance（SLR）occurs in the system due to the diffraction coupling between nanodisks.Therefore,in the aspect of MO response,the system obtains a MO signal exceeding 0.6 due to this plasmon mode.In the aspect of sensing performance,based on measuring the TMOKE,the system obtains a sensitivity of 717nm/RIU and a Fo M close to 7000 RIU^-1.As compared with the magneto-plasmonic based on gratings,the higher sensitivity arises from the more enhanced localized optical field supporting by noble metals;the higher Fo M arises from the MO signal with narrower line width induced by the SLR mode.MO systems can able to have a circumvention of loss of materials,increasing tremendously the sensing performance of the same type of plasmon-based sensorsFinally,we provide analyses and perspectives for the development of magneto-plasmonic sensors.