Study On Refractive Index Sensor Based On Magneto-optic Surface Plasmon Resonance (MOSPR)

Surface plasmon resonance（SPR）is a new sensing technology which has been developed rapidly in recent years,because of its high sensitivity towards the surrounding refractive index environment,label-free,nondestructive,real-time detection and some other characteristics,it has been widely used in biological sensing,food safety and environmental monitoring,etc.Magneto-optic surface plasmon resonance（MOSPR）is the result of the combination of SPR and magneto-optic（MO）effects,which can obtain a higher signal-to-noise ratio and limit of detection.At present,the sensitivity of the MOSPR sensor is commercially available,but it needs to be further improved,so how to further improve the sensitivity is a subject worthy of study.In this work,based on the principle that the sensitivity of MOSPR structure is influenced by the MO constant of the magnetic material,we use the Fe_xCo_1-x which has a higher MO constant than the currently used magnetic material（Fe,Co）as the magnetic layer in MOSPR structures,the sensing performance of different components of Fe_xCo_1-x in the Au/Fe_xCo_1-x MOSPR structures was studied.Through the characterization of Fe_xCo_1-x thin films,the dielectric constant and magnetic constant of Fe_xCo_1-x were calculated by the least square fitting method.According to the obtained optical parameters,we simulate the Au/Fe_xCo_1-x structures by 4×4 transfer matrix method,and optimize the structure of the device.With the experimental verification,we find that the highest sensitivity of Au/Fe_xCo_1-x bilayer transducer is clearly correlated to Fe_xCo_1-x’s MO response.In all structures,Au/Fe0.7Co0.3 shows the best sensing performance owing to its highest MO response,suggesting Fe_xCo_1-x as a promising candidate for ultrasensitive MOSPR sensors.In addition,due to the high magnetostrictive effect of Fe_xCo_1-x,the MOSPR structures such as Au/Fe_xCo_1-x can be developed as MOSPR strain sensor and applied to the field of surface detection,surface roughness mapping in the future.In this work,we also study the enhancement effect of all-dielectric structure,which has been proposed in these years,on the MO effect.Similar to the localized surface resonance（LSPR）,resonance effect also exist in all-dielectric structure,and there is a new magnetic resonance in compare with LSPR.Due to the low loss of the dielectric itself,we believe that there is a sharp enhancement of the MO effect around the resonance peak.Through the finite element software COMSOL,we confirmed that the enhancement does exist.According to this principle,we propose a new type of refractive index sensor with high figure-of-merit by all-dielectric structure in the future.