| Surface plasmon polaritons（SPPs）are a kind of electron sparse and dense waves propagating along the metal surface（metal-dielectric media interface）formed by the collective oscillation of free electrons caused by the interaction of free electrons on the metal surface with the electromagnetic field.SPPs have the advantage of overcoming the diffraction limit in conventional optics,while also greatly enhancing the electromagnetic field at the interface.The phenomenon of surface plasmon resonance（SPR）will occur if the wave vector of SPPs matches the wave vector of the incident light.SPR can strongly enhance the electromagnetic field at the interface between the metal and the dielectric.The electromagnetic field energy is maximal at the metal-dielectric interface and decays exponentially perpendicular to both sides of the interface.Therefore,SPR is very sensitive to the change of refractive index at the interface between metal and medium,and it has a wide range of applications in biosensors.SPR sensors based on noble metals have been widely studied.Researchers have proposed various methods to improve the sensitivity of sensors.Although the sensitivity can be improved from external factors,the loss of noble metals itself cannot be ignored.Therefore,the resonance linewidth of noble metal-based SPR sensors will be wider,resulting in a smaller figure of merit（FOM）of the sensor.Alkali metals have been considered as good plasmonic materials for a long time due to their small intrinsic loss.Due to their active chemical properties,almost no researchers have done comprehensive research on them.This paper is based on this background,we explore the characteristics of the sodium-based plasmon resonance sensor,design a variety of different sodium-based plasmon sensor structures,use simulation and theoretical methods to analyze device performance.The main research contents are as follows:1.A SPR sensor based on a planar sodium replacing noble metals film wrapped with polymer PMMA is studied,and a prism-coupled Kretschmann structure refractive index sensor is proposed.The role of PMMA is to prevent sodium from oxidation.COMSOL is used to simulate the structure and optimize each parameter.Transfer matrix method（TMM）is used to verify the simulation results to further confirm the reliability of the simulation.We get higher sensitivity sensing and narrower resonance linewidths.For the angular response,the sensitivity is 91.3 deg/RIU（refractive index unit,RIU）,the full width at half maximum is about 0.215°,and the FOM is 424.7/RIU;for the wavelength response,the sensitivity is14.9μm/RIU,the full width at half maximum（FWHM）is about 0.031μm,and the FOM is480.6/RIU.In addition,based on the existing research,we also tried to add graphene to the sensor.Compared with the case without graphene,the sensitivity and FOM are improved while the introduction of graphene could not significantly enhance the performance of the sensor.2.To further enhance the sensitivity of the sensor,we design a model of sodium sinusoidal grating structure.Consistent with the expected results,the sensing performance is significantly improved by introducting metal layers with different micro-nano structures.The structure can realize high angular sensitivity sensing,and the angular response of the structure has a self-reference function.By optimizing the structural parameters,the maximum angular sensitivity can be obtained as 840 deg/RIU.Here,we use rigorous coupled wave analysis（RCWA）to prove the validity of the simulation results of the angular response.The resonance angles gained by the simulation are basically in accord with those calculated by RCWA.In addition,for wavelength response,the largest wavelength sensitivity is 2μm/RIU,and the resonance linewidth is about 7.5 nm.Finally,the impact of each parameter on the resonance is studied.In addition,we compare the sensing performance of traditional noble metal-based SPR sensors,and our proposed sodium-based grating structure sensor has more superior sensing performance.Especially in terms of resonance linewidth,sodium has less loss than noble metals in the near-infrared wavelength,so narrower resonance linewidths can be obtained.Therefore,sodium is expected to find wider applications in sensing.3.This paper also studies a sodium-based metal-insulator-metal（MIM）structure,where Na is used as a metal substrate,which is wrapped with PMMA,and a gold nanograting with rectangular cross-section is on the top.The simulation is also carried out using COMSOL.Our MIM nanograting can support a very narrow resonance line width.By analyzing the field distribution,it is proved that the narrow resonance results from the surface lattice resonance（SLR）.In addition,the MIM structure can generate a localized Gap Plasmon Resonance（GPR）.By adjusting the appropriate parameters,the linewidth of the SLR mode is less than 3 nm,and a good sensitivity of 1000 nm/RIU is obtained,and the FOM is greater than 300 RIU-1.Finally,we investige the impact of various structure parameters on the sensing performance,and the grating period and gold line width have a greater impact on the sensing performance of the sensor.In addition,compared with other noble metal-based structures supporting SLR mode,it is found that our proposed sodium-based structure has superior sensing performance.In conclusion,we have investigated various sensors with high sensitivity and narrow resonance linewidth realized by sodium-based SPR,and analyzed the influence of structural parameters on the sensing performance of the sensor.The simulation results show that the sodium-based plasmonic sensors can indeed achieve better characteristics than traditional noble metal-based sensors.Due to the small intrinsic loss of sodium in the near-infrared bwavelength,and a good application prospect is expected for sodium-based plasmonic components. |
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