Plasmonic Properties Of Periodic Nanostructured Metal Arrays

Recently,numerous different plasmonic nanostructures have been investigated extensively due to their unique optical properties.These plasmonic nanostructures consist of metallic nanoparticles,nanorings,nanorods(or nanobars),and so on.They have been utilized widely in optical devices and sensors.When plasmonic nanostructures interact with the incident electromagnetic waves if their surface plasmon wave vectors match with the surface light wave vectors,the electrons in plasmonic nanostructures will resonate with the same frequency as incident light,which is known as localized surface plasmon resonance.The electromagnetic field is enhanced and localized in the vicinity of the surface of the nanoparticles.Therefore,these plasmonic nanostructures can be used for enhancing the electric field in the terahertz regime and for sensing applications.By using plasmonic nanostructures,one can significantly enhance the ability of detecting bio-molecules with very low concentrations.In this thesis,three different plasmonic nanostructures,including nanoslit array with metal back plane,nanoslit array with micro-cavities,and metallic nanoring array with metal back plane are investigated.The possibilities of employing the properties and performances of the latter two structures in sensing applications have been explored.The first structure is an array of slits design that combines the metallic nano-slit grating and dielectric-metal,which can be used to obtain giant and tunable electric field enhancement in terahertz regime.It is found that the enhancement depends primarily on the stripe width,the nano-slit width,period of grating,as well as the thickness of spacer layer,when the thickness of grating and width of nanoslit are thiner than the skin depth.This property is particularly beneficial for the realization of ultra-sensitive nanoparticles detection in the terahertz range.The second structure is a metallic nanoring array combining with a dielectric spacer and a metallic plate.The absorption property is due to the gap plasmon resonance mechanism.The absorption property is dependent on dielectric and metal of the structure.For investigations about its absorption properties is few by other researchers,we investigate the influence of different dielectric media including SiO2,TiO2,TiN,and Al2O3,while metals are Au,Ag,Al,and Cu,respectively.Operated as a perfect absorber and sensor,resonant wavelenght has redshift when the refractive index of the environment is increased.We also investigate the different media effect on the sensing performance.This structure has potential in chemical and biomedical sensing applications.Additionally,we investigate the narrowband absorption property of the second structure based on surface lattice resonance.Its absorption is up to 90% while linewidth is narrower than10 nm.The spectrum with a sharp absorption dip depends strongly on the refractive index of the media surrounding the nanorings.This feature can be explored to devise a refractive index sensor.Bulk sensitivity is 500 nm/RIU and FOM is 25,which is one order larger than that based on gap resonance mode,while the surface sensitivity factor FOM = 0.42 can be two times larger.The third structure is a perfect infrared absorber structure with narrow bandwidth consisting of a metal grating with nanoslits,a dielectric spacer layer,and a metal back plate.There is a slit with several nanometers wide in one unit cell of the top grating.Its bandwidth and perfect absorption are narrower than 10 nm and about 95%,respectively.Therefore,large field enhancement can be achieved in nanoslits with perfect absorption through this structure.The thickness of the nanobars and the spacer,and the width of the nanoslits are primary parameters determining the absorption performance.The refractive index of environment effects strongly on the resonant properties of the structure.This structure not only has narrow bandwidth but also can obtain the giant electric field enhancement in the tiny volume of the nanoslits.Operated as a refractive index sensor,this structure has figure of merit as high as 25.It has potential in biomedical sensing applications.