Localized Surface Plasmon Resonance Based Metal Nanostructure Refractive Index Sensing

High sensitive refractive index sensing structures have broad applications at chemical and biological sensing. Because metal nanostructures show significant electromagnetic enhancement due to their surface plasmon resonance (SPR), and hence have attracted considerable interests in highly sensitive sensing applications. Two types of SPR are used for refractive index sensing applications:propagated SPR (PSPR) and localized SPR (LSPR). PSPR sensing is based on very smooth metal surface, which requarets high machining precision. Therefor, we only discuss refractive index sensing based on LSPR here. Enhanced localized electric field at LSPR can be created by sharp apexes of metal nanostructures, so the metal nanostructures with sharp apexes have high sensitivities. In the first part of this dissertation we investigate an X-shaped metal nanohole array structure which has four sharp apexes for LSPR sensing. Experimental and simulated results reveal that the structure has unusually high refractive index sensitivity. Furthermore, metamaterials also show greatly enhanced localized electric field at magnetic resonance, so they can be used in high sensitive refractive index sensing. The rest of this dissertation is to fabricate metamaterial structures for sensing applications. Metal ring-shaped disk structure has large spatial overlap between the localized electric field and the surrounding medium to be sensed. X-shaped metal nanostrucuture has four sharp tips which have enhanced localized electric field at LSPR. Thus such two structures mentioned above have high sensitivity. Base on these points, we fabricate two metamaterial structures constructed with a dielectric layer supported metal ring-shaped disk (MRD) array on a metal film and a dielectric layer supported metal X-shaped array on a metal film. Experimental and simulated results show that such two structures have urtrahigh sensitivity. The work of this dissertation is divided in the follwing parts:1. X-shaped metal nanohole array for refractive index sensingMetal nanostructures with sharp apexes have strong localized electric field at LSPR. This phenomenon leads strong interaction between the localized electric field and the surrounding medium to be sensed. Thus such structures have high sensitivity. Based on this point, we fabricated an X-shaped metal nanohole array with four opposite sharp apexes. The presence of the four sharp apexes can confine and enhance electric field well at LSPR. Transmission spectrum measurements show the refractive index sensitivity of such structure can reach945nm RIU-1, higher than other metal nanoparticals with high refractive index sensitivities based on transmission and reflection spectrum measurements like nanorings and nanocresents. We use dielectric layer to lift the X-shaped metal nanohole array away from the glass substrate to increase the spatial overlap between the localized electric field and the surrounding medium to be sensed and then reduce the distance between the opposite apexes to further enhance the localized electric field. After these two steps, an ultrahigh sensitivity of1398nm RIU"1is achieved at near infrared. Such ultrahigh sensitivity provides such structure potentials for the development of chip-based high sensitivite nanooptical biomedicine sensing and integrated devices.2. Ring-shaped disk metamaterial for refractive index sensingBecause of the magnetic resonance, metametarials show greatly enhanced localized electric field around them. This will lead strong interaction between the localized electric field and the surrounding medium to be sensed. Thus metametarials have high sensitivity. Metal ring-shaped disk structure has larger spatial overlap between the localized electric field and the surrounding medium to be sensed than other metal nanostructures like metal disks and spheres, so such structure has higher sensitivity. Based on the two points mentioned above, we fabricate a metamaterial structure constructed with a dielectric layer supported metal ring-shaped disk array on a metal film. A refractive index sensitivity of1304nm RIU-1is achieved by reflection spectrum measurements. We increase the height of the dielectric layer and the inner radius of the ring-shaped disk to increase the spatial overlap between the localized electric field and the surrounding medium to be sensed. The increasing of the inner radius can enhance the localized electric field around the ring at the same time. After the two steps the refractive index sensitivity of the structure can reach1842nm RIU-1, higher than other ordinary metal nanoparticals for sensing applications based on transmission and reflection spectrum measurements.3. X-shaped metamaterial for refractive index sensingX-shaped metal nanostructure has four sharp tips which have strong localized electric field at LSPR, so such structure has high sensitivity. Here we fabricate metamaterial structure constructed with a delectric layer supported X-shaped metal nanostructure on a metal film. Reflection spectrum measurements show the sensitivity of such structure is2487nm RIU-1. The sensitivity can reach2835nm RIU-1by increasing the size of X-shaped nanostructure. Such ultrahigh sensitivity provides the structure big potentials for chip-based nano-optical biosensors and integrated devices.