Novel nanostructured plasmonic sensing devices : fabrication, modeling and application

Abstract

The research work presented in this thesis focuses on the fabrication and application of plasmonic sensors. The content describes (1) nanofabrication on the optical fiber facet to create a plasmonic probe using nanoimprint lithography, (2) plasmonic sensing and pH sensing applications using the nanostructured fiber facet, (3) morphologically gradient nanostructures fabricated by fiber-optic interference lithography following transfer of the structure from photoresist to a transparent mold, and (4) spectrometer-free plasmonic sensing of the refractive index of a liquid and hydrogen gas based on morphologically gradient nanostructures. In the first section, two-dimensional nanostructures are patterned on polymer molds using thermal nanoimprint lithography, and subsequently, a thin gold layer is evaporated on the nanostructured polymer mold surface. The nanostructured thin gold layer is then transferred to an optical fiber facet with 200/230 µm diameter by ultraviolet nanoimprint lithography. The prepared nanostructured fiber facet has plasmonic optical response in its reflection spectrum. Its local electric field distribution and far field spectrum are also modeled numerically. In the second section, the plasmonic fiber facet is applied to measure the aqueous refractive index, showing excellent thermal stability (137 pm/℃). Then, a pH-responsive hydrogel is prepared and directly coated onto the fiber facet by fiber-guided internal UV exposure. The hydrogel-integrated fiber pH sensor is capable of detecting acidic liquid with pH values ranging from 2 to 7.5. The fiber pH sensor has excellent reversibility and stability. In the third section, a custom-built fiber-optic interference lithography system is introduced. Using the Gaussian laser beam from a single mode optical fiber, concentric gradient nanostructures can be fabricated on 3×3 cm2 silicon wafer carrying photoresist. Important modules of the interference lithography are introduced, containing shutter, center alignment module of stage rotation and laser exposure. To obtain a transparent plasmonic mold with concentric nanostructures, two methods are applied. The first method transfers the two dimensional nanostructure with 370 nm pitch from photoresist to a polymer mold. Subsequent gold evaporation completes the plasmonic device. The other method places concentric nanostructures with 440 nm pitch on ITO carrying photoresist. After electroplating, plasmonic nanostructures are fabricated on the ITO surface. In the fourth section, the concentric gradient nanostructures on a polymer mold are used to perform plasmonic sensing applications. Under narrow-band light source illumination, the change in intensity curve, captured by CMOS and analyzed by computer, is used to present measurements of a series of refractive index liquids with 0.02 RIU interval on the chip surface. When under broad-band light illumination, we apply the hue value from HSV color space to follow the refractive index changes. We also use palladium-coated concentric gradient nanostructures to detect hydrogen gas. In this way, optical hydrogen sensing can be performed by image recognition without using a spectrometer.