Optimizing the Refractive Index Sensitivity of Extraordinary Optical Transmission Based Sensors

Extraordinary optical transmission involves the resonant coupling of incident electromagnetic radiation with surface charge density oscillations called surface plasmons. The resulting propagating surface modes are known as surface plasmon polaritons. In extraordinary optical transmission the coupling mechanism is a periodic array of sub-wavelength holes perforating a thin metallic film. As light shines on the metal surface energy accumulates within the surface modes, tunnels through the holes, and is re-scattered into the far field on the opposite side of the film. The resonance condition depends intimately on the profile of the metal film, geometry of the hole array, and the optical properties of the metal film and adjacent dielectric. These surface modes are evanescently constrained to the metal-dielectric interface, and therefore make excellent probes of the local refractive index. This thesis describes a series of studies aimed at optimizing the refractive index response of nanohole arrays in thin gold films. I designed and optimized these sensors to detect the refractive index changes caused by antibodies secreted by live, microfluidically-trapped immune cells, binding to functionalized arrays. In calibration studies, the minimum detectable concentration of antibody in cell growth medium was 3 ± 1 μg/ml. In live cell studies, I was able to detect antibodies secreted from 200 trapped cells, detecting a peak shift of 10 nm above that of a control sample. Detection from lower numbers of cells was unreliable, likely due to competing reactions from non-specific binding. In the quest to improve the device sensitivity, the influence of the array geometry and the role of the nanohole array lattice on the transmission spectrum of square and hexagonal arrays were clarified. With these insights and improvements, the minimum bulk refractive index resolution (glucose solutions) increased from 2.5 ± 0.3 x10-3 to 1.5 ± 0.1 x10-3 units, ultimately limited by the optical system and the 1 nm resolution of the spectrometer used in the measurements. A superior data analysis technique based on an integrated response analysis of the entire transmission spectrum was introduced. Finally, I demonstrated a process to recycle the delicate nanohole arrays without destroying their physical and optical properties.