Optical Characterization of Plasmonic Anisotropic Nanostructures by Modeling and Spectroscopic Verification

This thesis attempts to characterize the optical properties of plasmonic anisotropic nanostructures through modeling and verification. Two nanostructures with important applications are selected for characterization. First, uniaxially aligned gold nanorods (AuNRs) embedded in polyvinyl alcohol (PVA) films are realized by determining suitable heating conditions during stretching, using PVA of high molecular weight mixed with plasticizer to improve the plastic deformability, and stretching the composite film. A high stretch ratio of seven is attained and the induced alignment of the rods is quantified statistically by an order parameter of 0.92 and an average angle of 3.5°. The stretched composite film is shown to have dichroic optical properties, which confirmed the good alignment. Since the statistical quantification requires destructive examinations, a novel non-destructive method is developed based on a probabilistic approach, computational simulations, and spectrometric measurements. The new method yields results in agreement with the statistical method and applies to all dichroic particles. The second nanostructure is a gold nanostar (AuNS) – polydimethylsiloxane (PDMS) composite platform. This nanostructure is characterized by using a typical AuNS of average dimensions and idealized as consisting of a sphere and radially oriented truncated cones representing its core and branches. Using branches defined parametrically by their number, length, aperture angle and orientation, and gradually attaching branches to a core, their ensemble spectra of increasing complexity are simulated. The absorptive contribution of each component is analyzed, demonstrating the large tunability of the AuNS and allowing for finding the most effective way to tune its fundamental resonant excitation. Using plasmon hybridization theory, the plasmonic interaction between structural elements is demonstrated in three different geometries

Fabrication of a Gold Nanostar - Embedded Porous Poly(dimethylsiloxan) Platform for Sensing Applications

Porous poly(dimethylsiloxan) (PDMS) membranes have been fabricated by using sodium bicarbonate powder and polystyrene microspheres to generate the pores. Large gold nanostars (AuNSs) have been synthesized by a one-pot surfactant-free method by using gold seeds, and the stars have been embedded into the porous material by immersing the samples in the nanostars’ (NSs) aqueous solution. Sensitivity tests performed with samples prepared with the two porogens demonstrated the very high sensitivity toward the surrounding environment of gold nanostars embedded into the polymer. The sensitivity is found to be in the range of 400-550 nm/RIU, compared to approximately only 100 nm/ refractive index unit (RIU) for PDMS with embedded nanospheres. Absorbance spectra of nanostars embedded in the polymer are simulated by using the Finite Difference Time Domain (FDTD) method. A good agreement is found between the calculated and experimental spectra