Investigation of the unique tunable optical properties of noble metal nanocrescents

This dissertation describes the investigation of the unique optical properties of gold and silver nanocrescents fabricated on glass and silicon substrates by Nanosphere Templated Lithography (NTL). The NTL fabrication process enables production of ensembles of nanocrescents with good control of size, shape and orientation. Variation in template diameter and metal film deposition angle upon fabrication is used to control nanocrescent geometry. A broad geometrical tunability of nanocrescents leads to broad tunability of their optical properties as shown experimentally. The nanocrescents exhibit extinction spectra with localized surface plasmon resonances in a broad range from 600 to 3600 nm. Two of those plasmon resonances in the NIR and IR ranges are relatively narrow and quite sensitive to the dielectric environment demonstrating high sensitivity factors up to 880 nm/RIU and high ensemble sensing figure of merit up to 2.4. A new parameter, the relative sensitivity to dielectric environment, is introduced. The extinction efficiency of the strongest longitudinal plasmon resonance can be tuned up to 40, the highest experimental value reported. Tuning of the nanocrescent plasmon resonance wavelength in resonance with the analyte's probed vibration in Surface Enhanced Infrared Absorption (SEIRA) Spectroscopy produced the highest reported nanoparticle area normalized enhancement factor of 46000. The electromagnetic field decay near the nanocrescent surface is investigated in close and long range. A correlation between the sensitivity to refractive index changes near the nanocrescent surface and its aspect ratio is demonstrated. A local electric field enhancement as high as 400 is predicted near sharp nanocrescent tips by simulation. Electromagnetic field decay lengths as long as two hundred nanometers are estimated, but require additional experimental and theoretical verification. Two ways to expand the plasmon tunability range into the infrared (up to 5750 nm) by fabrication of gap controlled nanocrescents and fabrication of nanocrescents on high refractive index substrate are described. The preliminary results for the study of nanocrescent optical anisotropy and birefringence (e.g., phase shift about 45°) are reported, but require additional experimental verification. Nanocrescents' unique optical properties make them a strong candidate for plasmonic applications including SEIRA and LSPR spectroscopies, waveguiding and others.