Design Of Plasmonic Metal Nanostructures And Their Applications

Owing to the plasmonic resonances of metal nanostructures,their optical responses are quite different from those of bulk metal,which makes them have important applications in many scientific and engineering fields.This thesis focuses on the design of plasmonc metal nanostructures and their applications,which includes the designs of all-metal narrowband perfect absorber,dual-wavelength resonant nanoantennas and broadband optical nanoantennas.We also investigated their applications in surface enhanced Raman scattering(SERS)and fluorescence enhancement.Firstly,we proposed a novel all-metal based narrowband perfect absorber.Unlike traditional triple-layer Metal-Insulator-Metal film stacks,it has an ultranarrow bandwidth of absorption.Operated as a refractive index sensor,it has a much higher sensitivity and figure of merit(FOM)than the recently reported absorbers.Based on coupled mode theory,we investigate its localized field enhancement and link the far-field absorption with the near-field enhancement efficiently.We verify that the diluted field enhancement is proportional to the absorption,and thus perfect absorption is critical to maximum field enhancement.We also designed a novel dual-wavelength plasmonic resonant optical nanoantennas.Their local field hot spots are in the same gap region of nanostructures indicating high spatial overlap.By tuning the asymmetry of the structures,their resonance can be well controlled.Compare to the asymmetric dipole antennas,it has much narrower bandwidth and higher antenna efficiency,suitable for two-photon excited fluorescence enhancement.Based on the asymmetric nanoantennas,we proposed a dualband perfect absorber.It has strong local field enhancement at two different wavelength and can be used in SERS applications,one for the Raman excitation enhancement,and another for the Raman scattering.Finally,we proposed two types of broadband optical nanoantennas,including coupled broadband antennas and single-element broadband antenna.For research on coupled broadband antennas,we numerically investigate their plasmonic modes,and their applications in SERS and fluorescence enhancement.We also study its heat effect and demonstrate low heat generation when operating off resonance.We explained its broadband near field response with coupled harmonic models.For research on single-element broadband antenna,we investigate it numerically using finite-difference time-domain methods,and explain its performance using the analysis of charge distribution in addition to a multipole expansion.We further demonstrate that the proposed nanoantenna can provide broadband spontaneous emission rates and quantum efficiency enhancements when a low-quantum efficiency emitter is introduced.