Design And Characterization Of Plasmonic Nanostructures

In the last decade, metallic nanostructures have been of great interest because of their extraordinary optical properties caused by the collective oscillation modes of free electrons, the so-called plasmon resonances. On resonances, the plamonic nanostructures often exhibit extremely large optical cross-sections, consequently, large field enhancement and strong photothermal effects, which lead to many important applications ranging from surface-enhanced spectroscopy, bio-sensing, to integrated nanophotonic devices.Among them, ultrathin plasmonic absorbers show their potential applications in high efficient solar cell, antireflection and matrix-assisted laser induced desorption and ionization’. To achieve ultrathin, super-high efficiency, broadband absorption, many nanostructures were investigated in the past years. The most interesting nanostructure of broadband light absorbers is metal-insulator-metal nanostructure, that reported by Elbahri group and Qiu group They demonstrated that simply using a nanometer-thin random Au nanostructure as the top layer, instead of using complex designed nanostructure. The atypical broadband absorption spectra cannot be explained by the commonly used near-field coupling method. To address this problem, we did a systemic theoretically study. In addition to controlling the absorption of light, we also experimentally studied modulating the polarization of incident light by plasmonic chiral nanostructure. The thesis is mainly consist two sections that are arranged as follows:(1) We theoretically study the metal-insulator-metal (MIM) structure based ultrathin broadband optical absorber which consists of a metallic substrate, a dielectric spacer, and a nanostructured metallic top layer. From the perspective of permittivity, it is found that, treated the top metallic layer as a homogeneous one, there exists an effective permittivity, εnull, for the top nanostructured metallic layer which leads to unity-absorption (zero-reflection) of MIM structure. Importantly, this εnull exhibits abnormal dispersion. Both its real and imaginary parts increase monotonically with the wavelength. To obtain such naturally non-existing permittivity, we investigated the optical properties of two typical types of metal-dielectric nanocomposites, namely, thoroughly mingled composites using Bruggeman’s effective medium theory, and more realistic Au nanosphere-in dielectric structure using numerical permittivity retrieval techniques. We demonstrate that the εnull type dispersions, and consequently, perfect absorption can be obtained over a broad spectral range when the filling factor of metal component is close to the percolation threshold. The result not only explains the mechanism of the broad absorption just said above, but also provides theoretical guidelines for designing ultrathin broadband plasmonic absorbers for a wealthy of important applications.(2) 3D micro-helical structures were fabricated on aluminium cantilever using focused ion beam stress-inducing downward bending technology. We experimentally investigated the optical rotatory dispersion of single 3D micro-helical structure. In other words, when linear polarized light passed through a single 3D micro-helical structure with the direction along with the axis of helical structure, the rotation of major axis of transmitted light was measured. It is found that the maximum rotation is 6 degree at 800 nm. This work provides a method for measuring the optical activity of single chiral nanostructure.