Plasmonic Infrared Absorber And Surface Enhanced Infrared Absorption Spectroscopy

Recently, plasmonic nanostructures have attracted wide attention, because of their capability of controlling fundamental optical processes such as absorption, emission and refraction at the nanoscale. Surface plasmon polaritons (SPP) are surface modes at metal-dielectric interfaces caused by the coupling of free electrons and electromagnetic fields. With the help of plasmonic nanostructures, light can be guided, confined and enhanced at the nanoscale. This unique property leads to numerous applications, such as sensing, surface-enhanced spectroscopy, and nano-photonic devices.In recent years, considerable researches have been focused on the design and fabrication of ultrathin plasmonic optical absorbers. Plasmonic absorbers, especially in near and mid-infrared, are of great interests due to their board applications for biochemical sensing, thermal imaging, medical diagnostics, etc. In the past years, plasmonic thin IR absorbers have been achieved using various designs, such as metallic gratings, nanoparticles, and metal-insulator-metal (MIM) structures. Many of them have a single plasmonic resonant wavelength and many have a low absorption (below 50%). Here we design and fabricated a plasmonic infrared absorber, which has a tunable resonance, and experimentally investigated its optical properties, including absorption efficiency and surface enhanced infrared absorption spectroscopy of molecules absorbed on it. The thesis has four parts which are arranged as follows:(1) We designed an ultrathin infrared absorber, which consists of plamonic nanocavity array, for achieving perfect absorption. We simulated the absorption spectra of the structures using finite-difference time-domain (FDTD), and found that the resonant wavelength redshifts when the length of nanocavity increases. In order to tune the resonance wavelength, fifteen plamonic nanocavity array with increasing lengths were used in this work.(2) We successfully fabricated the plasmonic absorbers using electron beam lithography (EBL). In the process, we systematically optimized the parameters of the EBL, including exposure, development, evaporation, lift-off, etc. Particularly, the proximity effects were investigated in order to improve the resolution of the EBL.(3) We experimentally measured infrared absorption spectra of the plasmonic absorbers by using fourier transform infrared spectroscopy (FTIR). The results show that the resonance absorption wavelength is linearly dependent on the cavity length, consistent with our simulation results.(4) Because the plasmonic infrared absorbers exhibit a very high near-field enhancement, it can be used to study the surface enhanced infrared absorption effect of organic molecules. In this work, we investigated the surface-enhanced infrared spectra of allyl mercaptan molecules. However, no absorption peak from the molecule was observed, probably due to the extremely small number of the molecules adsorbed on the surface.