Infoscience

Thesis

Advanced Plasmonic Nanostructures for Engineering Fluorescence Applications

The appeal of plasmonics lies in the unique properties of coinage metal nanoparticles to strongly confine and enhance electromagnetic fields at the subwavelength. A simple 50 nm colloidal plasmonic particle can not only break the classical diffraction limit and produce subwavelength imaging, but it can also absorb light for killing cancer cells. A dipole gap antenna or a nanohole in a thin filmcan concentrate light below 10 nm, thus opening many perspectives, such as highly enhanced non-linear optics, extremely sensitive biosensors, Raman spectroscopy and single molecule detection. This thesis is oriented toward the design of new types of positive and negative plasmonic nanostructures can produce homogeneous plasmonic hot spots, where the electromagnetic field is dramatically enhanced. Electron beam lithography and focused ion beam milling are the twomethods used in this thesis to fabricate antennas. I have developed different original procedures to satisfy the specific fabrication requirements for different geometries. For instance, a dose adjustment technique is developed to achieve the effectively designed antenna gap and linewidth. Alignment accuracy is improved by adding one level of markers, which helps reaching the resolution limit of the system. The difficult liftoff problem caused by proximity effect is overcome by using single point exposure. Ion beam etching technique is applied to obtain perfectly symmetric three-dimensional sandwich structures. Small gaps for the negative antennas are obtained by adding a mask layer on top of soft metals. Most plasmonic nanostructures reported in the literature use gold asmetal, although silver is less absorbing and would provide even stronger enhancement factors. Unfortunately, silver deteriorates extremely rapidly. This has motivated me to investigate the deterioration mechanisms of silver. The common wisdom that oxidation and sulfidation cause the fast deterioration of silver antennas is proved questionable. On the contrary, I found that water or high relative humidity plays a crucial role for the deterioration of silver nanostructures. We have developed a dehydration process to remove the physically adsorbed water on the surface of the nanostructures; the silver antennas treated this way were found to be morphological stable with unaltered optical properties for over 14 weeks. The surface roughness of silver films treated in the same way were also significantly improved. These stable silver films and nanostructureswere used to fabricate different types of plasmonic antennas, including negative metallic nanohole antennas and sandwich cavity antenna, to study experimentally the interaction between fluorescent molecules and plasmonics nanostructures. The negative antennas exhibit a thickness-dependent fluorescence enhancement because the near-field enhancement becomes larger as the thickness of the antenna increases; the emission directionality is also improved for thick structures. Using surface chemical functionalization, molecules have also been placed directly in themiddle of the sandwich cavity, where the near-field enhancement is strongest. Unfortunately, the measured fluorescence enhancement is rather low; possibly caused by the non-uniform near-field enhancement in cavity and the quenching caused by the metal proximity. The last part of the thesis is dedicated to exploiting plasmonic hot spots with Fano-resonant plasmonic nanostructures.

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