Metallic Nanostructures for Opto-plasmonic and Electro-plasmonic Applications
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Surface Plasmon Polaritons (SPP) describe the coupling between the motion of free electrons in metals and electromagnetic waves in a dielectric. They can propagate at a metal-dielectric interface as a surface mode. This phenomenon enables concentration and manipulation of the electromagnetic field on the sub-wavelength scale and leads to an unprecedented enhancement of linear and nonlinear optical processes. It has application in areas like optical detectors, energy harvesting, and optical communication. However, the fabrication techniques used in research, such as Focused Ion Beam Lithography or Electron Beam Lithography restrict its widespread implementation and scalability. This is mostly associated with the high cost and the small area production of these methods. Additionally, the intrinsic losses induced by the inelastic scattering of free carriers in the metal limits the commercial success and development of these devices. This thesis aims at finding solutions to these problems. In Chapter 1, a method for a large area and low-cost fabrication based on optical interference lithography was developed. A coherent light source is split into two interfering beams, and the resulting intensity pattern is recorded by a photosensitive medium, which acts as a mask for a 1D and 2D plasmonic metallic structure. In Chapter 2, the resonance of the resulting metal nano-grating can be tuned by varying process parameters such as incident angle, incident power, and etching conditions.In Chapter 3, combining this plasmonic nano-grating with atomic layer deposition leads to an enhanced localized field in so-called nano-gaps, which is utilized as a molecule sensing platform.The mechanism and fabrication of it are laid out. Meanwhile, the fabrication of narrow and broadband metamaterial absorber based on optical lithography is demonstrated. However, all plasmonic nanostructures are affected by resistive losses in the metal which limits their use in real-world applications. Unlike previous strategies for loss mitigation such as reduction of surface roughness or using doped semiconductor in place of metal, it is shown here that it is possible to take advantage of these intrinsic losses in Chapter 4. The plasmonic activity can be studied by characterizing an induced resistance change electronically. Scattering processes induced by SPPs change the conductivity of the metal and can be determined by the change of an applied DC bias current. Additionally, since surface plasmons are wavelength and polarization dependent, the plasmonically induced resistance changes can be utilized as a new type of detector. It can fully determine the polarization state and is spectrally tunable from the visible to the IR by changing the geometry of the metal grating. This opens a pathway to exciting new applications for a CMOS compatible ultra-wideband plasmonic detector.