Terahertz Plasmonic Waveguides

Abstrakt: The talk would present work on a new class of terahertz (THz) waveguides based on structured metal geometries. The waveguides are designed with the core idea that adoption of planar layout in fabrication can lead to exponential growth in device capabilities, analogous to the growth in device capabilities based in electronics. From a functional point of view, the waveguides rely upon propagation of surface waves along the surface of metals. This approach is preferred, since dielectrics tend to be lossy at THz and the loss parameters scales almost quadratically with frequency for most dielectrics. The loss in propagating wave is minimized by utilizing metals, which are highly conducting at THz frequencies. Structuring the metal surface with periodic array of apertures of sub-⁠wavelength dimension allows bound surface wave to propagate as the wave can evanescently decay into the metal. This phenomenon is referred as the coupling of propagating wave to surface plasmon polariton (SPP) like mode at the interface of structured metal surface and air. Thus, these propagating THz waves are simply surface plasmon-⁠polaritons (SPPs). Similarly, complimentary structures that do not perforate the metal, but rather stand on the metal surface also support SPPs. The wavelength of SPPs can be controlled by changing the dimension of these apertures/⁠structures, since the dispersion relationship of the medium depends on the geometrical size. This engineering capability has been exploited in creating all the waveguides presented in this thesis. The devices presented are categorized based on the fabrication technique. Each technique is unique in its own regard and can be selected based on functional needs. A commonly adopted process of laser ablation covers a wide set of waveguides presented here. In one of the waveguides fabricated using ablation technique, the role of disorder is discussed. The waveguide with introduction leads to observance of localized mode with spectral and spatial feature like Anderson localized modes of photons. It is the first report of localized mode at THz frequency. 3D rapid prototyping involving 3D printer is used to create waveguides with complex layout that can allow for multi-⁠plane signal routing. This also is the first demonstration of 3D printing in the development of THz devices. In another approach a unique fabrication technique had to be developed to create waveguides mediums with negative index of refraction (NIM) as they require feature sizes which cannot easily be attained using conventional clean room techniques and 3D printing. This new fabrication approach uses a sacrificial layer technique that is used create an effective medium with negative index of refraction and length on the order of tens of wavelength.