Theoretical Studies On THz Metallic Grating Filter And The Effective Surface Plasmon-polariton On The Structured Metal Wires

Terahertz (THz) radiation can be used in a variety of ways because a number of natural phenomena are associated with the radiation frequency in this spectrum region. With the development of THz Optics, intensive researches have focused on the available THz Optical devices such as filters and waveguides.In this thesis, I theoretically analyzed the electromagnetic resonances of sub-wavelength rectangular metallic gratings, calculations were based on the modal expansion method. By employing the cavity resonance of the reflecting rectangular metallic gratings, we find that this metallic structure can be designed as a bandwidth tunable THz optical band-rejection filter. Moreover, the rejected THz photons can be highly localized in the grooves of the metallic grating and the extremely high electric field could be obtained with the excitation of cavity resonance, this property could be used to analyze nonlinear effect between THz radiation and molecules.The possibility of guiding electromagnetic waves at THz region with a lateral confinement below the diffraction limit is analyzed using an analytical model. We used a simple metal wire milled with arrays of sub-wavelength grooves to support propagation of effective surface plasmon polaritons and the location of equivalent surface plasmon frequencies can be modified at will. If the equivalent surface plasmon frequency was tuned to approach the THz region, the metal wire can readily have the ability to laterally confine the THz photons in a sub-wavelength region. Moreover, we note that for TM polarization the structured metal wires can be equivalent to a Guabo wire (a dielectric coated metal wire) with defined geometrical parameters and effective refractive index of the dielectric coating. We proposed some applications for this metallic structure such as optic delay-lines, couplers and filters which can be integrated in a simple metal wire waveguide. Interestingly, by tapering the structured metal wire, the THz photons can be focused in a sub-wavelength spot on the tip of the conical structure. We expect, in future, this metallic structure could have important applications in the THz optical research.