Houston, TX 77005
10:30 a.m. Friday, April 12, 2013
On Campus | Alumni
Several Novel optical devices, which were designed to manipulate terahertz waves for broadband near-field imaging, wave-guiding (invisible cloaking), and sensing (resonator), are presented in this thesis. The prototype of each device that is derived from the original working concepts of physics was demonstrated experimentally in our lab. The working concepts of physics were investigated in experiment, in simulation and in theoretical analysis. A tapered parallel-plate waveguide (PPWG) was exploited as a novel probe for broadband near-field imaging. This probe consists of two metal plates with the plate spacing gradually tapered from one end to the other. The tapering of the plate separation enables this probe to effectively propagate the broadband THz waves (with low-loss, no cut-off and nearly no dispersion) from the input end of large spacing into the narrow end of sub-wavelength spacing. Working in reflection mode, the tapered PPWG probe is able to differentiate the dielectric features as well as topographic information on the imaging sample. Combined with the methodology of filtered back projection, a two-dimensional image of a gold pattern on a GaAs chip was reconstructed by using this tapered PPWG probe. The smallest feature of ~100 µm is resolved by using the waves with average wavelength of 1.5 mm. The phenomenon of surface plasmon-polariton in THz range were studied on the platform of a parallel-plate waveguide (PPWG). In this thesis, we show the characterization of the waveguide mode of a finite-width parallel plate waveguide conducted by using an improved scattering-probe technique. An abrupt waveguide mode transition, from a traditional TEM mode to a plasmonic mode, was observed at a very narrow frequency range. This transition frequency was demonstrated as being determined by the material properties of the waveguide, the frequencies of the electromagnetic waves as well as the geometry of the waveguide. The capability of using the spoof surface plasmon to enhance the reflectivity of an interface between free space and a PPWG is exploited in this thesis. The reflection coefficient of this interface can be enhanced up to ~100 % at a designed frequency, by cutting a designed pattern of periodic rectangular groove on the output facet of the PPWG. When this enhanced reflection happens (almost a total reflection), a lateral shift and a phase shift of the reflected beam can be observed, which is a strong reminiscent of Goos-Hanchen shift. The experimental, simulation and theoretical characterizations of the lateral and phase shift are shown in the thesis. As an application, a prototype of a band-pass THz resonator is demonstrated in this thesis. The concept of a two-dimensional inhomogeneous artificial dielectric is exploited in this thesis. Basically this artificial dielectric is the space between the two metal plates of a PPWG working in TE1 mode. By using a ray-tracing and a full-wave simulation, a THz mirage device (or an invisible cloaking device) was designed, which contributed to the first experimental demonstration of such device. A metal coin of size several times larger than the working wavelength can be hidden in the device without casting any shadow. This work is in cooperation with Dr. Rajind Mendis and the author of this thesis contributed to the design and characterization of the device in simulations.