Novel Devices for Terahertz Wave Imaging, Wave-guiding and Sensing
Mittleman, Daniel M.
Doctor of Philosophy
Several novel optical devices, which were designed to manipulate terahertz waves for broadband near-field imaging, wave-guiding (invisible space), and sensing (resonator), are presented in this thesis. We developed the original working concepts of each device, and demonstrated the prototype experimentally in our lab. The working concepts of physics were investigated in experiment, in simulation and in theoretical analysis. We exploited a tapered parallel-plate waveguide (PPWG) as a novel probe for broadband near-field imaging. This imaging probe consists of two metal plates with the plate spacing gradually tapered from one end to the other. We proved that the space tapering enables this probe to propagate the broadband THz waves efficiently (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 a reflection mode, this imaging probe is proved to be able to differentiate the dielectric features as well as topographic information on the sample. Combined with the methodology of filtered back projection, we reconstructed a two-dimensional image of a gold pattern on a GaAs chip 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. We studied the phenomenon of surface plasmon-polariton in THz range 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 by using an improved scattering-probe technique. An abrupt waveguide mode transition was observed at a very narrow frequency range. We demonstrated that this transition frequency is determined by the material properties of the waveguide, the frequencies of the electromagnetic waves as well as the geometry of the waveguide. This result provides a good guidance for the waveguide design for THz transmission. We also exploited the capability of using the spoof surface plasmon to enhance the reflectivity of an interface between free space and a PPWG. We demonstrated that 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. A lateral shift and a phase shift of the reflected beam is observed in the experiment, which is a strong reminiscent of Goos-Hanchen shift. We carried out the experimental, simulation and theoretical characterizations of the lateral and phase shift. As an application, we designed and demonstrated a prototype of a band-pass THz resonator. We introduced the concept of a waveguide-based two-dimensional inhomogeneous artificial dielectric into THz range. This artificial dielectric is the space between the two metal plates of a PPWG working in TE1 mode. We designed a THz mirage device (or an invisible space device) by using ray-tracing and full-wave simulations, which contributed to the first experimental demonstration of such a 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 collaboration with Dr. Rajind Mendis and the author of this thesis contributed to the design and characterization of the device in simulations.