Date of Award

2016

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Electrical and Computer Engineering

Committee Chair

Robert Lindquist

Committee Member

Laurie L. Joiner

Committee Member

Patrick J. Reardon

Committee Member

Domenico de Ceglia

Committee Member

Mark Tillman

Committee Member

Junpeng Guo

Subject(s)

Photonics., Optoelectronics., Liquid crystal devices.

Abstract

The terahertz field is rapidly expanding due to technological progress over the last few years. While source and detectors are available in this frequency range, there is still no firmly established technology for tunable filters and modulators. In this work a number of transverse stratified configurations of metal and dielectric layers are studied for modulating Terahertz radiation in amplitude and phase. The first configuration analyzed is a set of metal plates placed on top of each other and separated by a thin layer of Liquid Crystal (LC). The period of the grating is such that the only propagating order is the order zero. The thickness of the LC is subwavelength and the propagation within the grating is TEM. The natural response of the structure is FP-like with periodic resonances that can be tuned by means of an applied voltage as a result of the electro-optic effect in the LC anisotropic medium. Pass-band flat-top response or high wide-band transmission is achieved using a cascade configuration or Brewster incidence respectively. A Brewster incidence is used to show the ability of the structure to steer an incoming THz beam. The single resonance FP response is enhanced by making cuts or placing metallic obstacles within the LC layer. In this way the periodicity of the resonances is altered due to a band-gap effect. As a result, the metallic structure reaches a flat band response at certain frequencies and a steep slope with high attenuation at other frequencies. A second transverse configuration is then studied. The structure is a set of dielectric slabs, in between two very thin layers of metal, placed on top of each other and separated by a layer of LC. The structure proves to be highly resonant in correspondence of certain frequencies at which it completely reflects the incoming radiation. At these frequencies, the transmission shows tunable periodic notches. The phenomenon is modeled using Structured Surface Plasmon and Bloch wave theory. The structure is modified with cuts in the thin metal layers to reduce their continuity. As a result, the response shows a wide flat band suitable for phase modulation. A filter design algorithm is defined to use these structures as building blocks of a complex THz filter. For each structure, an equivalent circuit model is generated and the transmission and reflection coefficients are calculated. Then the transmission coefficient is reformulated into a Bode form by linearizing the non-linear terms. A number of poles and zeros are identified to be used in the realization of custom designed filters. A filter design algorithm is developed in order to enable a direct implementation of classic analog filters, such as those of Butterworth and Chebyshev, by means of transverse stratified structures.

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