Candidate: Hadi Amarloo
Title: Terahertz Dielectric Waveguides and its Applications for THz Absorption Spectroscopy
Date: February 7, 2018
Time: 3:00 PM
Place: E5 5106 -28
Supervisor(s): Safavi-Naeini, Safieddin
Waveguides―components which convey electromagnetic waves between points―are essential building blocks within miniaturized systems for all ranges of frequency, including the Terahertz (THz) frequency gap. Although metallic THz waveguides have been available since several decades ago and have been used extensively for THz applications, dielectric waveguides could be a better choice for THz systems that need to be low cost and compact. Silicon-on-Glass (SOG) technology, a relatively recent concept, has demonstrated promising performance for frequencies up to 1 THz. In this thesis, two new THz dielectric waveguide structures are proposed and investigated theoretically and experimentally: 1) a THz line-defect photonic crystal waveguide based on SOG technology, and 2) a structure which uses benzocyclobutene (BCB) to create a Silicon-BCB-Quartz (SBQ) platform. Such THz dielectric waveguide structures could form the platforms for numerous different THz systems. For this thesis work, the application of these waveguide structures for waveguide-based THz absorption spectroscopy is studied and investigated experimentally.
The proposed THz line-defect photonic crystal waveguide is a step forward compared with available SOG technology. Pyrex is the waveguide substrate within this technology. Due to its high material loss when operating in the THz frequency range, pyrex is etched underneath the guiding channel to minimize waveguide loss. However, handling the guiding channel over the etched pyrex is quite challenging. The first advantage of the proposed THz line-defect photonic crystal waveguide is that the photonic crystal structure provides a mechanically stable platform for handling the guiding channel over the etched pyrex. Secondly, the photonic crystal structure provides a suspended platform to integrate other components, such as resonators or couplers, with the waveguide over the etched pyrex substrate. Moreover, the slow wave characteristics of this waveguide could be advantageous for sensing applications at THz frequencies.
The SBQ structure is proposed as an easy-to-fabricate and efficient platform for THz compact systems. Due to the low loss characteristics of quartz at THz frequencies, etching the substrate is not required for the SBQ platform. Therefore, the quartz substrate provides a solid substrate not only for the silicon guiding channel, but also potentially for other components besides the waveguide. The fabrication and experimental characterization of the proposed waveguide is presented. Based on measurement results, this waveguide structure achieved an attenuation constant as low as 0.026 dB/mm over 500 to 580 GHz, the lowest reported value for THz waveguide losses in the literature so far.
Two THz slot waveguides with highly confined fields are proposed: SBQ THz slot waveguide and THz plasmonic slot waveguide based on doped-GaAs. In sensing applications, these waveguide structures could enhance the interaction between the waveguide mode and the sample. The proposed SBQ THz slot waveguide is designed, fabricated, and investigated experimentally. The THz plasmonic waveguide is designed and characterized theoretically.
Despite the fact that free space setups are widely used for spectroscopy of different materials at THz frequencies, waveguide-based spectroscopy could provide a miniaturized setup for material characterization and sensing at this frequency band. To date, various metallic waveguides have been used for this purpose at the THz range. However, this thesis proposes the use of THz dielectric waveguides for absorption spectroscopy applications. The compatibility of the dielectric waveguide structures with available solid-state THz sources, in addition to the advanced fabrication facilities available for silicon based devices, open a pathway toward highly-miniaturized and low-cost THz spectroscopy systems. The performance of the proposed SBQ platform for waveguide-based THz absorption spectroscopy is studied experimentally using an exemplar test sample (lactose powder).