Terahertz Sensors Using Surface Waves in Periodic Metallic Structures

Terahertz range of frequency has found a fast growing number of applications in material characterization and sensing, imaging and extreme bandwidth communication. Different structures have been proposed for sensing at these frequencies. Surface plasmon waves have successfully been applied to ultra-high precision sensing at optical frequencies, because of their strong field confinement and enhancement. These waves are not as confined in THz due to metal properties over this range of frequencies. However, it has been shown that surface waves on properly designed periodic metallic structure have behavior very similar to plasmonic waves in optical range. These surface wave modes are called surface plasmon-like waves. Here we consider several periodic metallic structures, which support these surface plasmon-like modes, for THz sensing applications. The first one is a two dimensional array of metallic rods which is excited by prism. Many existing plasmonic sensing configurations use prism for plasmonic wave excitation. However, prism is too bulky for integration. Interests in integrated surface plasmonic devices at optical frequencies have been growing recently. As compared with free space configuration, integrated structures have distinct advantages such as small size and multi-channel sensing capabilities. An integrated sensing configuration using plasmonic-like wave is proposed. The new configuration uses a metallic grating that acts as a THz waveguide with a stop-band with a sharp transition edge. Excitation of such metallic grating waveguide through a dielectric waveguide will be described and analyzed. Moreover, it will be shown that the frequency of the transition edge between pass-band and stop-band is highly sensitive to the refractive index of the surrounding medium, and therefore it can be used for dielectric sensing. The excitation requirements of the proposed sensor and its sensitivity will be presented.1 yea

Frozen propagation of Reynolds force vector from high-fidelity data into Reynolds-averaged simulations of secondary flows

Successful propagation of information from high-fidelity sources (i.e., direct numerical simulations and large-eddy simulations) into Reynolds-averaged Navier-Stokes (RANS) equations plays an important role in the emerging field of data-driven RANS modeling. Small errors carried in high-fidelity data can propagate amplified errors into the mean flow field, and higher Reynolds numbers worsen the error propagation. In this study, we compare a series of propagation methods for two cases of Prandtl's secondary flows of the second kind: square-duct flow at a low Reynolds number and roughness-induced secondary flow at a very high Reynolds number. We show that frozen treatments result in less error propagation than the implicit treatment of Reynolds stress tensor (RST), and for cases with very high Reynolds numbers, explicit and implicit treatments are not recommended. Inspired by the obtained results, we introduce the frozen treatment to the propagation of Reynolds force vector (RFV), which leads to less error propagation. Specifically, for both cases at low and high Reynolds numbers, propagation of RFV results in one order of magnitude lower error compared to RST propagation. In the frozen treatment method, three different eddy-viscosity models are used to evaluate the effect of turbulent diffusion on error propagation. We show that, regardless of the baseline model, the frozen treatment of RFV results in less error propagation. We combined one extra correction term for turbulent kinetic energy with the frozen treatment of RFV, which makes our propagation technique capable of reproducing both velocity and turbulent kinetic energy fields similar to high-fidelity data

New Platforms for Terahertz Silicon Waveguides and Their Application in Absorption Spectroscopy

Waveguides (components which convey electromagnetic waves between points) are essential building blocks within miniaturized systems for all ranges of frequency and applications, including those in the Terahertz (THz) frequency gap. Although metallic THz waveguides have been available since a few decades ago and have been used extensively for THz applications, dielectric waveguides appear to be a more promising choice for THz systems that need to be low cost and compact. Silicon-on-Glass (SOG) technology, a relatively recent concept, has demonstrated remarkable 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 fundamental building blocks for numerous different THz systems. In this thesis research, the application of these waveguide structures for waveguide-based THz absorption spectroscopy is studied and investigated experimentally

Progressive augmentation of Reynolds stress tensor models for secondary flow prediction by computational fluid dynamics driven surrogate optimisation

Generalisability and the consistency of the a posteriori results are the most critical points of view regarding data-driven turbulence models. This study presents a progressive improvement of turbulence models using simulation-driven surrogate optimisation based on Kriging. We aim for the augmentation of secondary-flow reconstruction capability in a linear eddy-viscosity model without violating its original performance on canonical cases e.g. channel flow. Explicit algebraic Reynolds stress correction models (EARSCMs) for $k-\omega$ SST turbulence model are obtained to predict the secondary flow which the standard model fails to capture. The optimisation of the models is achieved by a multi-objective approach based on duct flow quantities, and numerical verification of the developed models is performed for various test cases. The results of testing new models on channel flow cases guarantee that new models preserve the performance of the original $k-\omega$ SST model. Regarding the generalisability of the new models, results of unseen test cases demonstrate a significant improvement in the prediction of secondary flows and streamwise velocity. These results highlight the potential of the progressive approach to enhance the performance of data-driven turbulence models for fluid flow simulation while preserving the robustness and stability of the solver.Comment: 23 pages, 20 figure

Substrate integrated Bragg waveguide: an octave-bandwidth single-mode hybrid transmission line for millimeter-wave applications

We demonstrate an air-core single-mode hollow hybrid waveguide that uses Bragg reflector structures in place of the vertical metal walls of the standard rectangular waveguide or via holes of the so-called substrate integrated waveguide. The high-order modes in the waveguide are substantially suppressed by a modal-filtering effect, making the waveguide operate in the fundamental mode over more than one octave. Numerical simulations show that the propagation loss of the proposed waveguide can be lower than that of classic hollow metallic rectangular waveguides at terahertz frequencies, benefiting from a significant reduction in Ohmic loss. To facilitate fabrication and characterization, a proof-of-concept 20 to 45 GHz waveguide is demonstrated, which verifies the properties and advantages of the proposed waveguide. A zero group-velocity dispersion point is observed at near the middle of the operating band, which is ideal for reducing signal distortion. This work offers a step towards a hybrid transmission-line medium that can be used in a variety of functional components for multilayer integration and broadband applications

Influence of seismic incident angle on response uncertainty and structural performance of tall asymmetric structure

The estimation of critical seismic response of a structure is complicated in nature due to the uncertain distribution of the internal forces under multidirectional seismic excitations. One of the important concerns inherent to this complication is the uncertainty associated with the final design direction as different seismic directions result in different seismic responses. In this regard, this research provides a detailed examination of (1) response-to-response and record-to-record variability under varying seismic orientations, (2) quantification of seismic response uncertainties and (3) the seismic performance of asymmetric structure in the context of conservative/nonconservative seismic design. With the mentioned research objectives, a realistic case study asymmetric structure under the influence of varying bidirectional seismic excitations was evaluated. It has been argued in this research that the seismic excitations applied at structure's reference axes are very unlikely to demonstrate maximum response for all response quantities simultaneously, even if it results in a peak response for a particular seismic response quantity. This research is particularly helpful for the critical assessment of directionality influence on asymmetric structures prior to making any decision during the structural design process. Substantial arguments have been presented to emphasize the inclusion of the investigation of seismic response uncertainty in practical design of critical asymmetric structures

Comparative Study on Photocatalytic Performance of TiO2 Doped with Different Amino Acids in Degradation of Antibiotics

In this study, three different reusable photocatalysts containing different amino acids as a source of non-metals, including L-Arginine, L-Proline, and L-Methionine, have been synthesized for the first time. Using a kinetic study and degradation efficiency test, these visible driven photocatalysts were investigated for their photocatalytic activity in removing antibiotics, including metronidazole (MNZ) and cephalexin (CEX). The morphology, structure and optical properties of the fabricated catalysts were characterized by X-ray Powder Diffraction (XRD), Field Emission Scanning Electron Microscopy (FESEM), Energy Dispersive Spectrometry (EDS)/mapping, Fourier-Transform Infrared Spectroscopy (FTIR), Photoluminescence Spectroscopy (PL) and UV-Vis Diffuse Reflectance Spectroscopy (DRS) analyses. Based on the results of the PL analysis, it was confirmed that doping TiO2 with amino acids containing C, N, and S inhibited the recombination of induced electrons and holes. Among the three catalysts, L-Arginine-TiO2 demonstrated the highest photocatalytic activity for antibiotic degradation, followed by L-Proline-TiO2. According to the response surface methodology (RSM), the optimum operating conditions were a concentration of 50 mg/L MNZ, pH = 4, and catalyst concentration = 1.5 g/L under 90 min of irradiation time. At this condition, 99.9% of MNZ and 81% of TOC were removed. In addition, 97.2% of CEX and 75% TOC were eliminated at the optimum conditions of 1g/L catalyst concentration, 50 mg/L CEX concentration, at neutral pH, and after 120 min irradiation. L-Arginine (1 wt.%)-TiO2 was tested for stability and reusability, and it showed that after five cycles, 10% of its performance had been lost. The role of reactive species in photocatalysis was identified and •OH had the most significant impacts on MNZ and CEX photodegradation. Antibiotic degradation efficiency was adversely affected by the presence of anions and humic acid, but this reduction was not significant for inorganic anions, as only 13% of degradation was lost