This paper presents and evaluates a new terahertz metamaterial absorber (MMA) for sensing applications. Because of its unique properties, metamaterial-based sensors are widely employed in a variety of applications. The reported structure comprises of two identical metallic patches, a dielectric spacer and a ground metal plane. The finite element approach has been utilized to simulate and analyse the design. It is found that the MMA offered a prominent resonant peak with near 100% absorbance at frequency 4.5 THz due to the resultant effect of coupling between the two identical patches. In addition, surface current distribution, absorption mechanism and structural parametric analysis has also been investigated. The peak is designated as 'A', with a line width of 0.02 THz and a quality factor (Q-factor) of 225, which is sensitive to the refractive index of the environment (RI). As a result of its highly sensitive sensing capabilities, the proposed design can be employed as a sensor for refractive index, having 1.6 THz per refractive index unit (RIU) sensitivity and figure of merit (FoM) of 80 in terms of change in RI of the environment. The majority of biomedical samples have RI of 1.3 to 1.36, which is worth highlighting. Thus, biomedical applications may be possible with the suggested sensor.

Electromagnetic waves in the terahertz spectral range have gained significant research focus due to their applications in various fields of science. To effectively generate and integrate terahertz waves in systems, appropriate waveguide design is critical. Conventionally waveguides have been used to control the propagation of electromagnetic waves. A waveguide with low loss and dispersion is always preferred. But achieving these characteristics is quite challenging especially if operating in the terahertz spectral range. There are inherent material and geometric limitations that exist for terahertz waveguides. It is therefore important to optimize the design to enable their use in applications efficiently. This thesis investigates the characteristics of three primary terahertz waveguides based on the underlying theory and results obtained from simulations. The three waveguides are parallel-plate waveguides, two-wire waveguides, and coplanar striplines. The work in this thesis mostly focuses on coplanar striplines, optimal for building a highly efficient commercial and portable terahertz system-on-chip (TSOC). The contribution of the thesis is around the use of different types of passive components mounted on a thin commercial Silicon Nitride membrane. A bias tee is introduced which is a combination of interdigitated electrodes and a meander inductor. The length of the interdigitated electrodes and the gap between them are 55 um and 5 um, respectively. The S21 parameter for this structure ranges from -24 dB/mm at near-zero frequencies to -0.8 dB/mm at 1 THz. This indicates that the designed bias tee can appropriately block low frequencies. Split-ring resonators are also used to act as band-stop filters. The resonant frequency of the resonator depends on the radii of the split-rings. In the optimized design, the internal radius of the outer ring is 25 um and the external radius of the inner ring is 20 um. This results in a narrowband band-stop filter with its resonant frequency centered at 701 GHz. The optimized final TSOC design discussed in this work uses these passive components placed on the Silicon Nitride membrane and is shown to have a total loss that is 3 dB/mm less than any of the previous work for terahertz frequencies. / Graduate

THz light has important applications in medical imaging, chemical sensing, industrial quality control, and future wireless communications. However, the widespread adoption of these applications is currently limited by the lack of practical sources of THz radiation that can operate at or near room temperature. Graphene is a promising materials system for basic studies and device applications in THz optoelectronics, with several key functionalities, including photodetection and optical modulation, already demonstrated in recent years. This thesis work is focused on the use of graphene for the THz light emission. In particular, I have demonstrated for the first time the generation of gate-tunable THz radiation from graphene nanoribbons under current injection. The underlying radiation mechanism involves the excitation of graphene plasmonic oscillations by the injected hot carriers and their subsequent radiative decay at the nanoribbon resonance frequency. Combined with suitably designed optical elements, this approach is promising for the development of compact THz sources for imaging and sensing applications. In addition, I have also investigated alternative radiation mechanisms that can provide higher efficiencies but require more complex ultra-high-mobility graphene samples. These mechanisms include Smith-Purcell emission by the graphene electron gas in the vicinity of a periodic grating and interminiband transitions in graphene superlattices produced with a periodic external potential. Finally, I have designed and investigated numerically a graphene-nanoribbon metasurface platform that can provide arbitrary wavefront shaping functionalities for incident THz light, such as beam steering and focusing. Importantly, this device can be actively reconfigured by varying the nanoribbon gate voltages, which makes it particularly attractive for applications in wireless communications beyond 5G.

The purpose of this thesis is to examine the radiation from high-temperature superconducting mesas of Bi2Sr2CaCu2O8+ (BSCCO). This is motivated by the need for coherent sources of continuous wave terahertz (THz) emission capable of radiating practically in the THz frequency band. As BSCCO has been shown to be tunable from 0.5–2.4 THz (i.e., through the entire socalled terahertz gap centered about 1 THz), and has a higher peak operating temperature near 1 THz than most alternative sources, it is a good candidate for imaging and spectroscopy device applications [1]. When a static DC voltage is applied to a BSCCO mesa, the stack of Josephson junctions intrinsic to this type-II layered superconductor synchronously radiate. Adjustment of the bath temperature and applied voltage allows for the high degree of tunability observed for such an emitter [2]. To determine the angular dependence of radiation from BSCCO mesas, the dual source model from antenna theory is employed, and Love’s equivalence principle is used to simplify this framework. The total emission power obtained in this manner for the pie-shaped wedge is then fit to experimental results for a thin isosceles triangular mesa using the method of least squares, resulting in a standard deviation of = 0:4657. Additionally, symmetry is shown to play a significant role in the emissions for the transverse magnetic (TM) cavity modes of the equilateral triangular mesa. When the full group symmetry is imposed, the density of allowed modes is heavily diminished, and the original first excited even mode becomes the C3v symmetric ground state. These results for the equilateral triangle suggest, along with earlier experiments on the regular pentagonal mesa [3], that symmetry breaking effects can be used for purposes of tuning the characteristic frequency and angular dependence of the power radiated from BSCCO mesas with a high degree of symmetry.

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Yes / This paper proposes a new solution to improve the performance parameters of on-chip antenna designs on standard CMOS silicon (Si.) technology. The proposed method is based on applying the metasurface technique and exciting the radiating elements through coupled feed mechanism. The on-chip antenna is constructed from three layers comprising Si.-GND-Si. layers, so that the ground (GND) plane is sandwiched between two Si. layers. The silicon and ground-plane layers have thicknesses of 20μm and 5μm, respectively. The 3×3 array consisting of the asterisk-shaped radiating elements has implemented on the top silicon layer by applying the metasurface approach. Three slot lines in the ground-plane are modelled and located directly under the radiating elements. The radiating elements are excited through the slot-lines using an open-circuited microstrip-line constructed on the bottom silicon layer. The proposed method to excite the structure is based on the coupled feeding mechanism. In addition, by the proposed feeding method the on-chip antenna configuration suppresses the substrate losses and surface-waves. The antenna exhibits a large impedance bandwidth of 60GHz from 0.5THz to 0.56THz with an average radiation gain and efficiency of 4.58dBi and 25.37%, respectively. The proposed structure has compact dimensions of 200×200×45μm3. The results shows that, the proposed technique is therefore suitable for on-chip antennas for applications in system-on-chip for terahertz (THz) integrated circuits. / Innovation programme under grant agreement H2020-MSCA-ITN-2016 SECRET-722424; UK Engineering and Physical Sciences Research Council (EPSRC) under grant EP/E0/22936/1.

Terahertz (THz) spectroscopy is a powerful tool that uses ultrashort pulses of light to study the properties of materials on picosecond time scales. THz light can be generated through a variety of methods. In our lab, we generate THz through the process of optical rectification in nonlinear optical (NLO) organic crystals. THz light can be used to study several phenomena in materials, such as spin precession, electron acceleration, vibrational and rotational motion. The work presented in this dissertation is divided into two parts: (1) the generation of THz light and (2) applications of THz light. The first portion of this work shows how THz light is generated, with an emphasis on the generation through optical rectification. We also show how to improve the generation of THz light by creating heterogenous multi-layer structures with yellow organic THz generation crystals. Additionally, we show that crystals used for THz generation can also be used to generate second-harmonic light. In the second half of this work, we show that THz light can be used to study the vibrational motion of molecular systems. We model how resonant vibrational modes in a fluorobenzene molecule can be excited with a multi-THz pump to transfer energy anharmonically to non-resonant modes. We also show that we can use two-dimensional (2D) THz spectroscopy to excite infrared-active vibrational modes and probe Raman-active modes in a CdWO4 crystal to obtain a nonlinear response. We show that the nonlinear response is due to anharmonic coupling between vibrational modes and we can quantify the relative strengths of these anharmonic couplings, which previously was only accessible through first-principles calculations.

This project was born out of the work performed by a group of researchers at the New Jersey Institute of Technology (NJIT) [1] [2] [3] working on interferometric imaging with a spiral array. Their investigation stopped at two dimensional imaging with a two dimensional array. In this thesis, their idea was developed further into the significantly more complex imaging with a three dimensional array. The general design of the NJIT [1] [2] [3] experiment was reproduced, studied, and modified in a manner that was theorized to enhance the experiment with the added ability to perform three dimensional imaging. The NJIT team [1] [2] [3] has developed their experiment to where they were able to accurately perform two dimensional imaging of two sources of equal intensity located at different distances from a spiral array. In this thesis, the equations used for two dimensional imaging are extrapolated into a three dimensional array application. This three dimensional imaging concept is simulated with MATLAB and the results presented and compared to the NJIT experimental results. [1] [2] [3] A proof of concept physical experiment is conducted and the results are compared to the MATLAB simulation. The results show that additional spatial information can be obtained from a three dimensional array that can enhance the information gleaned from images. / Master of Science