|Abstract: ||Terahertz (THz) radiation (≈ 0.1−10×10^12 Hz) is non-ionizing and its photon energies correspond to the rotational and vibrational modes of many complex molecules. Hence many substances of interest for biological and security applications can be detected using THz light; making THz spectroscopy an ideal tool for bio- and security sensing. However, a size mismatch between the photon wavelength and the size of many commonly sensed targets, and a lack of powerful sources hamper the progress of THz technology towards more widespread real-world applications.
The focus of this thesis is to use novel concepts in the field of metamaterials to overcome or side step some of these challenges. In particular, the use of confined electromagnetic surface modes, such as lattice resonances and spoof plasmons, on metamaterial surfaces to conduct THz sensing is investigated. Different ways in which sensing information can be extracted from these specially structured metamaterial surfaces are explored so as to demonstrate the feasibility and versatility of metamaterial surfaces in THz sensing applications.
The application of lattice resonances to detect refractive index changes caused by various fluids on an array of metallic rods is first reported in this thesis. This can be seen as a prelude to the work presented in later chapters where strongly confined spoof plasmons are employed for THz sensing. A metamaterial surface supporting spoof plasmons (simply termed as a Spoof Plasmon Surface (SPS)), consisting of a linear array of metallised sub-wavelength grooves, is filled with various fluids and shown to be capable of high performance refractive index sensing in an Otto prism setup. Sharp phase changes, readily available from THz time-domain spectroscopy (THz-TDS), associated with the coupling of THz radiation to spoof plasmons are used as the readout response in this case to indicate changes in the refractive indices of the fluids filling the grooves. Building upon the initial work on spoof plasmon sensing, further investigations demonstrate the feasibility of SPSs as a versatile platform on which various forms of sensing information can be extracted by using THz radiation that is coupled in and out of spoof plasmons via the scattering edge coupling scheme. The time-domain signal from the SPS is analysed using a short- time Fourier transform (STFT), enabling the extraction of the broadband spoof plasmon dispersion with a single measurement as well as the attenuation coefficient with a minimum of two measurements. Broadband sensing is demonstrated, again by filling the grooves with various fluids, which results in changes in the spoof plasmon dispersion and attenuation coefficients. In addition, the observation of the absorption peak of α-lactose monohydrate at 1.37 THz due to the enhanced light- matter interactions on an SPS is demonstrated and opens the door towards a more spectroscopic approach to THz sensing using SPSs.|