In our thesis work, the primary focus has been to investigate terahertz plasmonic and metamaterial structures for applications in sensors and modulators. Plasmonic waveguides evoke both fundamental and higher-order modes that depend on the shape and size of their structures, which can be important for analyte detection. Due to the longer interaction of the terahertz modes with the groove pattern of the waveguide, it can detect an analyte with higher sensitivity. Therefore, the sensing capabilities of plas- monic waveguides are investigated, taking pyramidal groove patterns into account. In addition, the propagation of terahertz surface plasmons was studied by pattering the metals with asymmetrical rectangular apertures in close proximity. Two resonators address this aspect of near-field coupling with slightly different sizes. Next, electro- magnetically induced transparency phenomena in waveguide structures were investi- gated, where the transparency effect can be controlled as a function of the shape and size of its components. In the case of terahertz metamaterials, a tunable metamate- rial absorber based on an array of graphene nanoribbons is being investigated. The study could make it feasible to design the metamaterial absorber for acquiring better absorption characteristics. The whole thesis has been divided into seven chapters. The organization of this thesis chapters are as follows:
Chapter 1 gives a general introduction of the THz waves, sources, detectors, and their applications in the different areas. We discussed THz plasmonics, the generation of surface plasmons at THz frequencies, and the applications of the THz plasmonic waveguide. In this chapter, we illustrated the numerical design, fabrication, and char- acterization of THz plasmon and metamaterial structures. This chapter also describes
the vital idea of this thesis work. Moreover, it additionally summarizes the thesis con- tribution and presents a quick outline of the thesis organization.
Chapter 2 investigates the sensing capabilities of a planar plasmonic waveguide comprising of periodically arranged pyramidal corrugations. This designed THz plas- monic waveguide supports highly confined terahertz modes and offers a much longer interaction between the analyte and the terahertz radiation. The dispersion properties of the fundamental and higher-order modes of the waveguide were examined to en- sure the plasmonic properties of the waveguide. The waveguide transmission spectra for different lengths of the corrugations have been investigated. We validate the nu- merical results by employing a semi-analytical transmission line model to our waveg- uide geometry. We thoroughly analyze the various waveguide sensing parameters such as quality factor, the figure of merit, frequency shift, and sensitivity.
Chapter 3deliberates the near field coupling of a surface plasma wave in a planar THz plasmonic waveguide comprising a 1D-array of periodically arranged asymmetri- cal rectangular apertures positioned along the transverse direction. The structures are assumed to be periodically patterned rectangular apertures in a thin sheet of metal and ensure the plasmonic response of the waveguide. The unit cell of the waveguide geom- etry comprises two subwavelength scale rectangular apertures with different lengths adjacent to each other along the transverse direction. We have investigated the near field coupling of surface plasmons from the asymmetric size resonances via the exci- tation of each aperture simultaneously. The findings are supported by a theoretical model based on the three-level plasmonic system.
Chapter 4 discusses the plasmon induced transparency (PIT) effect and its modu- lation in the terahertz guided systems. The waveguide geometry is composed of two slots in parallel slabs in the transverse direction. A particular gap is maintained be- tween the slabs for terahertz transverse magnetic mode propagation. One rectangular slot is filled with a dielectric material, and the other one is empty. We examined the corresponding transmission amplitude and the change of phase value of the transmit-
ted terahertz signal. Further, we have calculated group index values corresponding to the phase values. Also, a theoretical model based on coupled harmonic oscillator has been devised to explain the PIT effect of the waveguide to validate the numerical findings. The refractive index of the dielectric material is varied to observe the tunable control of the PIT effect. A comprehensive picture of modulation of the effect with the refractive index is presented through the contour plot.
Chapter 5explores the tunability and switching of the induced transparency win- dow in an air dielectric grooved parallel plate terahertz waveguide. The structure consists of two parallel pyramidal grooved patterned metal blocks that maintain a par- ticular gap. One of the two pyramidal grooves is filled with a dielectric material of varying refractive index to study the modulation of the PIT window; the other one re- mains empty. We examine the terahertz transmission characteristics where these two pyramidal grooves support resonant modes at two different frequencies adjacent to each other. The strong coupling between modes causes the PIT effect in the proposed configuration. We have verified the PIT effect with the help of a three-level plasmonic model. To achieve modulation of the PIT effect, we varied the refractive index of the dielectric material filled in one of the grooves. We have further demonstrated that the resonance and PIT effect can be tuned by varying the conductivity of the Si sheet placed between the grooves.
Chapter 6examines the tunable terahertz absorption modulation in graphene-based dielectric metamaterial. The unit cell of the metamaterial absorber consists of a stack of four layers. From top to bottom, they are: SiO2 layer of frustum-shaped geome- try, a periodic array of graphene nanoribbons, the SiO2 spacer layer, and a layer of a gold metallic reflector, respectively. All the layers are stacked together to form a unit cell of the metamaterial absorber. Broadband absorption modulation is observed for various values of the chemical potential of graphene nanoribbons. We examined the effect of each layer on the performance of the proposed configuration. We have first investigated the role of free-standing graphene nanoribbons on broadband absorption
and combined it with the SiO2 dielectric spacer and gold metallic ground plane. We employ the theoretical approach based on effective medium theory and transfer ma- trix method to verify the numerical findings. Electric field distribution has also been studied to understand the role of both dielectric layers, which reveals their importance in broadband absorption.
Chapter 7presents the final comment based on the work presented in the previous chapters. All the objectives have been reviewed, and also the findings have been high- lighted. In addition, future papers have been outlined to expand the research reported therein.
Thin lm sensing in terahertz 2
2.1 Introduction . . . 32 2.2 Design of THz waveguide with inverted pyramidal corrugations 33