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In recent years, topological photonic systems have garnered a lot of attention owing to their robust behaviour in the presence of defects and disorders, which could be exploited for applications such as backscattering-free waveguides, splitters, slow light devices, etc [1-3]. Although numerous investigations have been reported using microwave  and optical systems , there is still a lack of research using terahertz (THz) photonic systems [6, 7]. THz photonic systems have huge potential in the field of high-speed communication and information transfer . But, the performance of these systems is limited by challenges such as loss and signal distortion resulting from back-reflections at sharp corners, lack of easily integrable systems, etc. THz topological photonic systems could overcome these challenges and pave the way in realizing integrable devices for 6G mobile communication and several other applications.
Therefore, we investigate robust THz transmissions in a valley photonic crystal (VPC). Here, we examine the role of asymmetry in the robust characteristics of THz topological edge modes in an all dielectric VPC with a nontrivial topology . THz topological edge modes are excited through a zigzag domain boundary formed at the interface of two kinds of nontrivial VPCs with different band topologies. The proposed THz VPC is made up of air patterned on a Si slab (r =11.56) with height ‘h’ on the top of a substrate (r = 2.1). Figure 1(a) shows the proposed topological VPC, where the red dashed line represents the honeycomb lattice while the black solid line denotes the zigzag domain boundary. The proposed unit cell consists of two cylindrical air holes with diameters d1 and d2 and lattice constant ‘a’ is depicted in Fig. 1(b). A parameter defined as) represents the asymmetry in the structure.
For our analysis, we consider a = 250, h = 220 while d1 and d2 can vary from 20 to 110. The band structure has a degeneracy at K (K’) points when (standard honeycomb lattice with C6 symmetry) which is lifted by changing the diameter of one air hole while fixing the diameter of the other hole in the VPC unit cell ). Due to the introduction of this asymmetry, the C6 symmetry of the VPC transforms to a C3 symmetry, ultimately lifting the degeneracy at f = 0.336 THz. Figure 1(c) illustrates the band structures for TE mode for the case of , . Here, it is noteworthy to mention that signs of CV for VPC-I and VPC-II are non-zero and opposite to each other . Consequently, THz topological edge modes excitation is guaranteed by the bulk-edge correspondence principle in the VPC structure.
To examine the role played by asymmetry in the robust behaviour of THz topological edge modes, we vary the parameter for a straight and an -type domain boundary. For lower asymmetry values (i.e., 0.04a, 0.12a and 0.2a in our analysis) it is observed that the edge states suffer large scattering close to the domain boundary of the VPC. The presence of a narrow bulk bandgap at lower asymmetry causes intervalley scattering within the bands of the VPC. This leads to weakly guided edge modes at the domain boundaries for lower values of . However, it is observed that the robustness of the edge modes increases with increasing asymmetry. When increases, the edge modes shift away from the bulk bands and towards the middle of the bulk bandgap, which causes an increase in the confinement of the edge modes within the domain boundary of the VPC (see Fig. 1(d)). The robust behaviour of the edge modes is also attributed to the increase of the bulk bandgap of the VPC with increasing, reaching close to 10% for = 0.28a. The large bandgap at assures a complete suppression of the intervalley scattering in the VPC, ultimately leading to a robust guiding of the THz topological edge modes within the domain boundary. As a result, a robust transport of THz topological edge modes is achieved even when sharp corners/bends are present in the domain boundary. The THz transmission and Electric field confinement corresponding to is illustrated in Figs. 1(f) to (1(g). The most robust THz transport and the highest confinement of THz topological edge modes are achieved when . Such comprehensive study can be beneficial for realizing loss-free waveguides for 6G mobile communication and several other integrated devices at THz frequencies.
How to Cite
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