Polarisation Beam Splitting in the Terahertz Spectrum Using Topologically Engineered Photonic Structures

Topological photonics has been a main focus of research since the breakthrough in Topological Insulators (TIs), largely because of its intrinsic properties such as robustness against disorders and defects. In topological photonics, valleytronics exploits the valley degree of freedom to enable robust and directional light propagation, thus opening avenues for innovative integrated photonic applications. The valley degree of freedom arises from the existence of several energetically similar extrema-often referred to as "valleys"-in a material’s band structure. In many two-dimensional materials, these valleys occur at distinct locations in momentum space (for example, the K and K<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">′</sup> points in hexagonal lattices) and can be considered an extra binary characteristic, much like spin. By leveraging this property, topological photonics can achieve robust, valley-specific control of light, a principle that underpins the field of valleytronics. This enables retaining maximum fraction of the incoming signal to the output, ensuring high signal integrity and efficiency. We utilize this concept of valleytronics and design a polarization beam splitter structure which exhibits superior transmission in the common band gap range of 742 – 750 GHz for the transverse electric (TE) and transverse magnetic (TM) modes. Upon breaking the symmetry of the Valley Photonic Crystals (VPCs), valley-locked edge states appear inside the multi bandgaps. With special focus on designing the waveguide paths, the two modes are separated in the common band gap range. This could have potential applications in integrated terahertz topological passive devices for 6G technology.