Thin Film Sensing near Exceptional Point utilizing Terahertz Plasmonic Metasurfaces

Non-Hermitian quantum systems along with engineered metasurfaces enable a versatile podium for sensor designs from industrial to medical sectors. The singularity points known as Exceptional points (EP) can be realized in such non-Hermitian systems. EP demonstrates a square root topology on minute perturbations, hence promising to be a potential candidate to sense external parameters, such as temperature, thermal fluctuations, refractive index, and biomolecules. Hence, in this work, through numerical and analytical investigations, we explore the sensing capabilities in the vicinity of EP utilizing suitably designed terahertz metasurfaces. Here, we propose a non-Hermitian metasystem comprising two orthogonally twisted square split ring resonators coupled by near-field EM (Electromagnetic) interactions that can exhibit dark-bright modes. In such a system, the presence of an active (photo-doped) material in the split gap of one of the resonators opens up an effective avenue to introduce controllable asymmetric losses, ultimately leading to the emergence of exceptional points in the polarization space. Hence, thin film sensing at the proximity of the emerged exceptional point is investigated for different refractive indices by coating with an overlayer atop the metasurface. In such a configuration, the sensitivities of the eigenstates are calculated in terms of the Refractive Index Unit, which turns out to be - 0.044 THz/RIU and - 0.063 THz/RIU when the system is perturbed near EP. Our proposed metasurface-inspired EP-based sensing strategy can open up novel ways to sense the refractive index of unknown materials besides other physical parameters.