Harnessing Topological Valley-Hall States in Photonic Crystals for Label-Free Refractive-Index Discrimination of Cancer Cell Lines
Topological photonic crystals (TPhCs) are the photonic analogs of topological insulators, inspired by the quantum Hall effect and its variants, including the integer, spin, and valley Hall effects. Among these, valley-Hall photonic crystals (VPhCs) achieve topological protection by breaking inversion symmetry, making them closely related to valleytronics. Like other TPhCs, VPhCs enable robust, backscattering-free light propagation, even in the presence of structural imperfections, sharp bends, or defects. In this study, we introduce a silicon VPhC design near 6~THz that demonstrates exceptional resilience in guiding light through both linear and $Ω$-shaped waveguides ($-1.12$ and $-1.27$~dB insertion loss), even under significant structural disorder ($0.47$~dB worst-case degradation). Additionally, efficient light confinement is demonstrated within a hexagonal resonant cavity, further underscoring the versatility of the platform. On this basis, a VPhC-based biosensor is proposed, designed to achieve high sensitivity and a strong quality factor, thereby enabling carcinoma cell detection ($Δn = 0.011$~RIU). The work demonstrates that the sensor achieves a maximum quality factor of 285{,}338, a maximum sensitivity of 24{,}300~nm/RIU, and a figure of merit of 132{,}813~RIU$^{-1}$. This work holds significant promise for advancing medical and clinical diagnostics, paving the way for innovative applications in healthcare.