On-chip terahertz biosensing with topological Mach–Zehnder interferometer

Terahertz photonic sensing provides a powerful platform for probing biological materials on-chip due to its strong sensitivity to molecular resonances. Here, we demonstrate a complementary metal-oxide-semiconductor-compatible, silicon-based topological Mach–Zehnder interferometer operating in the THz regime for robust refractive-index sensing of biological materials, in which one arm of the interferometer is functionalized as the sensing region. The device is implemented on a valley photonic crystal platform supporting topologically protected waveguiding, enabling low-loss and scattering-resilient propagation, even along sharp bends. Owing to its interferometric architecture, we demonstrate that a thin polyimide film induces a measurable resonance frequency shift of up to 1.7 GHz, while biological samples with high water content, such as a Heliconia psittacorum leaf, induce amplitude attenuation of up to 25 dB. The interferometric dual-signature approach provides a flexible sensing scheme in which either the spectral shift or intensity modulation can be exploited, depending on the measurement configuration. These results establish the topological THz Mach–Zehnder interferometer as a promising architecture for integrated biosensing and environmental monitoring, with potential applications in tissue hydration monitoring and biological-material characterization.