| Summary: | An ultrasensitive photonic sensor is vital for sensing matter with absolute specificity. High specificity terahertz photonic sensors are essential in many fields, including medical research, clinical diagnosis, security inspection, and probing molecular vibrations in all forms of matter. Widespread photonic sensing technology detects small frequency shifts due to the targeted specimen, thus requiring ultra-high quality (Q) factor resonance. However, the existing terahertz waveguide resonating structures are prone to defects, possess limited Q-factor, and lack the feature of chip-scale CMOS integration. Here, inspired by the topologically protected edge state of light, we demonstrate a silicon valley photonic crystal based ultrasensitive, robust on-chip terahertz topological insulator sensor that consists of a topological waveguide critically coupled to a topological cavity with an ultra-high quality (Q) factor of 𝑄=0.14×106. Topologically protected cavity resonance exhibits strong resilience against disorder and multiple sharp bends. Leveraging on the extremely narrow linewidth (2.3 MHz) of topological cavity resonance, the terahertz sensor shows a record-high figure of merit of 4000 RIU mm−1. In addition to the spectral shift, the intensity modulation of cavity resonance offers an additional sensor metric through active tuning of critical coupling in the waveguide-cavity system. We envision that the ultra-high Q photonic terahertz topological sensor could have chip-scale biomedical applications such as differentiation between normal and cancerous tissues by monitoring the water content.
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