Label-Free Nucleic Acid Amplification in a Self-regulated All-Fiber Photothermal Microcavity

Nucleic acid amplification commonly relies on fluorescent labels and electrically driven thermal control, limiting simplicity, scalability, and remote deployment. Here, we establish a label-free optical strategy based on amplification-associated refractive-index (RI) evolution within a confined optothermal microcavity. During amplification, the accumulation of nucleic acid products alters the composition and molecular polarizability of the reaction medium, contributing to a progressive increase in its effective RI. This bulk RI change is converted into interferometric phase accumulation under thermally stabilized conditions, enabling real-time optical readout without labels. We implement this strategy in a 0.94 nL fiber Fabry-Perot microcavity that integrates localized optothermal regulation and phase-sensitive interrogation within a single optical pathway. The confined geometry enables rapid optothermal heating, reaching a 1 °C temperature rise in 15 ms, nearly three orders of magnitude faster than conventional electrical heating. Using hepatitis B virus (HBV) DNA as a model target, we achieve a detection limit of 5.24 copies/nL and quantitative agreement with fluorescence-based assays, while isothermal amplification reduces the total assay time by approximately 50% relative to commercial qPCR. Frequency-division multiplexing enables parallel multi-target analysis, while stable transmission is maintained over 20 km fiber links. Our results establish amplification-associated RI evolution as a promising label-free transduction mechanism for fiber-integrated molecular diagnostics.