High-Contrast Interferometric Imaging of Single-Molecule Dynamics on Optical Fibers

Direct observation of individual biomolecular events offers mechanistic insights into biological processes and provides a powerful tool for early disease diagnosis. However, the fundamental scale mismatch between optical wavelengths and molecules restricts the application of label-free techniques, leading to a poor signal-to-noise ratio (SNR). Here, we propose a high-contrast, label-free approach based on interferometric imaging, utilizing the strong evanescent field supported on a microfiber surface to provide near-field illumination. We observed unique interference patterns generated by in-plane scattering from natural defects, which enabled the high-contrast detection of localized phase changes induced by single molecules. The results indicate an approximately 38 dB enhancement in the SNR over the conventional fluorescence methods without employing any plasmonic or microcavity-based amplification techniques. This approach was further applied to track molecular dynamics, capturing both the conformational transition and binding behaviors of individual protein molecules. Meanwhile, the stimulus response of single molecules to acoustic waves was investigated, demonstrating the ultimate miniaturization of an acoustic sensor at the single-molecule scale. By enabling direct observation of molecular dynamics and mechanical responses at the single-molecule level, this approach provides a versatile platform for probing fundamental biological processes and developing ultrasensitive biosensors. Moreover, this approach lays the foundation for coupling optical and acoustic waves on the molecular scale, opening new avenues for next-generation single-molecule diagnostics and molecular physics studies.