Topological Vortex Transition Induced by Spin Hall Effect of Light for Tunable Humidity Sensing and Imaging

초록

Dynamic switching between edge detection and bright-field imaging modes is advantageous in optical imaging, particularly for biomedical diagnostics and material characterization. However, conventional approaches necessitate complex setups or intricate fabrication processes, limiting their practicality. This study demonstrates a humidity-responsive optical imaging system, enabling reversible and tunable transitions between edge-enhanced and Gaussian-like bright-field imaging modes by leveraging humidity-induced variations in the spin Hall effect of light. Utilizing polyvinyl alcohol films that exhibit reversible humidity-dependent changes in thickness and refractive index, the Fresnel reflection coefficients (rp$r_{\mathrm{p}}$ and rs$r_{\mathrm{s}}$) are effectively modulated, leading to asymmetric spin-dependent beam splitting in both x- and y-directions. These humidity-driven variations disrupt the initial symmetric vortex conditions, namely that the in-plane and out-of-plane shifts induced by the spin Hall effect of light are equal, transforming the topological vortex beam into a quasi-Gaussian distribution. Consequently, the imaging performance shifts from edge-enhanced mode to quasibright-field mode as the relative humidity increases. Experimental validation using customized resolution targets and biological tissue samples (planaria and small intestine) demonstrates reliable and reproducible imaging mode switching without requiring mechanical adjustments or complex fabrication. Thus, the proposed system offers improved simplicity, operational convenience, and cost-effectiveness compared to existing methods (e.g., metasurface-based techniques), underscoring the potential of humidity-tunable spin Hall effect of light-based polymer optics for practical and versatile imaging applications.

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