Guided-Mode-Resonant Colorimetric Metasurfaces for All-Optical and Nondestructive Structural Characterization of Polymeric Nanofibers

Metasurfaces have pioneered significant improvements in sensing technology by tailoring strong optical responses to weak signals. When designed with narrow-bandwidth, guided-mode resonances, metasurfaces can exhibit high sensitivity to changes in the intensity or polarization of light. Leveraging this to quantify structural alignment in fibrous materials unveils an alternative to destructive characterization methods. This work introduces metasurface-enhanced polymeric alignment detection (Meta-PAD), which employs polarization-tunable, guided-mode-resonant colorimetric metasurfaces to characterize molecular and bulk alignment of poly(ε-caprolactone) (PCL) nanofibers in a far-field configuration. PCL nanofibers drawn at 0%, 400%, and 900% ratios were interfaced with the metasurfaces. Metasurface resonances coinciding with the intrinsic nanofiber resonances─confirmed by Stokes polarimetry─produced the strongest colorimetric enhancement, resulting from alignment-specific nanofiber reflectivity. The enhancement degree corresponded with molecular and bulk alignments for each draw ratio, as measured through differential scanning calorimetry and scanning electron microscopy. Thus, Meta-PAD presents an all-optical, nondestructive, quantitative measurement of nanofiber alignment.