Metasurface-Enabled Helical Wavefront Fingerprint Metrology for Quantitative Retrieval of Liquid Optical Constants.

Optical constants of liquids encode intrinsic signatures of molecular composition and electromagnetic response, forming the physical basis for quantified optical analysis. However, conventional ellipsometric measurements are fundamentally constrained by interface-induced beam distortion arising from gas-liquid meniscus effects that limit the measurement precision and reliability, and necessitate bulky optical instrumentation. Here, we propose and experimentally validate a Metasurface-enabled Helical Wavefront Fingerprint (MHWF) metrological paradigm for quantitatively specific readout of liquid refractive index and extinction coefficient by exploiting the features of wavefronts as information carriers. MHWF is constructed from the wavefronts of a focused vortex beam generated by a dielectric metasurface embedded in the liquid. The topological preservation of the vortex beam with orbital angular momentum ensures the integrity of the MHWF wavefront features, forming a unique three-dimensional optical fingerprint. Specifically, the fingerprint carries wavefront profile, focal length evolution, and spatial intensity distribution information governed by the complex dielectric response of the liquid, which are deciphered using beam quality spatial variance, hyperbolic geometric propagation, and Fresnel diffraction theory. Furthermore, MHWF enables high-sensitivity refractive index sensing with multi-response. This work establishes an ultra-compact optical metrology platform, paving the way for light-matter interaction studies in liquid-state photonics.