Hyperbolic-enhanced Raman scattering in van der Waals MoOCl2: from Fano resonances to picomolar detection
Abstract
Natural van der Waals (vdW) crystals with hyperbolic dispersion challenge artificial metamaterials but remain confined to the mid-infrared spectrum. The emergence of MoOCl2, a quasi-one-dimensional metal with in-plane hyperbolicity, overcomes this spectral limit, shifting the focus to the practical visible range. Here, utilizing angle-resolved Raman spectroscopy, we uncover a highly anisotropic vibrational response characterized by pronounced Fano resonances and polarization switching, which serve as signatures of strong coupling between phonons and the metallic continuum. Harnessing this interaction, we demonstrate "Hyperbolic-Enhanced Raman" (HypER) scattering, where MoOCl2 provides polarization-tunable enhancement factors exceeding 107 and picomolar-level detection down to 100 pM, without any nanostructuring. These results establish MoOCl2 as a simple, air-stable, wafer-compatible platform for visible-range hyperbolic nanophotonics and lithography-free sensing.
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