Harnessing Interfacial Field Localization in Hexagonal GaP Mie Resonators

Gallium Phosphide (GaP) is a premier material for integrated nanophotonics, combining prominent second-order nonlinear properties with near-perfect lattice matching with silicon for CMOS compatibility. However, leveraging these advantages in scalable bottom-up architectures requires understanding optical properties of the geometries defined by the fabrication methods. Here, we reveal the formation of a highly confined electromagnetic "hot spot" at the nanowire-substrate interface in self-assembled GaP nanowires (NW). This robust interfacial field localization is driven by the fundamental TM11 Mie resonance, which is selectively supported by the hexagonal geometry of the epitaxially grown NW. Through full-wave simulations and polarization-resolved dark-field spectroscopy, we find that the hexagonal cross-section also suppresses higher-order modes, yielding a simplified and spectrally clean resonant landscape. Experimental measurements confirm the diameter-dependent tunability of these modes across the visible spectrum. These findings establish a scalable platform for nanoantennas deterministically concentrating light at the substrate interface, ideal for employing the intrinsic nonlinearity of GaP for frequency conversion and enhancing light-matter interactions for quantum emitters and biosensing.