Unexpected Near-Infrared to Visible Non-linear Optical Properties from Two-Dimensional Polar Metals

Nonlinear optical (NLO) processes have revolutionized laser technology, chemical sensing, and telecommunications. Emerging 2D semiconductors and multiple-quantum-well structures have redefined the state of the art for NLO performance through large second-order susceptibilities ${\chi}^{2}$. However, the largest ${\chi}^{2}$ values are limited to the mid-infrared and cryogenic temperatures, precluding implementation at visible and near-infrared (NIR) frequencies essential to telecommunications. Here, we report room-temperature NIR ${\chi}^{2}$ approaching 10 nm/V from a heterostructure of crystalline, atomically thin group-IIIa metal films that are epitaxial to a SiC substrate and capped with graphene. The surprising ${\chi}^{2}$ values are the result of intrinsic axial symmetry breaking unique to the heterostructure; the individual components are not expected to yield second-order responses due to their inversion symmetry. Metal bonding character transitions from covalent at the SiC interface to metallic to non-bonded at the graphene interface, with variation in the atomic spacing and an electrostatic gradient over 2-3 atomic layers. This leads to a theoretically predicted axial dipole that is confirmed by angle-dependent second harmonic generation (SHG) measurements, verifying that these are 2D polar metals (2D-PMets). Polarization-dependent SHG resolves structural influences of the substrate that are persistent into the metals, which adopt the hexagonal symmetry of the substrate. DFT calculations predict ${\chi}^{2}$ consistent with experiment, as well as electronic inter-band resonances, which may be leveraged to further amplify the nonlinear response. The exceptional nonlinear response of 2D-PMets, along with their electronic properties, environmental stability, and applicability to many metals, lays a foundation for further breakthroughs in vis/NIR NLO technology.