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Near-infrared-to-visible second harmonic generation from air-stabletwo-dimensional polar gallium and indium metals is described. The photonicproperties of 2D metals - including the largest second-order susceptibilitiesreported for metals (approaching 10nm$^2$/V) - are determined by theatomic-level structure and bonding of two-to-three-atom-thick crystallinefilms. The bond character evolved from covalent to metallic over a few atomiclayers, changing the out-of-plane metal-metal bond distances by approximatelyten percent (0.2 $\unicode{x212B}$), resulting in symmetry breaking and anaxial electrostatic dipole that mediated the large nonlinear response. Twodifferent orientations of the crystalline metal atoms, corresponding to lateraldisplacements < 2 $\unicode{x212B}$, persisted in separate micron-scaleterraces to generate distinct harmonic polarizations. This strong atomic-levelstructure-property interplay suggests metal photonic properties can becontrolled with atomic precision.

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