  Near-infrared-to-visible second harmonic generation from air-stable
two-dimensional polar gallium and indium metals is described. The photonic
properties of 2D metals - including the largest second-order susceptibilities
reported for metals (approaching 10nm$^2$/V) - are determined by the
atomic-level structure and bonding of two-to-three-atom-thick crystalline
films. The bond character evolved from covalent to metallic over a few atomic
layers, changing the out-of-plane metal-metal bond distances by approximately
ten percent (0.2 $\unicode{x212B}$), resulting in symmetry breaking and an
axial electrostatic dipole that mediated the large nonlinear response. Two
different orientations of the crystalline metal atoms, corresponding to lateral
displacements < 2 $\unicode{x212B}$, persisted in separate micron-scale
terraces to generate distinct harmonic polarizations. This strong atomic-level
structure-property interplay suggests metal photonic properties can be
controlled with atomic precision.
