Unexpected Near-Infrared to Visible Nonlinear Optical Properties from 2-D Polar Metals

Area of Science:

• Materials Science
• Condensed Matter Physics
• Nanotechnology

Background:

• Two-dimensional (2D) materials offer unique electronic and optical properties.
• Nonlinear optics in metals is typically limited due to centrosymmetric structures.
• Exploring 2D metals for advanced photonic applications is an emerging field.

Purpose of the Study:

• To investigate near-infrared-to-visible second harmonic generation (SHG) in air-stable 2D polar metals.
• To correlate atomic-level structure and bonding with observed photonic properties.
• To demonstrate control over nonlinear optical responses through atomic manipulation.

Main Methods:

• Fabrication of two-to-three-atom-thick crystalline films of 2D gallium and indium metals.
• Characterization of atomic structure, bonding evolution, and symmetry breaking using advanced microscopy and spectroscopy.
• Measurement of second-order nonlinear optical susceptibilities and harmonic generation efficiencies.

Main Results:

• Achieved significant SHG from 2D gallium and indium metals, exhibiting the largest second-order susceptibilities reported for metals (approaching 10 nm/V).
• Demonstrated that the evolution of bond character from covalent to metallic over a few atomic layers breaks symmetry, creating an axial electrostatic dipole responsible for the nonlinear response.
• Observed distinct harmonic polarizations generated by different crystalline orientations on micrometer-scale terraces, attributed to lateral atomic displacements <2 Å.

Conclusions:

• Air-stable 2D polar metals exhibit strong nonlinear optical properties mediated by atomic-level structural and bonding characteristics.
• Precise control over atomic arrangement in 2D metals allows for tailored photonic functionalities.
• This work opens avenues for designing novel metal-based photonic devices with tunable responses.