Stacking symmetry governed second harmonic generation in graphene trilayers

Area of Science:

• Condensed Matter Physics
• Materials Science
• Optics

Background:

• Crystal symmetry is crucial for physical phenomena like nonlinear optical second harmonic generation (SHG).
• SHG requires the breaking of inversion symmetry, a property absent in individual graphene layers.
• Graphene trilayers offer a platform to explore stacking-dependent symmetry properties.

Purpose of the Study:

• To investigate stacking-induced second harmonic generation (SHG) in graphene trilayers.
• To explore the role of stacking sequence (ABA vs. ABC) on SHG.
• To demonstrate the potential of SHG for mapping crystal domains in graphene.

Main Methods:

• Fabrication of graphene trilayers with controlled stacking sequences (Bernal ABA and rhombohedral ABC).
• Optical characterization using second harmonic generation (SHG) microscopy.
• Analysis of SHG intensity dependence on stacking order and crystal symmetry.

Main Results:

• Observed strong SHG in Bernal ABA-stacked graphene trilayers, indicating broken inversion symmetry.
• Found vanishing SHG in rhombohedral ABC-stacked trilayers, consistent with preserved inversion symmetry.
• Demonstrated the ability to distinguish and map ABA and ABC domains using SHG contrast.
• Measured a large second-order nonlinear susceptibility in ABA trilayers, comparable to exciton-enhanced 2D semiconductors.

Conclusions:

• Stacking order in graphene trilayers uniquely induces or suppresses nonlinear optical SHG.
• SHG serves as a sensitive probe for crystal symmetry and stacking domains in few-layer graphene.
• This finding opens avenues for exploring novel physics in stacking-engineered graphene systems.