Universal Imaging of Full Strain Tensor in 2D Crystals with Third-Harmonic Generation

Area of Science:

• Materials Science
• Condensed Matter Physics
• Nanotechnology

Background:

• Accurate strain mapping in 2D materials is crucial for understanding their properties and designing new functionalities.
• Existing optical techniques like photoluminescence, Raman, and second-harmonic generation spectroscopy have limitations, restricting their application to specific material types or symmetries.
• A universal, non-invasive optical method for comprehensive strain analysis in all 2D crystalline materials is needed.

Purpose of the Study:

• To develop and demonstrate a universal optical methodology for quantitatively measuring the full strain tensor in any 2D crystalline material.
• To establish a direct correlation between nonlinear optical response and strain via the photoelastic tensor.
• To provide a comprehensive tool for strain characterization across all 2D materials, facilitating device engineering.

Main Methods:

• Utilizing polarization-dependent third-harmonic generation (THG), a third-order nonlinear optical process.
• Leveraging the universal third-order nonlinear optical response of 2D crystals.
• Establishing a one-to-one correspondence between nonlinear susceptibility and the strain tensor through the photoelastic tensor.

Main Results:

• Successfully determined the photoelastic tensor for both D3h WS2 monolayer and D3d WS2 bilayer.
• Quantitatively mapped the strain tensor distribution in homogeneously and randomly strained monolayer WS2.
• Developed an atlas of photoelastic tensors for strain monitoring in 2D materials across all 32 crystallographic point groups.

Conclusions:

• Polarization-dependent third-harmonic generation offers a universal optical approach for full strain tensor measurement in any 2D material.
• This technique overcomes the limitations of previous spectroscopic methods, enabling precise strain analysis.
• The developed methodology and atlas will accelerate the design and application of 2D-material-based electronic, optoelectronic, and photovoltaic devices.