Reflection-based refractive index measurement of van der Waals materials

Area of Science:

• Materials Science
• Optics
• Condensed Matter Physics

Background:

• Characterizing transparent van der Waals (vdW) materials is crucial for advanced electronics and photonics.
• Mechanical exfoliation yields small vdW flakes, posing challenges for conventional optical characterization methods like ellipsometry.
• Ellipsometry typically requires large, uniform sample areas and predefined spectral models for accurate refractive index determination.

Purpose of the Study:

• To develop a reflection-based method for measuring the in-plane refractive index of transparent vdW materials.
• To enable characterization of small exfoliated vdW flakes (down to 10 μm²) on non-transmissive substrates.
• To provide a spectral-model-free approach for refractive index determination.

Main Methods:

• A reflection-based optical technique is employed.
• The method relies on measuring the spectral position of reflection minima.
• Accurate knowledge of flake thickness and substrate complex refractive index is required.

Main Results:

• The in-plane refractive index of hafnium disulfide (HfS₂) was successfully retrieved across the visible spectrum.
• The technique was demonstrated on small exfoliated flakes (10 μm²).
• The method is independent of any assumed spectral shape for the refractive index.

Conclusions:

• The developed reflection method offers an accessible alternative for vdW material characterization.
• It overcomes limitations of conventional ellipsometry for small, exfoliated flakes.
• The findings validate previous measurements and establish a new standard for refractive index analysis.