Recent advancements in the synthesis of single-crystal van der Waals (vdW) materials and supported 2D layers have led to the widespread availability of high-quality, large-area commercial samples. These developments have opened new avenues for exploring a range of physical properties—structural, electronic, optical, magnetic, thermal, and vibrational—that are highly sensitive to parameters such as thickness, strain, and layer stacking. However, the same level of control has not yet been achieved in tailoring their elastic behavior, highlighting a critical knowledge gap. A central question arises: Are current experimental methods sufficient to provide reliable and comprehensive insights into the mechanical properties of vdW systems, ranging from bulk crystals to individual atomic layers and complex nanoscale architectures?

In this work, we explore the anisotropic acoustic and elastic behavior of over 20 vdW single crystals using angle-resolved Brillouin light scattering (BLS). This technique, dating back over a century, remains one of the most effective non-invasive tools for probing phonons in the long-wavelength regime. Spontaneous BLS measures the frequency shift of monochromatic light scattered by thermally excited hypersonic (GHz) acoustic waves. The observed shifts are directly linked to the velocity and wavevector of these phonons, providing a window into the material’s elastic properties.

Among the suite of methods for assessing elastic anisotropy, non-contact techniques such as BLS are particularly advantageous when dealing with ultra-thin or delicate samples. We demonstrate the suitability of BLS for micrometer-scale vdW materials and introduce two experimental protocols—customized for transparent and opaque crystals—paired with data analysis strategies aligned with their optical characteristics. These findings provide a robust experimental foundation for probing complete elastic as well as photo-elastic and dielectric tensors, advancing our understanding of vdW materials, and addressing long-standing experimental gaps in this field.

References

V. Babacic, D. Saleta Reig, S. Varghese, et al. Advanced Materials 33 (23), 2008614, (2021)

Acknowledgements: This work was supported by the EIG CONCERT-JAPAN/9/91/PETITE/2023 project.