Nat. Commun. 9, 516 (2018)

Two-dimensional materials are often very bendy and can withstand strains larger than 10%. But it can be difficult to measure the local deformations experimentally, either due to optical resolution issues, or because microscopy techniques can be destructive. Now, Lukas Mennel and co-workers have shown that it is possible to map the local strain of two-dimensional crystals non-invasively — by probing their nonlinear optical response.

Second harmonic generation is a coherent process in which a material absorbs two photons, then emits one with twice the frequency and a polarization that is linked to the symmetry of the crystal lattice. Applying strain to the crystal will break these symmetries, and so monitoring the polarization gives access to the strain tensor. Crucially, this approach can resolve all the components of the strain tensor, providing more information than other techniques.

Mennel et al. demonstrated that this can work for molybdenum disulphide with sub-wavelength spatial resolution of a few hundred nanometres, but the approach might also be used on a sub-picosecond timescale to probe transient crystal deformations.