  Strain engineering is widely used in material science to tune the
(opto-)electronic properties of materials and enhance the performance of
devices. Two-dimensional atomic crystals are a versatile playground to study
the influence of strain, as they can sustain very large deformations without
breaking. Various optical techniques have been employed to probe strain in
two-dimensional materials, including micro-Raman and photoluminescence
spectroscopy. Here we demonstrate that optical second harmonic generation
constitutes an even more powerful technique, as it allows to extract the full
strain tensor with a spatial resolution below the optical diffraction limit.
Our method is based on the strain-induced modification of the nonlinear
susceptibility tensor due to a photoelastic effect. Using a two-point bending
technique, we determine the photoelastic tensor elements of molybdenum
disulfide. Once identified, these parameters allow us to spatially image the
two-dimensional strain field in an inhomogeneously strained sample.
