In a crystal, for example silicon, every atom has a fixed place. When the material is stretched or compressed, the distance between the individual atoms changes, and this distance has an influence on the electronic properties of the material. This phenomenon has been used in semiconductor technology for years: silicon crystals, for example, can be grown so that they are permanently under internal mechanical stress and thus show the desired properties. Two-dimensional materials such as graphene which consist of only one or a few atomic layers, are significantly more flexible than crystals, and thus their electronic properties can be strongly changed.
For years they have been considered to be a promising area in the field of material sciences, and demonstrate remarkable properties that open up completely new technical possibilities, from sensor technology to solar cells. Up until now, if one wanted to measure stresses present in this type of material, it was necessary to rely on extremely complicated measurement methods. The new measuring method is based on so-called frequency doubling, an effect which is already known. In this case a material is irradiated with a suitable laser beam of a certain color, and then reflects back light of a different color. Two photons in the incoming laser beam are combined to form one photon with double the energy, which is emitted from the material.