In general, the dipole moment of a molecule can be used for Second Harmonic generation. This phenomenon comes from the non-linear optical properties of a molecule that exhibits a magnetic or electric dipole moment. As molecular dipoles are non-centrosymmetric, they exhibit the property when two photons of whathever frequency come in onto that molecule, a new photon is generated with a frequency which is the sum or the difference between these frequencies of the incoming photons (no absorption happens, only scattering). When the 2 incoming photon frequencies are the same, they call it Second Harmonic Generation (SHG). This also appears to be the case for an ensemble of molecular dipoles. However, when the ensemble forms a centrosymmetric structure, the SHG will not happen. It only appears to work for centrosymmetric structures. Therefore, combining Second Harmonic Generation imaging with fluorescence imaging, scientists can recover whether a certain structure is centrocymmetric or not. This is especially interesting for cellular imaging as membranes are typically centrosymmetric.