used for studies of the cornea and lamina cribrosa sclerae, both of which consist primarily of collagen. Second-harmonic generation can be produced by several non-centrosymmetric organic dyes; however, most of the organic dyes also generate collateral fluorescence along with second-harmonic generation signals. Until now, only two classes of organic dyes have been shown which do not produce any collateral fluorescence and works purely on second-harmonic generation. Recently, using two-photon excited fluorescence and second-harmonic generation-based microscopy, a group of Oxford University researchers showed that organic porphyrin-type molecules can have different transition dipole moments for two-photon fluorescence and second-harmonic generation, which are otherwise thought to occur from the same transition dipole moment. Second-harmonic generation microscopy is also used in material science, for instance to characterize nanostructured materials. Characterization of crystalline materials Second harmonic generation is also relevant to characterize organic or inorganic crystals since is one of the most discriminant and rapid technique to detect non-centrosymmetry. In addition, this technique can be used on single crystal as well as on powdered samples. One should recall that SHG is only possible (from the bulk) in non-centrosymmetric (NC) crystals. The part of non-centroysmmetric crystals in Nature is much lower than centrosymmetric crystals (circa 22% of the Cambridge structural database), but the frequency of NC crystals increases by a lot in pharmaceutical, biological and electronic fields because of the particular properties of these crystals (piezoelectricity, pyroelectricity, polar phases, chirality,...). In 1968, (7 years after the first experimental evidence of SHG on single crystal), Kurtz and Perry started to develop a SHG analyzer to rapidly detect the presence or not of inversion center in powdered crystalline samples. The detection of a SHG signal has been shown to be reliable and sensitive test for the detection of crystalline non-centrosymmetry with the confidence level higher than 99%. It is a relevant tool to resolve space group ambiguities that can arise from Friedel’s Law in single-crystal X-ray diffraction. Furthermore, the method is referenced in the International Tables for Crystallography and is described as a “powerful method of testing crystalline materials for the absence of a symmetry center. One possible application is also to rapidly discriminate chiral phases such as conglomerate that are of particular interest for pharmaceutical industries. It could also be used as a technique to probe the structural purity of material if one of the impurities is NC reaching a detection threshold as low as 1 ppm