Relevance of the Second Harmonic Generation to Characterize Crystalline Samples

The application of powder second harmonic generation (P-SHG), temperature-resolved SHG (TR-SHG), and SHG microscopy (SHGM) in the characterization of bulk crystalline samples is illustrated. P-SHG applied to powder samples can be an extremely sensitive approach to detect the absence of an inversion center in crystalline structures, TR-SHG serves to monitor temperature-induced phase transitions, and SHGM is used in the detection of non-centrosymmetric zones inside a heterogeneous material. These methods are of great relevance, e.g., in the pharmaceutical industry where crystalline active pharmaceutical ingredients are often made of a single enantiomer and are therefore non-centrosymmetric. Herein, several examples are provided to describe how a given SHG signal should be interpreted. A general procedure to carry out a P-SHG experiment is illustrated in detail.