Symmetry Point Group Determination Using Third Harmonic Generation Microscopy

This PhD will focus on multiphoton microscopy, a technique based on nonlinear optics to gain structural insights into bulk samples. Using high intensity infrared lasers, a wide variety of samples, such as biological tissue or crystals, can be probed, resulting in optical responses known as second and third harmonic generation (SHG and THG), where the emitted light frequency will be double or triple the frequency of the incoming light, respectively. The initial focus of the research will be on symmetry point group determination using THG. An intuitive method for determining crystal symmetry has previously been developed using SHG, but it is limited to non-centrosymmetric crystals. Furthermore, some point groups cannot be distinguished using this approach. The research group has been developing theoretical models describing the third harmonic responses in order to determine symmetry. To validate these models, a variety of compounds, from small molecule to macromolecule, should be crystallised in different point groups and studied under the microscope. By extending the existing symmetry determination method to include THG, the project aims to develop an intuitive, accurate, and high-throughput technique that enables symmetry determination in both centrosymmetric and non-centrosymmteric crystals. Such a technique would be of significant interest to the pharmaceutical industry. Certain compounds have the ability to crystallise in multiple symmetry point groups, a phenomenon known as crystal polymorphism. This can affect properties like the solubility, bioavailability and toxicity of a drug. Traditionally, crystal symmetry is determined using X-ray diffraction, a highly detailed but low-throughput method that requires large, high-quality crystals to be examined individually. In comparison, the multiphoton microscope allows for the imaging and subsequent symmetry determination of multiple crystals simultaneousy, each sample set only taking a few minutes. It also shows promise for future automisation. As noted, this topic will serve as the starting point of the research. However, additional applications of multiphoton microscopy may also be explored. For instance, the imaging of collagen in different tissues, such as skin and muscle, or a structural study of biological materials by polarized SHG and THG microscopy in general.