Applications of calculated second harmonic generation tensors on monomolecular and bimolecular systems

The field of nonlinear optics (NLO) has proven itself as a relatively recent and powerful area of science in the areas of microscopy and high-sensitivity quantification measurements. In particular, symmetry selective NLO processes such as second harmonic generation (SHG) take advantage of symmetry rules to cancel background noise almost completely, leading to both high resolution images and limits of detection (LOD) in the photon counting range. Background noise in SHG is negated as result of signal cancellation in systems containing inversion symmetry; generally isotropic sources. Quantum mechanical computational chemistry has enriched the field of NLO by enabling studies on hypothetical systems and environments that are otherwise unobtainable in experiments. Such systems are of benefit to study on account of their simplicity and the removal of common sources of experimental interference. In the presented work, three particular examples are presented. In the first example, simulated SHG response is investigated for noncentrosymmetric dimers constructed from nominally centrosymmetric monomers whilst a theoretical explanation is developed for the origin of said response. In the second example, calculated SHG tensor elements are used in a sum-over-orientations algorithm to approximate experimentally bright SHG responses on various crystallized active pharmaceutical ingredients (APIs). By comparison with experiments, the predictive capability of time-dependent Hartree Fock (TDHF) is assessed with promising results. In the third example, SHG tensor elements calculated using TDHF simulations are utilized to predict polarization dependent two-photon absorption (PDTPA) for monomolecular systems. This is done by relating each SHG element to the product of the two-photon absorption element with the polarization dependent transition dipole