Quantifying the effect of morphology on second  harmonic generation in amino acids

Nonlinear optics studies the interaction between intense light and materials. Second harmonic generation (SHG) is the simplest frequency conversion process in which two low energy degenerate photons are converted to one higher energy photon. This process occurs only in non-centrosymmetric materials and so offers high specificity with inherent optical contrast. Analysing SHG can reveal information on the material’s internal crystal structure through a non-contact measurement, Accurately quantifying the material property generally responsible for SHG, the second order susceptibility tensor, can be helpful for characterizing crystallization in pharmaceutical compounds or monitoring disease progression through characterizing the building blocks of biomolecules, amino acids.. Though with the increasing interest in using organic materials for devices such as waveguides or nonlinear antennas, the effect of morphology on the signal recorded is often neglected.

In this thesis, we study the effect of morphology on the quantification of the second order susceptibility in some amino acids. The effect is separated into two scenarios. In the first, we developed an analytical model with a quantitative framework to quantify the second order susceptibility tensor of crystalline materials. This model was used to characterize the β and γ phases of glycine microneedles. Accounting for morphology showed an increase in contribution for the tensor longitudinal susceptibly elements at the expense of the others. The maximum susceptibility elements were d33 = 15 pm/V and d33 = 5.9 pm/V for the β and γ phases respectively.

The second scenario studies the effective susceptibility of amino acid cocrystal microparticles. We compare the difference in effective susceptibility magnitudes found using a simple ratio verses a volume normalized measure. We apply this method to six novel cocrystals using centrosymmetric sulfamic acid with Alanine, Cysteine, α-Glycine, Histidine, Methionine and Phenylamine. The effective susceptibilities found were 0.59 pm/V, 0.14 pm/V, 1.93 pm/V, 0.13 pm/V, 0.43 pm/V, and 0.08 pm/V respectively. These values are significantly higher than the pure crystallized amino acids. The volume normalized method produced slightly lower magnitudes but with a smaller standard deviation. The comparative study and widely applicable model presented here could be advantageous for characterizing functional biocompatible nonlinear materials.