Defesa de Tese de Doutorado #389 – Renan Souza Cunha – 25/11/2021

The study of light-matter interactions is a highly comprehensive research area, describing optical processes from reflection on specular surfaces in geometric optics to the dynamics of atoms and photons in cavity quantum electrodynamics. Within that framework is nonlinear optics, which encompasses phenomena that arise due to the incidence of intense fields in matter. As this year marks sixty years since the birth of modern nonlinear optics, research in this area has evolved into different topics, ranging from fundamental studies to applications in material science and even in biomedicine. This thesis lies within the scope of the last group: nonlinear optics for imaging biological tissues and 2D materials.
We first present a study based on multimodal imaging for histopathological characterization of amyloid-β (Aβ) plaques in an animal model of Alzheimer’s disease (AD). We explore label-free two-photon excitation fluorescence (TPEF), second harmonic generation (SHG), spontaneous Raman scattering (SpRS), coherent anti-Stokes Raman scattering (CARS), and stimulated Raman scattering (SRS) to describe and compare images of Aβ plaques, the main hallmark of AD, in brain tissues of transgenic mice. The obtained images allow for locating different plaque structures reliably and discussing the still unclear origin of SHG in the plaque. We also study the feasibility of SRS imaging of the plaque based on two intrinsic optical biomarkers unexplored with coherent Raman scattering techniques.
In a second approach, we study the effects of structural changes in a 2D material on SHG. In particular, we study defects-induced optical modifications in hexagonal boron nitride (h-BN). Performing linear photoluminescence imaging and spectroscopy, we found defective h-BN flakes whose SHG exhibited deviations from the expected behavior for pristine h-BN. Our results show that defects modify the intensity profile, polarization dependence, and efficiency of SHG in h-BN. Additionally, we characterized the second-order susceptibility of h-BN with defects with a value of one order of magnitude larger than for pristine h-BN.