Nonlinear Optical Studies of Complex Materials

This dissertation presents an in-depth exploration of nonlinear optical investigations into complex material systems, employing second harmonic generation (SHG) and complementary bulk measurement techniques. The first project focuses on surface symmetries and phase transitions in IrTe2. Using SHG and electrical resistance measurements, we systematically investigate temperature-induced symmetry changes and reveal significant shifts in transition temperatures due to thermal cycling. Comparison with bulk X-ray diffraction confirms that these shifts are surface-related, providing a new interpretation of inconsistencies in reported transition temperatures. Our findings emphasize the effectiveness of SHG in capturing subtle surface phenomena, with future directions involving straincontrolled studies to manipulate symmetry phases, including below 10 K near the superconducting regime. The second project investigates the magnetoelectric (ME) coupling in composite multiferroic nanofibers consisting of barium titanate (BTO) and cobalt ferrite (CFO). SHG polarimetry measurements under different magnetic fields reveal strong ME coupling and an unexpected semicylindrical alignment of fibers, attributed to their hierarchical structure. This study demonstrates SHG as a crucial tool for uncovering unique properties in complex materials. The third project briefly examines the topological semimetal PtBi2−x, integrating SHG with various bulk measurements to identify temperature-dependent surface phenomena and anomalies potentially linked to superconductivity. Overall, this dissertation highlights the versatility of SHG as a powerful method for probing structural, magnetic, and coupling phenomena in complex materials, offering new insights and pathways for future research.