Abstract

Terahertz (THz) light is at the resonant frequency of important fundamental excitations within crystalline materials such as carrier dynamics, phonons, and spin-wave excitations called magnons. THz light can be produced at high field strengths using optical rectification in nonlinear optical (NLO) crystals. N-benzyl-2-methyl-4-nitroaniline (BNA) is one such crystal commonly used to produce THz when pumping with 800 nm light. Here, we improve upon the design of the molecular building blocks of BNA by replacing a hydrogen atom with a fluorine atom, leading to improved THz generation and a higher crystal damage threshold. Later, we focus on using THz light to strongly drive nonlinear processes within a variety of materials to begin to unpack energy transfer pathways. Two-dimensional (2D) THz spectroscopy is a crucial tool in beginning to unpack these complicated dynamics for future use in technological advancements such as ultrafast switching. In the centrosymmetric crystal cadmium tungstate (CdWO4), we identify two sets of trilinear couplings between vibrational modes. Although the vibrational mode frequencies within these couplings appear inefficient, we show that the THz pulse itself lends bandwidth to the atomic motions to make the coupling possible. We push the limit of vibrational coupling identification in the complicated crystal β-barium borate (BBO) by combining a series of experimental techniques to limit the possible causes of our nonlinear signals from 521 couplings to 16. Later, we explore how THz light interacts with the lowest E(TO1) phonon-polariton in lithium niobate (LiNbO3) and show that a single THz pulse can excite various regions on the dispersion curve simultaneously, while a Raman-excitation can only excite a relatively narrow range of wavevectors. By exciting the phonon-polariton E mode using perpendicular THz pulses with a delay between them, we can drive the ions in LiNbO3 to move in a circular motion, which generates a magnetic field in this material with no innate magnetic ordering. Finally, we use 2D THz spectroscopy on bismuth ferrite (BFO), an antiferromagnetic material with both magnons and phonons within our THz frequency range. We identify nonlinear signals due to the coupling between phonons and phonons, magnons and phonons, and magnons and magnons.

Degree

PhD

College and Department

Computational, Mathematical, and Physical Sciences; Chemistry and Biochemistry

Rights

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