UC San Diego

Nonlinear optical phenomena reveal information about electronic structure and symmetry that is inaccessible to linear spectroscopy. At terahertz frequencies weak light–matter coupling has historically limited nonlinear studies to a narrow range of materials. This thesis demonstrates that plasmonic near-field enhancement can overcome this limitation through two complementary platforms. The first uses thin films of cadmium arsenide, a three-dimensional Dirac semimetal with high carrier mobility, patterned into square-packed disk arrays that support a localized terahertz plasmon resonance. Under intense terahertz excitation generated by optical rectification in lithium niobate, the arrays produce a measurable second-harmonic response despite the centrosymmetric crystal structure of cadmium arsenide. This response is attributed to strongly confined and spatially nonuniform plasmonic near fields at the disk boundaries, which locally break inversion symmetry and enable even-order nonlinear processes. High-field excitation also broadens the plasmon resonance, consistent with reduced carrier mobility and velocity saturation, establishing the disk arrays as a platform for both inducing and probing nonlinear terahertz behavior in quantum materials. The second platform consists of gold nanoslot arrays that concentrate terahertz fields within deeply subwavelength apertures, producing field enhancements approaching two orders of magnitude and driving nonlinear changes in the underlying substrate without requiring the active material itself to be patterned. Together, these results establish plasmonic near-field engineering as a versatile approach for accessing and characterizing nonlinear terahertz phenomena in quantum materials.