UC Berkeley

This thesis explores excitons as sensitive probes of ultrafast electromagnetic and vibrational dynamics in solid-state systems. By combining concepts from nonlinear optics, ultrafast spectroscopy, and device engineering, we develop and implement on-chip pump–probe techniques to study light–matter interactions on femtosecond to picosecond timescales.Part of the thesis focuses on excitonic electro-optic effects in GaN. We investigate the excitonic response to applied electric fields, demonstrating a quadratic exciton Stark shift. Building on this, we develop a time-resolved measurement scheme where optical excitation generates terahertz-frequency voltage transients that propagate along on-chip transmission lines. By monitoring the exciton response under bias, we directly probe these transients in the time domain. This approach enables a pathway toward efficient on-chip terahertz emitters. Extending these ideas, we demonstrate hybrid GaN–monolayer transition metal dichalcogenide (TMD) electro-optic devices where transient gating of the TMD is achieved without reliance on slow contact-limited transport, revealing routes toward GHz-bandwidth modulation.The second part of the thesis investigates exciton–phonon interactions in van der Waals heterostructures that contain moiré superlattices of TMDs. Using coherent acoustic phonons, we observe the coupling between excitons and coherent lattice vibrations that arise from the moiré effect called, moiré phonons, in angle-aligned WS2/WSe2 heterobilayers. We identify an emergent layer-selective coupling between moiré excitons and moiré phonon modes. These results establish moiré engineering as a means to control exciton–phonon coupling and enable new pathways for manipulating non-equilibrium dynamics in two-dimensional materials.Taken together, this work demonstrates that excitons can serve as versatile, high-resolution probes of ultrafast phenomena, bridging the gap between fundamental many-body physics and the development of integrated optoelectronic and photonic devices.