2025 Theses Doctoral

Enhancement of energy carrier transport by coherent phonons in van der Waals semiconductors

Cheng, Shan-Wen

Understanding and controlling how phonons, or lattice vibrations, affect the behavior of photo-excited energy carriers is a longstanding goal of physical and materials chemistry. Thermal phonons, the observables in steady-state measurements such as infrared (IR) and Raman spectroscopies, are random fluctuations in nuclear displacements that scatter carriers, reducing electronic transport efficiency, and resulting in Joule heating. In contrast, coherent phonons exhibit synchronized atomic motion with maintained phase relations over extended spatial scales, often resulting in long-range vibrational energy propagation at well-defined group velocities.

This thesis demonstrates that coherent phonons can enhance the transport of photo-excited electronic carriers, including electrons and excitons – strongly bound electron-hole pairs – through dynamic trapping by the propagating deformation potential they create. Our primary material platform is tungsten diselenide (WSe₂), a van der Waals semiconductor prone to strong carrier–phonon interactions due to dimensional confinement and large deformation potential interactions. Using ultrafast spatiotemporal microscopy, we directly image nonequilibrium electronic carrier transport in the presence versus the absence of either coherent acoustic phonons or phonon-polaritons.

First, we show that laser-generated coherent acoustic phonons (sound/strain waves) in multilayer WSe₂ can carry free charges and excitons over microns at the speed of sound, paving the way for the miniaturization of surface acoustic wave-driven devices that are gaining increasing attention in quantum information science. Second, we show for the first time that hyperbolic phonon-polaritons in boron nitride can drag excitons in monolayer WSe₂across a van der Waals gap, leading to exciton transport at light-like speeds approaching the exciton Fermi velocity over picosecond scales. These studies highlight the potential of leveraging coherent species such as phonons to achieve orders-of-magnitude enhancement of electronic carrier transport via inter-particle interactions.