UC Santa Barbara

The dynamics of photo-excited charge carriers near defects and interfaces plays a fundamental role in determining the efficiency and performance of a wide variety of optoelectronic and solar devices. Conventional optical pump-probe optical spectroscopies possess femtosecond temporal resolution, but their spatial resolution is constrained by the optical diffraction limit. One option to achieve simultaneously high temporal and spatial resolution is to integrate femtosecond lasers with electron microscopes. By coupling femtosecond photon pulses to the cathode in an electron microscope, it is possible to generate ultrafast, sub-picosecond electron pulses that are not constrained by optical diffraction.

Scanning ultrafast electron microscopy (SUEM) is a recently developed photon-pump electron-probe technique that uses short electron pulses to image the response of a sample surface after the impact of a photon pulse. SUEM is highly sensitive to surface charge dynamics, and has been used to study photocarrier diffusion in uniform materials and near interfaces. This dissertation describes the development process of SUEM at UCSB and documents various technical challenges encountered during its construction.

SUEM is then used to visualize the diffusion of photocarriers in BAs, a semiconductor with a unique phonon band structure and ultrahigh thermal conductivity. We observed ambipolar diffusion at low optical fluence with persistent hot carrier dynamics for above 200 ps, which can likely be attributed to the large frequency gap between acoustic and optical phonons, the same feature that is responsible for the high thermal conductivity. At higher optical fluence, we observed spontaneous electron-hole separation. Our results show BAs is an attractive optoelectronic material combining high thermal conductivity and excellent photocarrier transport properties.

Once thought to be an ordinary semiconductor, RuCl3 is a Mott-Hubbard insulator and host to a variety of exotic physics, including predictions of a photoinduced insulator-to-metal transition. We use SUEM to image the response of RuCl3 to photoexcitation. At low optical fluences, SUEM images show uniform bright contrast and slow carrier diffusion. At higher fluences, the material response becomes non-linear, potentially due to the predicted phase transition.

This thesis establishes SUEM as a platform to study the spatio-temporal evolution of excited carriers on extreme time and length scales. It then demonstrates its potential to study photocarrier transport in emerging and scientifically relevant systems. Future iterations of SUEM can continue improve upon spatial and temporal resolution and integrate other SEM detectors in order to be expanded into a versatile characterization platform.