The light induced emission of electrons from a material, the photoelectric effect, has historically been of interest to study due to its potential applications in fields such as microscopy, accelerators, and free electron lasers. At high enough intensity, the electric field of a laser is strong enough to control the motion of an emitted electron. We call this the strong-field regime. In atomic and molecular systems, the interaction of electrons with strong laser fields can give rise to interesting phenomena such as attosecond streaking and high harmonic generation. Another source of strong-field investigations is around nanostructures, such as sharp nanotips. Nanotips provide a link between atomic and bulk systems as they are a microscopic object, but act in an atomic manner due to their shape. They also have the benefit of the natural optical field enhancement that arises from their sharp geometric shapes. Nanotips can be used a source of ultrashort electron pulses with high spatial and temporal coherence for use in matter wave experiments, with applications for femtosecond imaging and as ultrafast electron diffraction sources.

My research aimed to combine existing electron spectroscopy techniques and laser development expertise with novel tip materials to understand and identify various emission mechanisms. By changing the various parameters of the experiments — laser factors, applied voltage, tip composition, etc. — we were to explore the different regimes of electron emission. Each regime gives a unique signature that can be identified by the photoelectron energy spectrum.

From spectral features we were able to extract information about the system such as the photoemission mechanism and the enhancement factor of the electric field of the laser. For example, I confirmed the observation of above threshold photoemission (ATP) peaks from a tungsten nanotip, characterized by prominent peaks emerging in the photoemission spectra each separated by the photon energy of the driving laser. I detected the first laser induced electron emission from a carbon-based nanotip formed around a single carbon nanotube, presenting as thermal enhanced emission determined by the narrow energy range of the spectra. Finally, I observed a plateau in the electron spectra from a silver nanotip, the signature of electron recollision and rescattering in the tip in the strong-field regime.

Learn More