Scaling of ultrafast photon-triggered field emission cathodes composed of arrays of sharpened single-crystal Si pillars

Abstract

Ultrafast (<1 ps-long pulses) cathodes with spatially structured emission are an enabling technology for exciting applications such as free-electron lasers, tabletop coherent x-ray sources, and ultrafast imaging. In this thesis, we explore the scaling down and multiplexing limits of ultrafast photon-triggered field emission cathodes composed of arrays of nano-sharp high-aspect-ratio silicon pillars. We are interested in exploring how the geometry of the array and the morphology of the pillars affect the emission of the electrons. Both the multi-photon regime and the strong-field tunneling regime of the emission process were studied using near-IR pulses at various pulse energies. We model the structure using 2D and 3D COMSOL Multiphysics, collect the charge-energy characteristics of the actual devices, compare and interpret the results from both simulations and experiments. We find that the field factor is a strictly increasing function of the pitch at the fixed height, and the field factor saturates when the pitch is very large. The field factor is also strictly increasing with respect to the height with a diminishing return. In addition, due to the trade-off between the field factor and the pitch scaling, there exists an optimum pitch at 2.5 [mu]m, which yields the highest emission of electrons. This work also confirms earlier work by the group on the emission characteristics and polarization dependence: at low pulse energy, the emission is in the multiphoton regime and has an sin² ([theta]) dependence on the polarization; at high pulse energy, the emission is in the strong-field tunneling regime and has an sin²([theta]) dependence on the polarization.