Partial Quantum Coherence, Ultrashort Electron Pulse Statistics, and a Plasmon-Enhanced Nanotip Emitter Based on Metallized Optical Fibers

The present dissertation covers two related research projects. The first topic was initiated with the ultimate goal of observing quantum degeneracy in ultrashort free electron pulses. This constitutes a thorough theoretical analysis of the problem involving partial quantum coherence and spin polarization of the source in light of a path-integral treatment of the phenomenon of matter-wave diffraction-in-time. Subsequently, results of a trailblazing experiment, to be superseded by a Hanbury Brown-Twiss type conclusive test of free fermion antibunching with electrons, is reported. In this experiment, the statistical distribution of the emitted electrons is studied taking advantage of a double-detector coincidence detection technique. The utilized electron emitters are ultrafast photoemission tungsten nanotip needle sources which are known to procure large spatial coherence lengths. The emission statistics is found to be sub-Poissonian. The second project involves introduction and full characterization of a novel laser-driven electron nanotip emitter based on metallized fiber optic tapers in which the emission mechanism is found to be assisted by surface plasmon resonance excitation as predicted prior to the design of such sources. It is shown that gold-coated fiber optic nanotips can emit electrons using both low-power continuous wave lasers as well as femtosecond pulsed lasers tuned to or near the surface plasmon resonance excitation wavelength of the system. The final chapter entails a proposal of a spin-polarized electron photoemitter based on the spin Hall effect for which such a fiber optic nanotip source is exploited. The optically switchable spin-polarized tip source for the first project with the Pauli exclusion principle at its core is based on GaAs and is developed and characterized in-house by Mott polarimetry. Another novel spin-polarized emitter based on our proposed fiber optic nanotip sources therefore constitutes a confluence of the two research programs considered here and will wrap up the present dissertation. The reported findings in this work are of direct relevance to electron source and beam physics, ultrafast electron microscopy, and foundations of quantum mechanics in relation to the burgeoning field of free electron quantum optics