Electron source brightness and degeneracy from Fresnel fringes in field emission point projection microscopy

Abstract

Fresnel edge fringes observed in a lensless point projection field-emission electron microscope operating at 90 eV have been studied and found to be formally equivalent to the fringes observed in transmission electron microscopy (TEM) under weak scattering conditions at the edge of an opaque object. The tip-to-spectrum distance z1 plays the role of the objective lens defocus setting Δf in conventional TEM. The image magnification, effective source size, transverse coherence width, instrumental resolution, and source brightness are all obtained from an analysis of the fringe spacings and intensity. The quantum mechanical upper limit on source brightness, as well as relationships among beam brightness, coherence parameters, and degeneracy, are discussed, and the degeneracy measured from experimental Fresnel fringes.