Aberration Corrected Photoemission Electron Microscopy with Photonics Applications

Abstract

Photoemission electron microscopy (PEEM) uses photoelectrons excited from material surfaces by incident photons to probe the interaction of light with surfaces with nanometer-scale resolution. The point resolution of PEEM images is strongly limited by spherical and chromatic aberration. Image aberrations primarily originate from the acceleration of photoelectrons and imaging with the objective lens and vary strongly in magnitude with specimen emission characteristics. Spherical and chromatic aberration can be corrected with an electrostatic mirror, and here I develop a triode mirror with hyperbolic geometry that has two adjacent, field-adjustable regions. I present analytic and numerical models of the mirror and show that the optical properties agree to within a few percent. When this mirror is coupled with an electron lens, it can provide a large dynamic range of correction and the coefficients of spherical and chromatic aberration can be varied independently. I report on efforts to realize a triode mirror corrector, including design, characterization, and alignment in our microscope at Portland State University (PSU). PEEM may be used to investigate optically active nanostructures, and we show that photoelectron emission yields can be identified with diffraction, surface plasmons, and dielectric waveguiding.Furthermore, we find that photoelectron micrographs of nanostructured metal and dielectric structures correlate with electromagnetic field calculations. We conclude that photoemission is highly spatially sensitive to the electromagnetic field intensity, allowing the direct visualization of the interaction of light with material surfaces at nanometer scales and over a wide range of incident light frequencies.ii

Photoemission Electron Microscopy for Analysis of Dielectric Structures and the Goos-Hänchen Shift

“…Comparing the results given by Eqs. (32)(33)(34)(35)(36), we see it is precisely identical to the equations obtained in the plane wave expansion method.…”

“…The aperture angle is directly related to the maximum obtainable resolution based on the diffraction limit [37]. The 30 µm aperture in our system has an aperture angle of approximately 0.4°, which gives a theoretical maximum achievable resolution on the order of a few nanometers [36]. Currently PEEM is far from this resolution, with aberration being the limiting factor rather than the diffraction limit.…”

“…Aberrations can theoretically be reduced even further by going from a diode mirror to a three or four electrode mirror giving more freedom to adjusting the parameters [35,36].…”

“…From this profile the distance it take the field to decay by 1/e divided by the scale factor q from Eqs. (35)- (36) gives the penetration. The penetration depth then produces a shift through simple geometry of ray tracing as in Eq.…”