aberration-free imaging:

After 2π deflection the image returns exactly to its origin on the optical axis in the center of the magnetic deflector, independently on the sign of the starting angle. As a consequence, the object of filtration and its mirror symmetric image are invariant in the 2π deflection process. Additionally, the starting angles at the symmetry plane reverses the sign after 2p deflection, which indicates the mirroring- like effect after deflection by π radians. This mathematical analogy to the classical mirror operator is enriched by the further unique property of the α-SDA deflector: the direction of electron propagation before “reflection” at the symmetry (mirror) plane is conserved after the 2π deflection process is completed. As indicated previously, the electron beams at the entrance and output from the α-SDA are combined mathematically by the classical mirror operator. But contrary to the classical electrostatic mirror, the propagation direction on both side of the mirror plane is conserved i.e. energy filtered electron beam propagate along its initial direction also “behind” the mirror, which could be referred to as an unique “through the looking-glass” electron optical effect. Thus, the α-SDA imaging analyzer acts electron optically as a ”transparent” electrostatic mirror, exhibiting all of its advantages without the resignation of the beneficial linear geometry, i.e. the same PEEM alignment is used also after switching on the analyzer. The only difference is the energy filtered EF-PEEM real image that occurs now at the same screen. The α-SDA is the only one, unique example of the „transparent” electron optical mirror! Of course not only the real image, but also the reciprocal image undergoes the energy filtering, since both of them are created behind the dispersion aperture. Thus, the energy filtered k-pattern occurs in the second section of the imaging column behind the first transfer lens precisley at the location of the contrast aperture. It allows a bright or dark field real imaging without influencing the independent action of the second (“diffraction”) column.

transp mirroring effect1axial geometr aberr                                         chromatic aberr       image sim


link to the manuscript (click on the red title):

theoretical electron optical analysis

of the imaging spherical energy filter

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