Dear Kaleem,

A maximum magnification achieved by Bessel Beam Microscopy is up to 1000X. The degree of magnification achieved by the electron Bessel beams  could be improved simply by increasing krough, due to the corresponding decrease in beam central spot size. However, increases in krough are met by corresponding increases in the focal point of the first diffracted order. In order to ensure that the beam remains focused on the sample, it then becomes necessary to compensate for this effect by reducing lens excitation, therefore decreasing the demagnification factor of the system. The compensatory measures designed to re-focus the electron beam therefore ultimately cancel any benefit that might otherwise arise from an increase in the transverse wavenumber krough, This limitation could certainly be overcome by a more flexible illumination scheme, however. increasing the size of the hologram aperture would not improve the quality of images obtained in the configuration. The beam convergence is not determined by the size of the aperture in the Fresnel regime.

For more on this topic, please see the following publication:

Generation and Application of Bessel Beams in Electron Microscopy

Vincenzo Grillo, Jérémie Harris , Gian Carlo Gazzadi, Roberto Balboni

Erfan Mafakheri, Mark R. Dennis, Stefano Frabboni, Robert W. Boyd, Ebrahim Karimi

Abstract

We report a systematic treatment of the holographic generation of electron  Bessel beams, with a view to applications in electron microscopy. We describe in detail the theory underlying hologram patterning, as well as the actual electrooptical configuration used experimentally. We show that by optimizing our nanofabrication recipe, electron Bessel beams can be generated with efficiencies reaching 37±3%. We also demonstrate by tuning various hologram parameters that electron Bessel beams can be produced with many visible rings, making them ideal for interferometric applications, or in more highly localized forms with fewer rings, more suitable for imaging. We describe the settings required to tune beam localization in this way, and explore beam and hologram configurations that allow the convergences and topological charges of electron Bessel beams to be controlled. We also characterize the phase structure of the Bessel beams generated with our technique, using a simulation procedure that accounts for imperfections in the hologram manufacturing process. Finally, we discuss a specific potential application of electron Bessel beams in scanning transmission electron microscopy.

https://arxiv.org/ftp/arxiv/papers/1505/1505.07815.pdf

Hoping this will be helpful,

Rafik

@Rafik Karaman thanks sir a lot for your attention and answer, actually i am asking w.r.t the optical point of view, imaging of near field signals into the far field region, because BBM is one of the established technology which can image the possible  NF information into the FF region in a versatile way. Although there are other microscopes like PALM, STORM, RESOLFTs and SSIM etc which can image beyond the diffraction limit but they are only for flourescent samples.