Down the Nanohole: Probing the Nearfield Phase Distortions of a Nanohole via Relativistic Electrons

In order to map the nearfield and probe its response, we create a nearfield by illuminating one side of a single subwavelength aperture with continuous wave laser light, inside a Scanning Electron Microscope (SEM). We position a Fibre Optic termination adjacent to the hole (on the electron beam side) for light collection purposes and focus the SEM electron beam directly into the subwavelength aperture.

Scanning relativistic, accelerated electrons across the nanohole aperture permits probing the phase and field distribution of the optical nearfield, to which end we spectroscopically examine the resulting scattered photons.

By driving the electron velocity at various accelerating voltages (1 – 39 kV) we can examine any Doppler shift introduced (Normal or Anomalous). The input light is from a highly coherent source and is monitored via spectrometer, while the collected scattered photons are analysed via spectrometer and cooled CCD over some integration period.

The work was prompted originally by Engheta et al, [1] which shows that a curious inverse Doppler shift occurs as an observer approaches the nearfield, an experimental validation of which is further sought herein. This may lead to knowledge of the nearfield polarisation and range, as well as lead to determining the velocity of particles entering it.

A fibre optic collects light scattered by an electron beam entering the nearfield

[1] Engheta et Al. IEEE Transactions on Antennas and Propagation, vol.AP-28, No. 4, July 1980

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