Physicists have found a new way to confine electromagnetic energy without it leaking using nonradiating anapole modes.  Nonradiating electromagnetic sources continue to be of interest as a model for stable atoms and to understand why orbiting electrons do not radiate, and have potential applications for combatting energy losses and explaining dark matter.   The radiationless anapole mode is achieved by dividing the current between two different components, a conventional electrical dipole and a toroidal dipole.  The radiation or far-field scattering is cancelled out if these two configurations are out of phase rendering the feature invisible.


Scientists Miroshnickenko and colleagues tested this theory with near-field characterization of single silicon nanodisks of 50nm height and 200nm-400nm diameter, which were made effectively invisible by cancelling the disc’s scattering of visible light.  The existence of an optical anapole mode in these nanodisks was investigated in both the far field and near field; the near-field characterization was critical for the device becomes invisible and thus undetectible in the far-field at the mode's excitation.   A spectral dip in the far-field spectrum was observed corresponding to the dark anapole mode excitation, while near-field distribution of the disks was mapped at different wavelengths.  The image on the right shows (a) far-field scattering spectra and (b) near field map for a 310nm diameter disk, where the far-field spectrum dip is most pronounced at 620nm.


Near field characterization was done with a CryoView MP that provides optical access from the top and from the bottom.  It enables easy integration with all conventional optical microscopes for near field measurements in transmission, true reflection and collection modes.  Both tip scanning and sample scanning are possible in the same scanning head which is especially important for the optical measurements described in this paper.  The sample was illuminated in the far field using a supercontinuum source while transmitted light was collected in near field with a Nanonics cantilevered NSOM probe.  with aperture diameter 50 nm during tip scanning.   The Nanonics MV 4000 scanning head together with unique Nanonics cantilevered NSOM probes are the best tools for optical characterization of nanodevices in the near field with nanometric resolution.



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