Abstract
Near-field focusing properties of perfect black Fresnel zone plates
are analyzed by solving Maxwell equations using the vectorial angular spectrum
representation and the equivalent Rayleigh diffraction formulae of the first
kind given by the Luneburg vector diffraction theory. Linear and circular
polarizations are compared. As a result, an exact analytical expression for
axial intensity is obtained showing that, in the near field, the shift of
focal length is inherent in the diffraction process and independent of incident
field polarization. The numerical intensity distribution of the electromagnetic
field is also obtained, which allows us to demonstrate that when vectorial
analysis of the system is performed, subwavelength focusing cannot be achieved
for very short focal distances. This result is valid for linearly and circularly
polarized incident fields. Finally, it is shown that symmetry of the focalized
field is strongly dependent on the polarization of the incident field and
that the vectorial diffraction theory can be a good alternative to finite-difference
time-domain methods.
© 2011 IEEE
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