  Large surface plasmon polariton assisted enhancement of the magneto-optical
activity has been observed in the past, through spectral measurements of the
polar Kerr rotation in Co hexagonal antidot arrays. Here, we report a strong
thickness dependence, which is unexpected given that the Kerr effect is
considered a surface sensitive phenomena. The maximum Kerr rotation was found
to be -0.66 degrees for a 100 nm thick sample. This thickness is far above the
typical optical penetration depth of a continuous Co film, demonstrating that
in the presence of plasmons the critical lengthscales are dramatically altered,
and in this case extended. We therefore establish that the plasmon enhanced
Kerr effect does not only depend on the in-plane structuring of the sample, but
also on the out-of-plane geometrical parameters, which is an important
consideration in magnetoplasmonic device design.
