  We investigate how a periodic array composed of achiral isotropic
high-refractive index dielectric nanospheres generates nearfield over the array
surface reaching helicity density very close to its upper bound. The required
condition for an array of nanospheres to generate optimally chiral nearfield,
which represents the upper bound of helicity density, is derived in terms of
array effective electric and magnetic polarizabilities that almost satisfy the
effective Kerker condition for arrays. The discussed concepts find applications
in improving chirality detection based on circular dichroism (CD) at surface
level instead of in the bulk. Importantly the array would not contribute to the
generated CD signal when used as a substrate for detecting chirality of a thin
layer of chiral molecules. This eliminates the need to separate the CD signal
generated by the array from that of the chiral sample.
