  In superconducting thin films, engineered lattice of antidots (holes) act as
an array of columnar pinning sites for the vortices and thus lead to vortex
matching phenomena at commensurate fields guided by the lattice spacing. The
strength and nature of vortex pinning is determined by the geometrical
characteristics of the antidot lattice (such as the lattice spacing $a_0$,
antidot diameter $d$, lattice symmetry, orientation, etc) along with the
characteristic length scales of the superconducting thin films, viz., the
coherence length ($\xi$) and the penetration depth ($\lambda$). There are at
least two competing scenarios: (i) multiple vortices sit on each of the
antidots at a higher matching period, and, (ii) there is nucleation of vortices
at the interstitial sites at higher matching periods. Furthermore it is also
possible for the nucleated interstitial vortices to reorder under suitable
conditions. We present our experimental results on NbN antidot arrays in the
light of the above scenarios.
