  We present a symmetry-based theory of the depolarization induced by
subwavelength metal hole arrays. We derive the Mueller matrices of hole arrays
with various symmetries (in particular square and hexagonal) when illuminated
by a finite-diameter (e.g. gaussian) beam. The depolarization is due to a
combination of two factors: (i) propagation of surface plasmons along the
surface of the array, (ii) a spread of wave vectors in the incident beam.
