The Bloch modes of a periodic slit array in a metallic slab are identified, then used to
investigate the transmission of light through sub-wavelength slits residing in a finite-thickness slab.
Specifically, the Bloch mode method is used here to study Fabry-Perot-like resonances within
individual slits, in conjunction with the onset of surface plasmon polariton (SPP) resonances and in
the vicinity of the Wood anomalies. Although the results largely agree with our earlier numerical
simulations obtained with the Finite-Difference-Time-Domain (FDTD) method, there are
indications that the FDTD method has difficulty with convergence at and around resonances; the
points of agreement and disagreement between the two methods are discussed in the present paper.
When the period p of the slit array is comparable to (or somewhat below) the incident wavelength
λo, the Bloch mode method requires only the 10-20 lowest-order modes of the slit array to achieve
stable solutions; we find the Bloch mode method to be an effective tool for studying dielectric-filled
apertures in highly conductive hosts.