We investigated the optical transmission of periodic narrow slit arrays in thick metal films with finite-difference time-domain simulations. Transmission spectra of ideal slit arrays of various structure parameters (film thickness, period, slit width) were calculated, and compared to those of single slits with the same width in metal films of the same thickness. It is observed that the transmission of individual slits are determinative to the transmission pass-bands of the slit arrays; while the arrayed slit structures induce surface plasmon resonances, they also have negative roles on transmission. We propose that the in-plane surface plasmon resonances at the metal-dielectric interfaces are based on the Bloch waves of surface plasmons rather than the grating-assisted phase matching conditions. It is then concluded that the main features of the transmission spectra of periodic narrow slit arrays result from superposition of transmission valleys induced from in-plane surface plasmon resonances on the transmission spectra of corresponding individual slits.

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