Frozen mode from hybridized extraordinary transmission and Fabry-Perot resonances

Frozen modes arising in stacked subwavelength hole arrays are studied in detail. Their origin is proved to be connected with the interaction between the extraordinary transmission resonance and the Fabry-Perot cavity mode. The analysis is done for various situations that differ in metal plate thicknesses and sizes and shape of the holes. Dispersion results and finite-stack transmission spectra are in good agreement, both showing the features indicating hybridization. The boundaries of the hybridization are found in terms of the geometrical parameters. The effect of the number of stacked plates on the transmission has been demonstrated. Finally, it is shown that the group index of refraction n_g in the considered finite structures can be larger than 200. The obtained estimates of n_g, which are based on dispersion and transmission results, well coincide with each other.