We have investigated the mechanism of enhanced absorption intensities of vibrational bands of adsorbates on copper meshes with subwavelength holes by measuring and simulating temporal profiles of infrared pulses transmitted through the meshes. As reported previously [Williams et al., J. Phys. Chem. B, 2003, 107, 11871], the absorption intensities of CH stretching bands of alkanethiolate adsorbed on the mesh increase substantially with decreasing hole size. The enhancements of absorption intensities are associated with temporal delays of infrared pulses transmitted through the mesh. Finite difference time domain calculations reproduce the observed pulse delays as a function of hole size. These facts indicate that the delays of transmitted pulses are not caused by coupling of infrared radiation to surface plasmon polaritons propagating on the front and rear surfaces of the mesh, but they are caused by the reduction in group velocity owing to coupling to waveguide modes of mesh holes. Consequently, the strong enhancements of the absorption intensities are attributed to adsorbates inside the holes rather than to those on the mesh surfaces that have been proposed previously.
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