Metal films grown on an Si wafer perforated with a periodic array of subwavelength holes have been fabricated and anomalous enhanced transmission in the midinfrared regime has been observed. High order transmission peaks up to $\mathrm{Si}(2,2)$ are clearly revealed due to the large dielectric constant contrast of the dielectrics at the opposite interfaces. The $\mathrm{Si}(1,1)$ peak splits at oblique incidence both in TE and TM polarization, which confirms that anomalous enhanced transmission is a surface-plasmon-polariton (SPP) assisted diffraction phenomenon. Theoretical transmission spectra agree excellently with the experimental results and confirm the role of SPP diffraction by the lattice.

• Received 18 December 2006

DOI:https://doi.org/10.1103/PhysRevLett.99.066805