Anomalous terahertz transmission through double-layer metal hole arrays by coupling of surface plasmon polaritons

The transmission characteristics of the double-layer metal hole array are investigated in the terahertz (THz) region. We measured the transmission spectra with varying the layer spacing and the lateral displacement between the two layers, and observed the unexpected transmission characteristics in the range of the layer spacing below the wavelength (lambda similar to 1 mm) of the THz wave. When the second layer is shifted laterally by a half of the lattice constant with respect to the first layer, the transmittance is enhanced by 3 times compared with that without shift. The finite difference time domain simulation is used to understand this anomalous transmission phenomenon, and it is attributed to the near-field coupling of the surface waves excited on the metal surfaces of both layers.