Long-wavelength limit and Fano profiles of extraordinary transmission through metallic slit gratings in the THz range

Transmission measurements through thin metallic slit gratings deposited over a dielectric substrate are presented in the THz range below and above the limit at which radiative diffraction occurs. The results are analyzed owing to numerical and analytical modeling based on the modal method, especially in the long-wavelength limit. Rayleigh-Wood anomalies and extraordinary transmission are observed and explained by a pure interference mechanism, which gives rise to Fano profiles at discrete mode wavelengths over a diffraction background fed by evanescent diffraction orders.