Observation of a new type of THz resonance of surface plasmons propagating on metal-film hole arrays

Highly conducting metal-film subwavelength hole arrays, lithographically fabricated on high-resistivity silicon wafers in optical contact with thick silicon plates, have been characterized by terahertz time-domain spectroscopy with subpicosecond resolution and over a frequency range from 0.5 to 3 THz with 5 GHz resolution. A well-defined ringing structure extending to more than 250 psec is observed on the trailing edge of the transmitted THz pulse. In the frequency domain this ringing structure corresponds to a new type of extremely sharp resonant line structure between the fundamental surface plasmon modes of the hole array. A simple theoretical model is presented and shows good agreement with the experimental data.