Extraordinary Transmission of Bull’s Eye Structure Observed by Up-Conversion-Based THz-Wave Frequency-Domain Spectroscopy

Using optical up-conversion-based THz-wave frequency-domain spectroscopy, we observed the extraordinary transmission of a bull’s eye structure fabricated by a micromachining technique. The fabricated bull’s eye structure, which consisted of a circular aperture of 100-$\mu$m diameter and concentric periodic grooves of 200-$\mu$m periodicity and 20-$\mu$m depth, showed the surface plasmon-enhanced transmission peak at 1.48 THz and the subwavelength-scale confinement of the transmitted THz-wave beam.