  Extraordinary optical transmission (EOT) is a phenomenon of exceptional light
transmission through a metallic film with hole arrays enhanced by surface
plasmon (SP) resonance, which stimulates renewed research hotspots in
metamaterials, subwavelength optics, and plasmonics. Below the frequency of the
first order SP mode, f_pl0, the metallic film typically shows strong reflection
and no EOT. Here, we report an unusual EOT phenomenon below fpl0, i.e., beyond
the long-held spectral boundary of classic EOTs. It is induced by a novel bound
surface state in a Fabry-Perot(F-P) cavity comprising a holey gold film and a
silicon-air interface. By tailoring the cavity length, EOT phenomenon has been
pushed deep into the sub-wavelength region by a factor of as large as 20%, and
EOT frequency comb with cavity function has been achieved. Due to the enhanced
slow-wave effect as the frequency approaches fpl0, the cavity induced EOT
gradually merges with the first order SP EOT. Distinguishing from the classic
EOT phenomenon, no transmission zero is found between these two EOTs, which
dramatically broadens the EOT bandwidth by a factor of 10 at terahertz (THz)
frequencies. Furthermore, the EOT transmittance is actively modulated with
graphene, achieving a large modulation range from 0.5 to 0.25 under a sub-volt
bias from -0.3 to 0.5 V at 500 GHz. To the best of the authors' knowledge, both
the modulation range and the low bias are among the best for active EOT devices
with graphene to date. Such a structure provides a new strategy for
miniaturizing sensing devices, high-power sources, and broadband photonics as
well as their active control in the THz regime.
