  A planar Fabry-Perot cavity with inter-mirror spacing significantly shorter
than the probing wavelength is explored for its "zero-order mode" terahertz
transmission. The enhanced transmission observed as d approaches 0 indicates
that such cavities satisfy the resonance condition across a broad terahertz
bandwidth. The experimental signatures from this elusive, "technically
challenging" regime are evidenced using time-domain terahertz spectroscopy and
are complemented by numerical calculations. The results raise intriguing
possibilities for terahertz field modulation and pave new paths for strong
coupling of multiple transition frequencies simultaneously.
