(a) Terahertz pulse is incident onto the ring-shaped nano-slot antenna structure. (b-d) Top-view of SEM images for ring-shaped nano-slots. (gold thickness h = 8.2 nm; gap width w = 340 nm; lx = ly = 50 μm; px = py = 100 μm). (e) Normalized transmission amplitude and field enhancement spectra of the nanoslot array. The direct transmission of film is subtracted from slot array data. Field enhancement factor reaches 44 at 0.5 THz in MPTMS-coated sample, whereas uncoated sample has no distinct peak.