Enhanced transmission of THz radiation through subwavelength holes

We present measurements of the transmission of terahertz radiation through periodic arrays of holes made in highly doped silicon wafers. The size of the holes, $70\times 70\mu \mathrm{m},$ is significantly smaller than the radiation wavelength $\lambda \simeq 500\mu \mathrm{m}.$ Sharp resonances and enhanced transmissions are observed. This unusual transmission is attributed to the resonant tunneling of surface-plasmon polaritons that can be excited on doped semiconductors at terahertz frequencies.