  The terahertz spectral regime, ranging from about 0.1 to 15 THz, is one of
the least explored yet most technologically transformative spectral regions.
One current challenge is to develop efficient and compact terahertz
emitters/detectors with a broadband and gapless spectrum that can be tailored
for various pump photon energies. Here we demonstrate efficient single-cycle
broadband THz generation, ranging from about 0.1 to 4 THz, from a thin layer of
split-ring resonators with few tens of nanometers thickness by pumping at the
telecommunications wavelength of 1.5 micrometer (200 THz). The terahertz
emission arises from exciting the magnetic-dipole resonance of the split-ring
resonators and quickly decreases under off-resonance pumping. This, together
with pump polarization dependence and power scaling of the terahertz emission,
identifies the role of optically induced nonlinear currents in split-ring
resonators. We also reveal a giant sheet nonlinear susceptibility
$\sim$10$^{-16}$ m$^2$V$^{-1}$ that far exceeds thin films and bulk
non-centrosymmetric materials.
