  Plasmonic metamaterials and metasurfaces offer new opportunities in
developing high performance terahertz emitters and detectors beyond the
limitations of conventional nonlinear materials. However, simple meta-atoms for
second-order nonlinear applications encounter fundamental trade-offs in the
necessary symmetry breaking and local-field enhancement due to radiation
damping that is inherent to the operating resonant mode and cannot be
controlled separately. Here we present a novel concept that eliminates this
restriction obstructing the improvement of terahertz generation efficiency in
nonlinear metasurfaces based on metallic nanoresonators. This is achieved by
combining a resonant dark-state metasurface, which locally drives nonlinear
nanoresonators in the near field, with a specific spatial symmetry that enables
destructive interference of the radiating linear moments of the nanoresonators,
and perfect absorption via simultaneous electric and magnetic critical coupling
of the pump radiation to the dark mode. Our proposal allows eliminating linear
radiation damping, while maintaining constructive interference and effective
radiation of the nonlinear components. We numerically demonstrate a giant
second-order nonlinear susceptibility around Hundred-Billionth m/V, a one order
improvement compared with the previously reported split-ring-resonator
metasurface, and correspondingly, a 2 orders of magnitude enhanced terahertz
energy extraction should be expected with our configuration under the same
conditions. Our study offers a paradigm of high efficiency tunable nonlinear
metadevices and paves the way to revolutionary terahertz technologies and
optoelectronic nanocircuitry.

Optics;Mesoscale and Nanoscale Physics