# Nonlinearity in the Dark: Broadband Terahertz Generation with Extremely High Efficiency

## TL;DR

This paper introduces a novel nonlinear metasurface design that significantly enhances terahertz generation efficiency by eliminating linear radiation damping through destructive interference and perfect absorption, enabling high-performance terahertz devices.

## Contribution

The study presents a new metasurface concept combining dark-state resonances with symmetry control to drastically improve nonlinear terahertz emission efficiency.

## Key findings

- Achieved a giant second-order nonlinear susceptibility (~100 billionth m/V).
- Demonstrated a two-order magnitude increase in terahertz energy extraction.
- Proposed a paradigm for high-efficiency, tunable nonlinear metadevices.

## Abstract

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.