Nonlinear Metasurfaces

Nonlinear metasurfaces based on coupling a locally enhanced plasmonic response to intersubband transitions of n-doped multi-quantum-wells (MQWs) have recently provided second-order susceptibilities orders of magnitude larger than any other nonlinear flat structure measured so far. In order to fully understand these nonlinear structures, we proposed a comprehensive theory – based on the effective susceptibility approach – to characterize their electromagnetic response, providing a homogeneous model that takes phase-matching at the unit-cell level and the influence of saturation and losses into account. In addition, we derive the limits imposed by saturation of the MQW transitions on the nonlinear response of these metasurfaces, revealing useful guidelines to design devices with enhanced performance. These guidelines have been successfully applied to the design and experimental characterization of highly efficiency (>0.07%) MQW-based nonlinear metasurfaces with second-order nonlinear susceptibility over 106 pm/V, which is the largest nonlinearity reported to date from any condensed matter system in the THz-infrared spectral range and 5-6 orders of magnitude larger than traditional nonlinear crystals. Current efforts allows to extend these concepts to other nonlinear processes, such as differential frequency generation, aiming to achieve highly efficient THz radiation in room temperature, compact, devices.  

From the practical point of view, it would be highly desirable to simultaneously control the amplitude and phase of the generated nonlinear wavefront. To this purpose, we have extended the Pancharatnam-Berry (PB) concept to operate in the nonlinear regime. This approach – common in linear devices - introduces a topological phase difference between transmitted (or reflected) waves by simple changing the orientation of adjacent unit-cells. Applying this powerful concept to MQW-based nonlinear plasmonic metasurfaces allows to achieve advanced functionalities such as light bending and focusing, while simultaneously providing conversion efficiencies several orders of magnitude larger than any other planar nonlinear configuration.