We demonstrate experimentally a nonlinear plasmonic metasurface that exhibits strongly asymmetric second-harmonic generation (SHG), depending on the illuminating direction: when the metasurface is illuminated from one side, it produces a significant SHG signal, whilst this nonlinear response is strongly reduced (approx. 10 dB), upon illumination from the opposite side. This surprising behavior stems from the bianisotropic response of the system, as confirmed by a homogenization analysis and the extraction of the effective susceptibility tensor. At first sight, it could be tempting to interpret this asymmetric response as a non-reciprocal phenomenon, but we will show that actually it is time-reversal asymmetric.
|