Nonlinear Vectorial Metasurface for Optical Encryption

The unprecedented capability of the metasurface to manipulate light propagation is extended to the nonlinear optical regime, which enables a plethora of applications for nonlinear optical security and encryption. One of the key issues in this area is how to arbitrarily control the vectorial polarization profile of nonlinear waves. Here, we propose and experimentally demonstrate a nonlinear photonic metasurface that can generate a vectorial polarization profile for nonlinear imaging. The high-resolution grayscale image can be read out through a second harmonic generation (SHG) beam from an ultrathin nonlinear metasurface. The polarization of SHG waves can be locally manipulated by simply changing the orientation angle of the plasmonic meta-atoms. The grayscale image is revealed using a linear polarizer based on the Malus law. We believe that the rich polarization information based on nonlinear metasurfaces has significant potential for applications in nonlinear optical image encryption, security, anti-counterfeiting, and so on.