Fig. 1. (a) THG imaging based on Si membrane metasurfaces: the near-infrared signal passes through the target, then being converted into the visible signal via the metasurfaces, forming the target image on the CCD camera. (b1–e1) The images of the target and the metasurfaces under white light source illumination. (b2–e2) Transformed visible images of the target via membrane metasurfaces under NIR light illumination.¹³