2, Seoul National University, Seoul, , Korea (the Republic of)
Recently, multi-functional metasurfaces have been demonstrated based on polarization dependency, nonlinear optical effect and superposition of metasurfaces; nevertheless, conventional multi-functional metasurfaces have a severe limitation. We call them “coherent metasurfaces”. When incoherent light such as sunlight is shone on coherent metasurfaces, desired phase distribution cannot be developed; i.e., no information is obtained. In contrast, “incoherent metasurfaces” work only under incoherent light. They show colors or pseudo-holograms by controlling transmission or reflection spectra of incoherent optical waves. This incoherent response of the metasurfaces can be exploited for practical applications because incoherent light is more common than coherent light. However, no metasurface has achieved both coherent and incoherent functionality simultaneously.
Here we propose the first dual-mode metasurface that operates under both coherent and incoherent light simultaneously. The term “dual-mode” represents independent control of coherent response to transmitted light and of incoherent response to reflected light. Our dual-mode metasurface deploys parallel dielectric nanoantennas based on Pancharatnam-Berry phase to control spatial phase distribution of coherent light, and the reflection spectrum of incoherent light. Conventional metasurfaces based on Pancharatnam-Berry phase only control the orientation of each nanoantenna to manipulate phase distribution, but the reflection spectrum is also controllable by changing the sizes of nanoantennas. The nanoantenna sizes affect cross-polarization transmittance, so we find a pair of nanoantenna designs that have equal cross-polarization transmittance near the target wavelength of 635 nm. Based on the pair of designs, we design and experimentally demonstrate a crypto-display that contains encrypted information as an example of the dual-mode metasurface. Under incoherent white light the crypto-display works as a typical reflective display, whereas under coherent light, the encrypted information is revealed in the form of a hologram. Furthermore, the encrypted information does not affect the reflected image, so the information encoded in the crypto-display is not revealed unless coherent light is shone on it. Our device and design approach provide a way to develop novel security technologies such as steganography, anti-counterfeiting measures, and ghost imaging applications.