Forgeries could become next to impossible with a two-level encrypted holographic security device, built by researchers at Pohang University of Science and Technology (POSTECH). The platform for the technology is a bifunctional, pixelated metasurface that allows several properties of light, including wavelength, phase, and polarization, to be controlled by the same device. The bifunctionality of the metasurface comes from the bifunctional meta-atom in the material that acts as both a resonator and a waveguide, allowing the multiplexing of colors and phase at each spatial location in the metasurface. The pixilation in the metasurface was created by grouping meta-atoms together. This allowed the researchers to assign arbitrary polarization states to the output beam, which in turn allowed them to engineer multiple properties of light using the metasurface. Vectorial hologram image generation. Each numerical image has a unique polarization state, allowing selective information to be obtained through additional optical devices. Courtesy of POSTECH. The metasurface-based security device produces structural color printing and vectorial holography. In the two-step verification process, the structural color prints are observed under white light and the holograms are reconstructed under laser illumination. As the device creates a vectorial hologram label that displays a QR code containing a variety of colors that can be viewed with the naked eye or scanned with a camera, the device, under laser illumination, renders a polarization-encoded 3D holographic image. This image has a unique polarization state, and can be selectively turned on or off with a liquid crystal analyzer to make only certain encrypted numbers visible. The vectorial holograms can use multiple polarization channels, and each hologram retains a specific polarization to set it apart from previously rendered holograms. Users scan the QR code from the meta-optical device with a smartphone to generate a password composed of random numbers. When this password is applied to the meta-optical device as a voltage value, the second password is displayed as an encrypted holographic image. As a proof-of-concept, the researchers designed an optical security platform into which liquid crystals were incorporated. The researchers decrypted an image under white light to provide the first key. They used the corresponding information to fully unlock the encrypted information via projected vectorial holographic images. This electrically tunable photonic security platform is an optical version of the two-level, encrypted, one-time password (OTP) security mechanism used by banks and merchants, and could be a significant advance for two-level encryption; the color image must be decrypted to provide a key. This key must be used to fully unlock the encrypted information available through the projected vectorial holographic images. The metasurface-based security device could be used in optical sensors, holographic displays, security, and anticounterfeiting applications. Vectorial holographic color prints. With the naked eye, the device displays a two-colored QR code image, but with laser illumination, the encrypted vectorial hologram image is projected in space. This holographic image with a unique polarization state can be selectively turned on or off via a liquid crystal analyzer, which can make only certain encrypted numbers visible. Courtesy of POSTECH. According to the researchers, existing metasurface devices can modulate only one property of light. “This vectorial holographic color printing platform is more advanced than the metasurface devices reported so far, and has demonstrated that various degrees of freedoms of light can be modulated with one optical device,” professor Junsuk Rho, who led the research, said. “It is a highly perfected optical OTP device that promises to become an original optical encryption technology, applicable in designing and analyzing meta-atoms.” The research was published in Nature Communications (www.doi.org/10.1038/s41467-021-23814-5).