Information security is crucial for human society owing to its important role in protecting the properties and security of individuals, companies, governments, and other organizations, and thus, it has been a great concern for societies since ancient times. Among the technologies in this field, physical information security technologies possess a very high security level owing to the diversity and concealment of physical carriers and implementation principles. Currently, despite its security level, information capacity is a crucial aspect of information security technology owing to the rapid growth of information data in our society. Under these circumstances, photonic nanostructures have been introduced into physical information-security technologies. Apart from the high information capacity despite their small size, photonic nanostructures enable flexible nanoscale light-field manipulations using optical interference, plasmonic resonance, and Mie resonance, etc., providing a versatile platform for constructing various new physical information security techniques. In photonic nanostructure-based information technology, private information can be encrypted in various ways owing to the enhanced light-matter interactions and multiple degrees of freedom of light. For example, information can be encoded using light amplitude, wavelength, polarization, propagation direction, or their combinations. In addition, since the encoded information is interpreted differently from the directly perceived forms of its physical carriers, camouflages can be designed using photonic nanostructure-based information security technology to further improve concealment and security levels. A comprehensive review of this promising field can help us obtain an overview of its design principles, better understand its development trends, and develop new strategies for securing information.
We briefly review the background and development of physical information security technology based on photonic nanostructures and then illustrate its basic principles and recent progress. Three main types of physical information security technologies are introduced in detail: those based on surface structural color images, metasurface holography, and orbital angular momentum (OAM).
Regarding information security technology based on structural color images, private information can be hidden on a cover image, such that it cannot be identified without designed keys. This is implemented mainly by two mechanisms: one utilizing phase modulation inside nanostructures (Fig. 1) and the other utilizing the structural anisotropy of nanostructures (Fig. 2). The former hides information by modulating phases to control spectral shifts or spectrum regions, whereas the latter exploits the polarization-dependent response of anisotropic nanostructures, with the capability to construct multi-channel information encryptions. In addition, this type of technology can be realized by many other mechanisms (Fig. 3), such as the utilization of angle-dependent optical responses, reversible chemical reactions, and photoluminescence.
In information security technology based on metasurface holography, private information is encrypted into hologram images off the surfaces of physical carriers. Four typical strategies exist for realizing information encoding (Fig. 4): utilization of the resonance phase, geometric phase, propagation phase, and light amplitude. In metasurface holography-based technology, different freedoms of light, such as wavelengths, polarizations, and incident angles, can be designed as keys for information decryption in a single or combined manner to enable multi-channel information encryption (Fig. 5). In addition, tunable metasurfaces can be used in this type of information security technology to further improve security levels (Fig. 6). Furthermore, such holography-based technology can be combined with that based on structural color images, with the images on the surface working as camouflages and conveying real information encoded into the hologram images (Fig. 7).
In information security technology based on OAM, private information is encrypted using OAM, which theoretically possesses an infinite number of orthogonal charges, indicating unprecedented advantages for building super-multi-channel systems. An important aspect of this OAM-based technology is the design of photonic nanostructures that responds to OAM light with different charges, which can be realized using grating-based nanostructures, three-dimensional nanoparticle aggregates, or planar metasurfaces (Fig. 8). In particular, by applying the OAM to holography, a variety of highly secure encryption systems can be developed. For example, by exploiting a sampling array algorithm, multi-channel OAM encryption systems working in both linear and nonlinear regions have been achieved, and an OAM-encoded quantum holographic encryption system has also been developed (Fig. 9).
Information security technologies based on photonic nanostructures have provided numerous ways to construct high-security information systems and have generated enormous possibilities for the development of new information security techniques. These three types of nanostructure-based technologies exhibit both common and unique characteristics. For example, in both the technologies based on structural color and metasurface holograms, private information is interpreted in images. The former shows information on a physical surface and is easy to implement without complex optical systems, whereas the latter depicts information off the surface and is more flexible for designing complex keys involving different degrees of freedom of light. In the technology based on OAM, private information can be interpreted either in images or in a non-image manner, and in theory, its information capacity is extremely high owing to the infinite nature of the OAM charges.
The three types of security technologies involve various techniques and working mechanisms, the multiplexing of which is promising for developing new types of high-security information techniques. However, for the further development of photonic nanostructure-based technologies in real-world industries, some issues still need to be addressed, such as the complexity of implementation, concealment of information, and cost. Accordingly, lowering the complexity of implementation without sacrificing security can make the techniques more applicable in more scenarios, and improving the concealment of private information using the principle of steganography is valuable for protecting information from scrutiny. Regarding the issue of cost, it is of great importance to develop feasible low-cost fabrication techniques, such as nanoimprinting, self-assembling, high-resolution photolithography and the multiplexing of these techniques, other than high-cost techniques including electron beam lithography and focused ion beam etching. With the rapid development of modern nanoscience and nanotechnology, the reviewed technologies will eliminate the barriers hindering real-world scalable applications in the future and benefit the peace and prosperity of our society.
