Optically variable devices (OVDs) are in tremendous demand as optical indicators against the increasing threat of counterfeiting. Conventional OVDs are exposed to the danger of fraudulent replication with advances in printing technology and widespread copying methods of security features. Metasurfaces, two-dimensional arrays of subwavelength structures known as meta-atoms, have been nominated as a candidate for a new generation of OVDs as they exhibit exceptional behaviors that can provide a more robust solution for optical anti-counterfeiting. Unlike conventional OVDs, metasurfacedriven OVDs (mOVDs) can contain multiple optical responses in a single device, making them difficult to reverse engineered. Well-known examples of mOVDs include ultrahighresolution structural color printing, various types of holography, and polarization encoding. In this review, we discuss the new generation of mOVDs. The fundamentals of plasmonic and dielectric metasurfaces are presented to explain how the optical responses of metasurfaces can be manipulated. Then, examples of monofunctional, tunable, and multifunctional mOVDs are discussed. We follow up with a discussion of the fabrication methods needed to realize these mOVDs, classified into prototyping and manufacturing techniques. Finally, we provide an outlook and classification of mOVDs with respect to their capacity and security level. We believe this newly proposed concept of OVDs may bring about a new era of optical anticounterfeit technology leveraging the novel concepts of nano-optics and nanotechnology.

affects both its electrical conductivity and its absorption spectrum. These changes have obvious application as a sensor of relative humidity (RH).Structural colors found in nature [9] have received great scientific interest and been reproduced by diverse structural colors. [10][11][12] The simplest structural coloration mechanism consists of planar multilayered arrangements. [13][14][15][16][17] In particular, the metal-insulator-metal (MIM) configuration based on a Fabry-Pérot resonator has a high quality factor efficient band-pass filtering with scalability and cost-effective fabrication. [18][19][20] However, conventional planar MIM resonators lack tunable function, because the resonance depends only on the geometry and optical parameters of the insulating layer.Structural colors have been tuned using chemical, [21][22][23][24][25][26] mechanical, [27,28] or electrical stimuli, [29,30] polarization, [31,32] and phase-change materials. [33] However, tunable structural colors have a slow response, [21] complicated mechanism [27,28,33] and insufficient dynamic change. For those reasons, practical application of structural colors has been limited.In this work, we propose tunable color filter composed of MIM multilayer, in which the insulator is chitosan hydrogel. This color filter can serve as a humidity sensors when combined with a photovoltaic (PV) cell. The structure uses chitosan film sandwiched between two ultrathin silver (Ag) layers deposited on a glass substrate. The key element is the chitosan insulating layer, in which the effective optical thickness t eff and refractive index n c change in response to RH; this trait can be exploited to obtain optical tunability of the resonance wavelengths. The corresponding resonance peak shift induces output current change of a PV cell, which is proportional to a change in the RH value of the environment. The special features of the proposed sensor are simple development, incorporation into PV cell, and potentially zero power usage, that make it a promising material for devices that monitor RH in enclosed spaces, workplaces and storage areas. Results and Discussion Transfer-Matrix Method (TMM) Simulation of Ag-Chitosan-Ag Multilayer StructuresWe present a tunable MIM bandpass filter in which the sensitive insulating layer is composed of chitosan, which can adsorb A tunable Fabry-Pérot resonator is realized using metal-insulator-metal structure, in which the insulator is chitosan hydrogel. The chitosan swells in response to changes in relative humidity; this change affects transmissive structural color of the multilayer structure. This tunable resonator is utilized for a humidity sensor combined with a photovoltaic cell. The change in current through the photovoltaic cell provides rapid precise measurement of relative humidity, and the change in color of the multilayer provides an approximate, remotely-readable estimate. The response requires no power, so the device has numerous sensing applications.

The development of real-time and sensitive humidity sensors is in great demand from smart home automation and modern public health. We hereby proposed an ultrafast and full-color colorimetric humidity sensor that consists of chitosan hydrogel sandwiched by a disordered metal nanoparticle layer and reflecting substrate. This hydrogel-based resonator changes its resonant frequency to external humidity conditions because the chitosan hydrogels are swollen under wet state and contracted under dry state. The response time of the sensor is ~10 4 faster than that of the conventional Fabry-Pérot design. The origins of fast gas permeation are membrane pores created by gaps between the metal nanoparticles. Such instantaneous and tunable response of a new hydrogel resonator is then exploited for colorimetric sensors, anti-counterfeiting applications, and high-resolution displays.

Structural coloration using metasurfaces has been steadily researched to overcome the limitations of conventional color printing using pigments by improving the resolution, lowering the toxicity, and increasing the durability. Many metasurfaces have been demonstrated for dynamic structural coloration to convert images at the visible spectrum. However, the previous works cannot reach near-zero scattering when colors are turned-off, preventing it from being cryptographic applications. Herein, we propose a completely on/off switchable structural coloration with polarization-sensitive metasurfaces, enabling full-colored images to be displayed and hidden through the control of the polarization of incident light. It is confirmed that the nanostructure exhibits the polarization-dependent magnetic field distributions, and near-zero scattering is realized when the polarization of incident light is perpendicular to the long axis of the nanofins. Also, the metasurfaces are made up of triple-nanofin structures whose lengths affect locations of resonance peaks, resulting in full-color spectrum coverages. With such advantages, a QR code image, a two-color object image, and an overlapped dual-portrait image are obtained with the metasurfaces. Such demonstrations will provide potential applications in the fields of high-security information encryption, security tag, multichannel imaging, and dynamic displays.

Metasurface holography, the reconstruction of holographic images by modulating the spatial amplitude and phase of light using metasurfaces, has emerged as a next‐generation display technology. However, conventional fabrication techniques used to realize metaholograms are limited by their small patterning areas, high manufacturing costs, and low throughput, which hinder their practical use. Herein, a high efficiency hologram using a one‐step nanomanufacturing method with a titanium dioxide nanoparticle‐embedded‐resin, allowing for high‐throughput and low‐cost fabrication is demonstrated. At a single wavelength, a record high theoretical efficiency of 96.9% is demonstrated with an experimentally measured conversion efficiency of 90.6% and zero‐order diffraction of 7.3% producing an ultrahigh‐efficiency, twin‐image free hologram that can even be directly observed under ambient light conditions. Moreover, a broadband meta‐atom with an average efficiency of 76.0% is designed, and a metahologram with an average efficiency of 62.4% at visible wavelengths from 450 to 650 nm is experimentally demonstrated.