Invisible Thin‐Film Patterns with Strong Infrared Emission as an Optical Security Feature

Abstract

blueshifts as described by the Planck's law. The broad spectrum is useful for illumination purposes in the visible and as well as in the infrared. On the other hand, spectrally selective thermal emission is desired for applications like thermophotovoltaics, [1][2][3] radiative cooling, [4,5] sensing, [6][7][8] and near-infrared communication [9,10] by modifying the emissivity of the surfaces. The emissivity of a surface is equal to its absorbance in thermal equilibrium according to the Kirchhoff's law of thermal emission, ranging from 0 for a perfect mirror to 1 for a perfect absorber or a blackbody. A simple approach to modify the emissivity is finding a dielectric with phonon bands at the desired absorption wavelength range. A recent example is embedding glass microspheres in an infrared transparent polymer matrix for radiative cooling. [11] The dielectric dimensions can be as thin as micrometers when a back mirror, typically an optically thick metal layer is employed. This approach has been preferred for selective thermal emission since early 1980s, e.g., using ≈1 µm thick silicon monoxide on Al for radiative cooling. Owing to the simplicity of the structure, surfaces consisting of polar-dielectric-coated mirrors are still popular. [12] Stacking more than one type of polar dielectric layers broadens the absorption bands, that is especially useful for radiative cooling. [13] Using single [14] or multiple [1,15] lossless, that is infrared transparent, layers together with polar dielectrics introduce additional design parameters, hence offers better thermal-emission tunability.Despite the simplicity that the polar dielectrics offer, their emission spectra are limited with the absorption bands. An arbitrary emission spectrum can be achieved by optical resonators such as Salisbury screens that consist of a lossless dielectric layer between a bottom mirror and a thin lossy layer on the top. [16][17][18][19] The absorbance wavelengths (resonances) are mainly determined by the optical thickness of the lossless dielectric layer. Alternatively, the lossy dielectric can be coated directly on the mirror layer exhibiting strong interference effects. [20][21][22] The absorbance wavelengths in this case are primarily determined by the optical properties of the lossy layer and its thickness. The strong interference surfaces consist of a simple bilayer structure, but exhibit broad absorbance spectra. Nanostructured films of metal or lossy materials, e.g., micrometerscale gratings, [23] photonic crystals, [24] antennas, [15,25,26] provide a better control over the emission spectrum at the expense of the ease of fabrication. The emitter design can be as complex Spectrally selective thermal emission is in high demand for thermophotovoltaics, radiative cooling, and infrared sensing applications. Spectral control of the emissivity is historically achieved by choosing the material with suitable infrared properties. The recent advancements in nanofabrication techniques that lead to enhanced light-matter interactions enable optical pr...

Tunable Infrared Emissivity in Multilayer Graphene by Ionic Liquid Intercalation

“…All matter at a temperature above absolute zero emit infrared waves, the intensity of which is dependent on the temperature (T) and infrared emissivity (ε) of the surface materials [1]. This is widely used nowadays for night vision [2], infrared source [3,4], and temperature measurement [5]. Since thermal cycling is usually quite slow [6], mere temperature control could not satisfy the need for the further development of the infrared source [3,4], thermal management [7], and thermal camouflage [6,7,8].…”

Functional Micro–Nano Structure with Variable Colour: Applications for Anti-Counterfeiting

“…(d) Photograph and thermal image of alternative mirror layers. Adapted from [198]. SERS image of a panda was shown visibly with vibrational modes at 1078 cm −1 while invisibly at 1200 cm −1 .…”

“…e code of "KIST" could not be visually or microscopically identified, however, it could be seen with green luminescence by irradiation of NIR light. Bakan et al [198] constructed invisible patterns by ultrathin dielectrics which was transparent in the visible and exhibited strong infrared absorption in the spectral range of thermal cameras (see Figure 9(d)). To expand the obtained selectively by 365 nm UV light, then converted to a fully wrinkled surface.…”

Spectrally Tailored Light‐Matter Interaction in Lithography‐Free Functional Nanomaterials

“…Such cavities can be useful to design solar reflectors with efficient radiative cooling properties. As a second example, by tuning layer thicknesses, SiO 2 /CaF 2 /Al mirror cavities can at the same time reflect efficiently visible light and show a strong absorbance/emissivity in the mid‐infrared . These cavities are used as an encryption medium on which imprinted data are invisible to the eye but revealed by a thermal camera.…”