Multispectral Imaging with a Planar Cavity-Type Metasurface for Optical Security

Abstract

Multispectral imaging refers to capturing images in different wavelength ranges across the electromagnetic spectrum. Despite the potential impact of multispectral imaging, its widespread use has been limited by the poor spectral selectivity of naturally occurring materials beyond the visible range. In this study, we present a multilayered planar cavity structure to simultaneously record mutually independent visible and infrared (IR) images on solid surfaces. The structure consists of a color control unit (CCU) and an emission control unit (ECU). The visible color of the cavity is controlled by varying the thickness of the CCU, whereas its IR emission is spatially tuned by the laser-induced phase change of a Ge2Sb2Te5 layer embedded in the ECU. Because the CCU comprises only IR lossless layers, its thickness variation has negligible influence on the emission profile. This enables different color and thermal images to be printed in a single structure. The cavity structure can be fabricated on flexible substrates (plastic and paper) as well as rigid bodies. Furthermore, the printed images remain stable against bending. This study shows that the proposed multispectral metasurface is highly promising for use in the field of optical security, such as identification, authentication, and anti-counterfeiting.

Laser Dynamic Control of the Thermal Emissivity of a Planar Cavity Structure Based on a Phase-Change Material

“…Meanwhile, the recording of independent visible and thermal images in a single cavity structure has been reported. 10,49 As multispectral imaging provides a robust platform for optical security, the integration of dynamic emission control and structural coloration is an intriguing research topic.…”

“…3D control would be possible with a focused femtosecond laser, as its ultrafast characteristics enable a nonthermal interaction in localized regions through multiphoton absorption. Meanwhile, the recording of independent visible and thermal images in a single cavity structure has been reported. , As multispectral imaging provides a robust platform for optical security, the integration of dynamic emission control and structural coloration is an intriguing research topic.…”

“…An object at an absolute temperature ( T ) above zero emits infrared (IR) rays; this phenomenon is typically called “thermal emission” or “thermal IR radiation.” The Stefan–Boltzmann law states that the energy ( E ) emitted from an object is proportional to its surface emissivity (ε) and temperature: E = σε T 4 , where σ denotes the Stefan–Boltzmann constant. Materials with tailored emissivity can be widely applied, such as in personal thermal management, IR camouflage, development of energy-saving windows, radiative cooling, thermophotovoltaics, optical security, and multispectral imaging. − Long-wave IR light in the wavelength range of 8–13 μm is significantly less absorbed by the atmosphere than other IR wavelengths, and typical IR cameras detect radiation in this atmospheric window range. A blackbody in the temperature range of T = 300–400 K emits IR rays at 5–30 μm wavelengths, with an emission peak located at ∼10 μm.…”

Recent progress on structural coloration

“…Among these functionalities, the realization of two-dimensional or threedimensional holographic image reconstruction through phase modulation has garnered significant attention in display technology [252][253][254] . To extend the applications to areas such as advanced optical encryption and higher-density optical information storage, it becomes crucial to simultaneously modulate spatial phase and spectral response within a single-layer metasurface [255][256][257][258][259][260][261] .…”