Highly Flexible Infrared Emitter with Spatially Controlled Emissivity for Optical Security

Engineering the infrared (IR) emissivity of a material or structure is crucial for a variety of fields, including thermal camouflage, radiative cooling, personal thermal management, and optical security. For practical and wide-spread use, the emitter should possess high mechanical flexibility while maintaining the capabilities of space-selectively and dynamically controlling its emissivity. In this paper, an optical resonator consisting of a Ge₂Sb₂Te₅ (GST) layer on top of a thin metal reflector stationed on a flexible substrate is presented as an IR emitter that satisfies the aforementioned requirements. A laser-induced phase change from amorphous to crystalline GST enables dynamic tuning of the local emissivity of the resonator. This study illustrates that although GST is a brittle material, the emitters fabricated on plastic and paper substrates are highly robust against bending up to a radius of curvature of 0.5 cm. Moreover, visible light and IR images can be independently recorded in the same region by employing a spatially modulated laser beam owing to the multispectral properties of GST. The fact that a single emitter can exhibit different visible and IR images is particularly attractive for optical security applications, including anti-forgery. This feature is experimentally demonstrated using white paper and color-printed paper.