Visually invisible transparent emitters with tunable mid-IR emissivity for adaptive infrared camouflage

The integration of thermal imaging with optical structures that are both invisible and transparent in the visible spectrum is essential for advancing applications in passive radiative cooling, thermal management, smart windows, and secure infrared identification. However, achieving simultaneous optical invisibility and precise mid-infrared (mid-IR) emissivity control remains a significant challenge due to material and structural limitations. Here, we present a scalable and multifunctional transparent thermal emitter that enables tunable mid-IR emission while maintaining high visible transparency. The emitter is based on a planar Fabry–Pérot cavity composed of transparent conductive oxides, where nanometer-scale modulation of the top aluminum-doped zinc oxide (AZO) layer induces pronounced changes in mid-IR emissivity, particularly within the atmospheric transparency window (8–13 μm), while preserving over 85 % visible transmittance (500–800 nm). This allows spatially encoded thermal patterns that are visually imperceptible yet clearly resolvable via infrared imaging, providing a basis for adaptive infrared camouflage and secure thermal labeling. The emission remains angularly independent, ensuring uniform thermal contrast regardless of viewing direction. Furthermore, Joule heating through the transparent back electrode can be used as a stable heating method, enabling dynamic modulation under varying ambient conditions. Large-area scalability is also achieved without compromising optical or thermal performance. This work establishes a versatile platform for transparent thermal emitters that simultaneously achieve spectral selectivity, visual stealth, and active control, with broad potential in energy-regulating surfaces, smart windows, and covert infrared technologies.