Tunable Infrared Emissivity Using Laser‐Sintered Liquid Metal Nanoparticle Films

Abstract This paper describes laser exposure to tune the infrared (IR) emissivity of a film of eutectic gallium indium (EGaIn) particles. EGaIn – a liquid metal at room temperature – forms a native oxide that keeps particles of the metal from spontaneously percolating. Photothermal energy from a CO₂ laser percolates the particles into a conductive network. Here, it also causes a decrease in the IR emissivity of the film of particles from 0.4 to 0.24 over the range of 7.5–13 µm wavelength (measured by an IR camera) with the increase of laser fluence from 1.4 to 1.9 J cm⁻². The particles percolate most prominently at the bottom of the film, and thus, the apparent surface roughness does not change with laser exposure. This finding suggests the decrease in emissivity is not due to changes in the film's topography. Instead, the change in IR emissivity is attributed to a loss of the surface plasmonic resonance effect of EGaIn particles in the IR range after the sintering, which is confirmed by optical simulations. As a demonstration, it is shown that the ability to change the emissivity makes it possible to encrypt messages and camouflage laser‐processed patterns.