Infrared Absorption of Laser Patterned Sapphire Al₂O₃ for Radiative Cooling

The reflectance (R) of linear and circular micro-gratings on c-plane sapphire Al₂O₃ ablated by a femtosecond (fs) laser were spectrally characterised for thermal emission ∝(1-R) in the mid-to-far infrared (IR) spectral range. An IR camera was used to determine the blackbody radiation temperature from laser-patterned regions, which showed (3-6)% larger emissivity dependent on the grating pattern. The azimuthal emission curve closely followed the Lambertian angular profile ∝cosθa at the 7.5-13 μm emission band. The back-side ablation method on transparent substrates was employed to prevent debris formation during energy deposition as it applies a forward pressure of >0.3 GPa to the debris and molten skin layer. The back-side ablation maximises energy deposition at the exit interface where the transition occurs from the high-to-low refractive index. Phononic absorption in the Reststrahlen region 20-30 μm can be tailored with the fs laser inscription of sensor structures/gratings.