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 ∝ cos θa at the 7.5-13 µm emission band. The backside 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 backside 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.