Infrared Absorption of Laser Patterned Sapphire Al₂O₃ for Radiative Cooling

Area of Science:

• Materials Science
• Optics and Photonics
• Laser Physics

Background:

• Sapphire (Al2O3) is a key material in optical and electronic applications.
• Controlling thermal emission is crucial for infrared sensor technology.
• Femtosecond laser ablation offers precise micro-structuring capabilities.

Purpose of the Study:

• To spectrally characterize the thermal emission of femtosecond laser-ablated micro-gratings on sapphire.
• To investigate the influence of micro-grating patterns on infrared emissivity.
• To explore the potential for tailored phononic absorption using laser inscription.

Main Methods:

• Spectral characterization of reflectance (R) for linear and circular micro-gratings on c-plane sapphire.
• Utilizing an infrared (IR) camera to measure blackbody radiation temperature and emissivity.
• Employing a back-side ablation technique on transparent sapphire substrates.

Main Results:

• Micro-gratings exhibited (3-6)% larger emissivity compared to untreated sapphire.
• Azimuthal emission followed a Lambertian profile (∝cosθ) in the 7.5-13 μm band.
• Back-side ablation minimized debris and maximized energy deposition at the exit interface.

Conclusions:

• Femtosecond laser ablation is effective for enhancing thermal emission from sapphire micro-gratings.
• The back-side ablation method is advantageous for processing transparent substrates.
• Laser-inscribed gratings allow for tailored phononic absorption in the Reststrahlen region (20-30 μm).