Picosecond laser ablation of millimeter-wave subwavelength structures on alumina and sapphire. (October 2021) - Wen, Qi
Highlights: 100 W ps-laser ablation of mm-wave subwavelength structures on alumina and sapphire. Average ablation rate larger than 20 mm 3 /min on alumina and sapphire. Order of magnitude increase in average ablation rate of alumina and sapphire. Modeling of ultrashort-pulse laser ablation for fabricating pyramid structures. Abstract: We use a 1030 nm laser with 7 ps pulse duration and average power up to 100 W to ablate pyramid-shape subwavelength structures (SWS) on alumina and sapphire. The SWS give an effective and cryogenically robust anti-reflection coating in the millimeter-wave band. We demonstrate average ablation rate of up to 34 mm 3 /min and 20 mm 3 /min for structure heights of 900 μm and 750 μm on alumina and sapphire, respectively. These rates are a factor of 34 and 9 higher than reported previously on similar structures. We propose a model that relates structure height to cumulative laser fluence. The model depends on the absorption length δ, which is assumed to depend on peak fluence, and on the threshold fluence ϕ th . Using a best-fit procedure we find an average δ = 630 nm and 650 nm, and ϕ th = 2.0 - 0.5 + 0.5 J/cm 2 and 2.3 - 0.1 + 0.1 J/cm 2 for alumina and sapphire, respectively, for peak fluence values between 30 and 70 J/cm 2 . With the best fit values, the model and data values for cumulative fluence agree to within 10%. Given inputs for δ and ϕ th the model is used to predict average ablation rates as a function of SWS height and average laser power.