Picosecond laser ablation of millimeter-wave subwavelength structures on alumina and sapphire

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 mm3/min and 20 mm3/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 ϕth. Using a best-fit procedure we find an average δ=630 nm and 650 nm, and ϕth=2.0-0.5+0.5 J/cm2 and 2.3-0.1+0.1 J/cm2 for alumina and sapphire, respectively, for peak fluence values between 30 and 70 J/cm2. With the best fit values, the model and data values for cumulative fluence agree to within 10%. Given inputs for δ and ϕth the model is used to predict average ablation rates as a function of SWS height and average laser power.