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 mm${}^3$/min and 20 mm${}^3$/min for structure heights of 900 $\hat{I}¼$m and 750 $\hat{I}¼$m 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 $\hat{I}´$, which is assumed to depend on peak fluence, and on the threshold fluence $\ddot{I}{•}_{th}$. Using a best-fit procedure we find an average $\hat{I}´=630$ nm and 650 nm, and $\ddot{I}{•}_{th}=2.0_{-0.5}^{+0.5}$ 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 $\hat{I}´$ and $\ddot{I}{•}_{th}$ the model is used to predict average ablation rates as a function of SWS height and average laser power.

15 pages, 11 figures, submitted to Optics & Laser Technology