Kavli Affiliate: Tomotake Matsumura
| First 5 Authors: Qi Wen, Elena Fadeeva, Shaul Hanany, Jürgen Koch, Tomotake Matsumura
| Summary:
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 $mu$m and 750 $mu$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 $delta$, which is assumed to depend on peak fluence, and on
the threshold fluence $phi_{th}$. Using a best-fit procedure we find an
average $delta = 630$ nm and 650 nm, and $phi_{th} = 2.0^{+0.5}_{-0.5}$
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 $delta$ and $phi_{th}$ the model is used to
predict average ablation rates as a function of SWS height and average laser
power.
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