Front-side ablation was used with laser pulses traveling from air-into-Al
2O
3; back-side ablation occurs when the focus is placed onto the exit plane (Al
2O
3-to-air). This is usually possible with samples thinner than 0.5 mm and at less tight focusing with
. It took
min to ablate the profile of a 1-mm-diameter lens with a height of
m. Surface roughness of as-ablated surface was
nm and was reduced to
nm after 2 hr annealing (
Figure 1(c)). However, light scattering was still strong and the surface remained visibly white. Initial roughness was also dependent on the focusing conditions and pulse overlap during ablation. Other smoothing strategies need to be tested, e.g., ablation with more focused pulses. When ablation takes place at the very top of lens profile (surface of sapphire) there was a strong change in ablation conditions, which resulted in non-ablated plateau, which is ≈
m in diameter (
Figure 1). This is a known issue and can be circumvented by placing a micro-layer of a matching refractive index material, e.g., asilica film, that allows to ablate sapphire [
8]; the added layer is later washed out in solvent/acid.
The effect of HTA was examined through the ablation of crystalline flat c-plane Al
2O
3 which lead to better understanding of surface changes. Since the maximum temperature used was
C, which is much lower than the melting point of Al
2O
3C, surface restructuring by surface tension driven flow of the molten phase was absent and other contributions from surface diffusion at atomic level, coarsening of debris nanoparticles, as well as sublimation took place. Sapphire has a melting point of 2053
∘C and boiling temperature of 2980
∘C. It has recently been shown that lower temperatures with larger times can also yield surface re-flow, which can be understood from length of diffusion scaling
, where
[cm
2/s] is the surface diffusion coefficient and
t is time. Temperatures of 1100
∘C and 1200
∘C at 3, 6, 9, 18, and 30 hours (in air ambience) have been tested and showed smoothing of c-plane sapphire terraces in step configurations [
9]. Laser modified regions under high intensity irradiated in air also might have surface formation of ceramic ALON phase (AlN)
x (Al
2O
3)
with
. Generation of defects, vacancies, and interstitials under high intensity fs-pulses is taking place, e.g., F
+ color center of the oxygen vacancy with a trapped electron created by fs-laser machining of sapphire is fully annealed at 1100
∘ [
10]. Frenkel pair defects - interstitial-vacancy - observed earlier by MeV-electron irradiated high-pressure cubic-BN was induced by fs-laser pulses [
11] at similar conditions used in this study. Chemical bond breaking by high intensity laser pulses can create metal enriched regions on the surface of oxides and nitrides due to formation of volatile O
2 and N
2. Such laser ablated surfaces of oxides rich with defects, debris, and depleted of oxygen are expected to be restructured by HTA. We chose O
2 flow in order to facilitate surface migration of Al-atoms to form terraces of Al
2O
3 under oxygen supply. This is one probable contribution to formation of smooth crystalline surface.