7 Micro-hole Arrays and Net-like Structure Fabrication …
229
Fig. 7.18 Relationship
between squared diameter
D 2 and laser fluence
F 0 = 2E
πd
2
(4)
If the fluence F 0 surpasses the ablation threshold fluence F th , the squared diameter
D
2 of the ablation crater is associated by
D
2
= 2d
2 ln(F 0
F th )
(5)
As shown in Fig. 7.18, the single-shot ablation test of the fused silica sheet is
investigated and the single-shot irradiation is realized by controlling the repetition
rate of the machine. The squared diameter of the micro-holes is a function of the
laser fluence. The curve shows that relationship between the squared diameter D
2
and laser fluence F 0 is logarithmic because the X-axis is the logarithm of the laser
energy. The curve for the relationship between the squared diameter D 2 and the
logarithm of laser energy is linear [31, 32]. The extrapolation to zero of the linear
fit obtains the single-shot ablation threshold, and the ablation threshold for the fused
silica sheet is calculated as 2.36 J/cm
2 . Ablation is a function of the number of
pulses. It leads to the definition of an N-pulse ablation threshold when the sample is
irradiated by pulsed lasers with the number N 0 . N 0 < N, no ablation occurs, while
if N 0 > N, ablation occurs. This effect has been defined as an incubation effect [33,
34]. For the material-dependent “incubation effect,” [33] with an increasing number
of overlapping pulses, the ablation threshold is anticipated to decrease.