Page 351 - High Power Laser Handbook

320 So l i d - S t at e La s e r s Ultrafast Solid-State Lasers 321

Figure 12.15 Entrance (left) and exit (right) micrographs of hole drilled in
1-mm mild steel stock, using a cryogenically cooled, high-average-power
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Ti:sapphire amplifier system. Using this laser resulted in greater than 10X
reduction in drill time compared with previous efforts—1.5 seconds was
required to drill this hole.

material removal can be substantially different from that of longer
pulses, going from melt expulsion for microsecond and nanosecond
pulses to vaporization or sublimation for femtosecond pulses. These
dynamical differences produce concrete differences in the results
obtained during laser machining of surfaces (Fig. 12.15). Machining
with femtosecond laser pulses generally reduces the amount of debris
and surface contamination compared with that produced by longer
pulses, a feature that is partially responsible for making femtosecond
lasers the preferred tools for repairing photolithographic masks.
Femtosecond lasers also have advantages for micromachining inside
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transparent materials.
When a femtosecond laser pulse is focused inside the bulk of a
transparent material, the intensity in the focal volume can become
high enough to cause absorption through nonlinear processes, lead-
ing to optical breakdown in the material. Because the absorption is
strongly nonlinear, this breakdown is localized to only the regions of
highest irradiance in the focal volume, without affecting the surface.
The energy deposited in the bulk material then produces permanent
structural changes in the sample, which can be used to micromachine
three-dimensional structures inside the bulk of the material. More-
over, the threshold nature of a femtosecond pulse interacting with a
material allows ultrashort pulse machining with feature sizes below
the diffraction limit. Although micromachining in glasses and crys-
tals has many uses, another means of producing microscopic struc-
tures is to use light-induced polymerization, in which light initiates a
polymerization reaction to produce a solid polymeric object. Other
researchers have already used single-photon polymerization to fabri-
cate microrotors only 5 µm in diameter and to produce light-powered
micromachinery. Femtosecond lasers are also used for microfabrication