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The simplest
application of laser ablation is to remove
material from a solid surface in a controlled
fashion. Laser machining and particularly laser
drilling are examples; pulsed lasers can drill
extremely small, deep holes through very hard
materials. Very short laser pulses remove
material so quickly that the surrounding
material absorbs very little heat, so laser
drilling can be done on delicate or
heat-sensitive materials, including tooth enamel
(laser dentistry).
Also, laser
energy can be selectively absorbed by coatings,
particularly on metal, so CO2 or
Nd:YAG pulsed lasers can be used to clean
surfaces, remove paint or coating, or prepare
surfaces for painting without damaging the
underlying surface. High power lasers clean a
large spot with a single pulse. Lower power
lasers use many small pulses which may be
scanned across an area. The advantages are:
Another class of
applications uses laser ablation to process the
material removed into new forms either not
possible or difficult to produce by other means.
A recent example is the production of carbon
nanotubes.
Guo et al. (March
1995)
were the first to report the use of a
laser to ablate a block of pure graphite, and
later graphite mixed with catalytic metal (June
1995). The catalytic metal can consist of
elements such as Co, Nb, Pt, Ni, Cu, or a binary
combination thereof. The composite block is
formed by making a paste of graphite powder,
carbon cement, and the metal. The paste is next
placed in a cylindrical mold and baked for
several hours. After solidification, the
graphite block is placed inside an oven with a
laser pointed at it, and Ar gas is pumped along
the direction of the laser point. The oven
temperature is approximately 1200° C. As the
laser ablates the target, carbon nanotubes form
and are carried by the gas flow onto a cool
copper collector. Like carbon nanotubes formed
using the electric-arc discharge technique,
carbon nanotube fibers are deposited in a
haphazard and tangled fashion. Single-walled
nanotubes are formed from the block of graphite
and metal catalyst particles, whereas
multi-walled nanotubes form from the pure
graphite starting material.
A variation of
this type of application is to use laser
ablation to create coatings by ablating the
coating material from a source and letting it
deposit on the surface to be coated; this is a
special type of physical vapor deposition, and
can create coatings from materials that cannot
readily be evaporated any other way. This
process is used to manufacture some types of
high temperature superconductor.
Remote laser
spectroscopy uses laser ablation to create a
plasma from the surface material; the
composition of the surface can be determined by
analyzing the wavelengths of light emitted by
the plasma.
Finally, laser
ablation can be used to transfer momentum to a
surface, since the ablated material applies a
pulse of high pressure to the surface underneath
it as it expands. The effect is similar to
hitting the surface with a hammer. This process
is used in industry to work-harden metal
surfaces, and is one damage mechanism for a
laser weapon. It is also the basis of pulsed
laser propulsion for spacecraft.
Laser ablation
has biological applications and can be used to
destroy nerves and other tissues. For example, a
species of pond snails, Helisoma trivolvis can
have their sensory neurons laser ablated off
when the snail is still an embryo to prevent use
of those nerves. |
