| Ablation |
The
use of a laser to remove any material by vaporization. |
| Absorption
|
The
loss of light as it passes through a material, generally
due to its conversion to other energy forms (typically
heat). |
Avalanche
Ionization
|
Free
electrons colliding with a surrounding atoms, and breaking
off more free electrons, create additional free electrons
at an exponential rate. |
Conductivity
|
A
material property that is the inverse to its resistance
to the flow of electricity. |
Defects
|
Faults
which cause the material to be unusable for it intended
purpose. |
Features
|
While
we have yet to create features this small to date in materials,
the principle has been demonstrated. |
| Free
Electrons |
Electrons
in the outer orbit around the nucleolus of an atom, they
can be moved out of orbit comparatively easily. |
| Gigawatt
|
Most
large nuclear power plants produce Megawatts of average
power. This is a lot more energy than an ultrafast laser
can produce of average power - which is typically one
watt. So it is not necessary to have one thousand nuclear
power plants all connected to the ultrafast laser at the
same time to operate these ultrafast lasers! In fact,
most residential houses have enough electricity to run
one. The difference comes from the fact that in a nuclear
power plant, power is being delivered continuously, whereas
in these ultrafast lasers power is being compressed into
pulses that are less than a trillionth of a second in
duration. |
| Heat-Affected
Zone (HAZ) |
It
should be noted that under some conditions these effects
can be present. The process has a threshold. Below that
threshold energy from the laser pulse may be absorbed
into the material and converted to heat that will dissipate
into the surrounding material. Since the beam profile
typically does not have sharp edges, some energy in the
beam may be below the threshold for ablation. How much
gets into the surrounding material depends on the exact
beam shape, its relation to the threshold for ablation
and the repetition rate of the laser. Typically, however,
some set of conditions can be chosen to minimize these
effects. The price for this minimization may be through-put;
i.e. how fast material can be removed from the target.
In some cases this may be unacceptably slow. |
| Heat-diffusion
time |
All
tool bits deposit mechanical energy into the material
that is being machined, a portion of which is converted
to heat energy. Lasers deposit optical energy into materials
that they machine, some of which is also converted to
heat energy. This heat energy does not stay localized
where it was deposited initially. It moves away in a characteristic
time - the so-called "heat diffusion time".
This is a familiar phenomenon. If you turn on the heating
element on an electric stove, it will take a few seconds
to warm-up. The same happens at the microscopic level,
but the time scales involved are quite different. The
typical "heat-diffusion time" encountered in
laser machining is not counted in seconds but rather in
picoseconds. |
| Intensity
|
Flux
per unit solid angle. |
Ionized
|
The
gain or loss of one or more electrons in an atom, which
causes it to carry a negative or a positive charge. |
| Ions
|
An
atom that has gained or lost one or more electrons, and
as a result, carries a positive or a negative charge. |
| Plasma
|
A
plasma is a fourth state of matter well known to physicists
but not well known to the layman - the other three states
of matter being solid, liquid and gas. A plasma is a loosely
bound soup of highly charged atoms and electrons containing
so much energy that the forces that hold the material
together are obliterated. Ultrafast lasers can produce
this state of matter because they pack so many particles
of light called photons into so small a time interval
that when they interact with the atoms in the surface
of the material, they strip as many as 15 electrons off
the atom. Physicists call this process multiphoton ionization.
|
| Peak
Power |
The
maximum power supplied by a laser pulse. |
| Picosecond
|
A
fraction of a second (10-¹²). Abbreviated as
p. |
| Power
Density |
In
laser beam welding or heat treating, the instantaneous
laser beam power per unit area. This parameter is key
in determining the fusion zone profile (area of base metal
melted) on a work piece. |
| Recast
Layer |
Molten
metal which forms a layer of debris on the surface of
the material during picosecond machining. |
| Slag
|
The
unwanted material that is removed from metal when it is
heated to a liquid state. |
| Stent
|
A
device placed in a body structure, such as a blood vessel
or the gastrointestinal tract, to provide support and
to keep the structure open. |
| Terawatt
|
A
unit or power equal to one trillion watts. |
| Threshold |
So far we have talked only about ablation of materials.
There are other processes that are more generally defined
as 'physical and/or chemical changes in the structure
of materials' that have properties that are similar to
those associated with ablation - but differing thresholds.
For example, it is possible to locally change the index
of refraction of materials at the focus of an ultrafast
laser beam, inside the bulk of the material (see our section
on waveguides). This can have very useful consequences
for the creation of devices used in telecommunication
networks. |
| Ultrafast
|
As
it relates to micromachining, a laser capable of generating
light pulses that last only a few femtosecond's time.
This can be achieved by nonlinear filtering to increase
bandwidth and compress the pulse or by passive modelocking
or synchronous pumping in conjunction with pulse-shaping
techniques. |
| Ultrashort
|
See "Ultrafast." |
