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."