Laser ablation

# Overview
Laser ablation is the process of removing material from a solid (or occasionally liquid) surface by irradiating it with a laser beam.  At low laser flux, the material is heated by the absorbed laser energy and evaporates or sublimes.  At high laser flux, the material is typically converted to a plasma.  Usually, laser ablation refers to removing material with a pulsed laser, but it is possible to ablate material with a continuous wave laser beam if the laser intensity is high enough.
The depth over which the laser energy is absorbed, and thus the amount of material removed by a single laser pulse, depends on the material's optical properties and the laser wavelength.
Laser pulses can vary over a very wide range of duration (milliseconds to femtoseconds) and fluxes, and can be precisely controlled.  This makes laser ablation very valuable for both research and industrial applications.
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:
- No solvents are used, so it is environmentally friendly and operators are not exposed to chemicals.
- It is relatively easy to automate, e.g., by using robots.
- The running costs are lower than dry media or CO2 ice blasting, although the capital investment costs are much higher.
- The process is gentler than abrasive techniques, e.g. carbon fibres within a composite material are not damaged.
- Heating of the target is minimal.
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