Laser ablation

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For the medical technique, see Laser-induced thermotherapy.

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 sublimates. 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.

Fundamentals[edit]

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 and pulse length. The total mass ablated from the target per laser pulse is usually referred to as ablation rate.

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.

Applications[edit]

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). Several workers have employed laser ablation and gas condensation to produce nano particles of metal, metal oxides and metal carbides.

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.

In March 1995 Guo et al.[1] were the first to report the use of a laser to ablate a block of pure graphite, and later graphite mixed with catalytic metal.[2] The catalytic metal can consist of elements such as cobalt, niobium, platinum, nickel, copper, 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 argon 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 called pulsed laser deposition (PLD),[3] 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.

Propulsion[edit]

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.

Manufacturing[edit]

The laser ablation of electronic semiconductors and microprocessors is now being pioneered in the UK to keep electronic manufacturers designs confidential. The main reason is that it greatly reduces the risk of copying infringements.

Processes are currently being developed to use laser ablation in the removal of thermal barrier coating on high-pressure gas turbine components. Due to the low heat input, TBC removal can be completed with minimal damage to the underlying metallic coatings and parent material.

Applications in medicine[edit]

Laser ablation has biological applications and can be used to destroy nerves and other tissues. For example, a species of pond snail, Helisoma trivolvis, can have their sensory neurons laser ablated off when the snail is still an embryo to prevent use of those nerves.[4]

Laser ablation can be used on benign and malignant lesions in various organs, which is called Laser-induced interstitial thermotherapy. The main applications currently involve the reduction of benign thyroid nodules[5] and destruction of primary and secondary malignant liver lesions.[6][7]

See also[edit]


References[edit]

  1. ^ Guo T, Nikolaev P, Rinzler D, Tomanek DT, Colbert DT, Smalley RE (1995). "Self-Assembly of Tubular Fullerenes". J. Phys. Chem. 99 (27): 10694–7. doi:10.1021/j100027a002. 
  2. ^ Guo T, Nikolaev P, Thess A, Colbert DT, Smalley RE (1995). "Catalytic growth of single-walled nanotubes by laser vaporization". Chem. Phys. Let. 243: 49. Bibcode:1995CPL...243...49B. doi:10.1016/0009-2614(95)00825-O. 
  3. ^ Robert Eason - Pulsed Laser Deposition of Thin Films: Applications-Led Growth of Functional Materials. Wiley-Interscience, 2006, ISBN 0471447099
  4. ^ Kuang S, Doran SA, Wilson RJ, Goss GG, Goldberg JI (2002). "Serotonergic sensory-motor neurons mediate a behavioral response to hypoxia in pond snail embryos". J. Neurobiol. 52 (1): 73–83. doi:10.1002/neu.10071. PMID 12115895. 
  5. ^ Valcavi R, Riganti F, Bertani A, Formisano D, Pacella CM. (2010). "Percutaneous Laser Ablation of Cold Benign Thyroid Nodules: A 3-Year Follow-Up Study in 122 Patients". Thyroid. 20:11. 
  6. ^ Pacella CM , Francica G , Di Lascio FM , Arienti V , Antico E , Caspani B , Magnolfi F , Megna AS , Pretolani S , Regine R , Sponza M , Stasi R . (June 2009). "Long-term outcome of cirrhotic patients with early hepatocellular carcinoma treated with ultrasound-guided percutaneous laser ablation: a retrospective analysis". J Clin Oncol. 16:2615-21. 
  7. ^ Pompili M , Pacella CM , Francica G , Angelico M , Tisone G , Craboledda P , Nicolardi E , Rapaccini GL , Gasbarrini G . (June 2010). "Percutaneous laser ablation of hepatocellular carcinoma in patients with liver cirrhosis awaiting liver transplantation". European Journal of Radiology. 74(3):e6-e11.