Laser ablation or photoablation 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. Excimer lasers of deep ultra-violet light are mainly used in photoablation; the wavelength of laser used in photoablation is approximately 200 nm.
Video Laser ablation
Fundamentals
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. Such features of laser radiation as laser beam scanning velocity and the covering of scanning lines can significantly influence the ablation process.
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.
Maps Laser ablation
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). 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.
One of the advantages is that no solvents are used, therefore it is environmentally friendly and operators are not exposed to chemicals (assuming nothing harmful is vaporized). It is relatively easy to automate. The running costs are lower than dry media or dry-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.
Laser ablation is also used in removing rust from iron objects easily.
In March 1995 Guo et al. were the first to report the use of a laser to ablate a block of pure graphite, and later graphite mixed with catalytic metal. 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), 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 and laser crystals.
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.
Laser ablation is also used to create pattern, removing selectively coating from dichroic filter. This products are used in stage lighting for high dimensional projections, or for calibration of machine vision's instruments.
Propulsion
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
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.
Chemical analysis
Laser ablation is used as a sampling method for inductively coupled plasma mass spectrometry with forensic and biological applications.
Biology
Laser ablation is used in science to destroy nerves and other tissues to study their function. 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.
Another example is the trochophore larva of Platynereis dumerilii, where the larval eye was ablated and the larvae was not phototactic, anymore. However phototaxis in the nectochaete larva of Platynereis dumerilii is not mediated by the larval eyes, because the larva is still phototactic, even if the larval eyes are ablated. But if the adult eyes are ablated, then the nectochaete is not phototactic anymore and thus phototaxis in the nectochaete larva is mediated by the adult eyes.
Laser ablation can also be used to destroy individual cells during embryogenesis of an organism, like Platynereis dumerilii, to study the effect of missing cells during development.
Medicine
There are several laser types used in medicine for ablation, including argon, carbon dioxide (CO2), dye, erbium, excimer, Nd:YAG, and others. Laser ablation is used in a variety of medical specialties including ophthalmology, general surgery, neurosurgery, ENT, dentistry, oral and maxillofacial surgery, and veterinary. Laser scalpels are used for ablation in both hard- and soft-tissue surgeries. Some of the most common procedures where laser ablation is used include LASIK, skin resurfacing, cavity preparation, biopsies, and tumor and lesion removal. In soft-tissue surgeries, the CO2 laser beam ablates and cauterizes simultaneously, making it the most practical and most common soft-tissue laser.
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 and destruction of primary and secondary malignant liver lesions.
Laser ablation is also used to treat chronic venous insufficiency.
See also
- Parts cleaning
- Matrix-assisted laser desorption/ionization
- Laser induced breakdown spectroscopy
- Laser surgery
- Soft-tissue laser surgery
- LASEK
- LASIK
- Laser scalpel
- Dental laser
- Laser cutting
- Laser engraving
- Laser bonding
- List of laser articles
- Optical breakdown photoionization mode (OB) at Photoionization mode
- Soft retooling
References
Bibliography
- Oxford Concise Medical Dictionary,2002,6th edition,ISBN 0-19-860459-9
Source of article : Wikipedia
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