The word "laser" is an acronym for light amplification by stimulated emission of radiation and refers to devices that generate or amplify light through that principle. Lasers are used whenever a directed, sometimes very intense, beam of monochromatic (single wavelength) light is required. For a laser to function, the gas or solid of which it is composed must be excited into a non-equilibrium condition wherein the number of atoms or molecules in a highly energetic state is greater than the number in a less energetic state, a so-called inverted population. If a photon of light with energy equal to the difference between the two energy levels is introduced into the excited substance, either from the outside or by spontaneous decay of one of its own excited atoms or molecules, it can stimulate other excited atoms or molecules to decay to the less energetic state with release of additional photons. Emitted photons travel in the same direction, at the same wavelength, and in phase with the original photon, producing coherent radiation. Often mirrors (one fully reflecting and the other partially Reflecting) are placed parallel to each other on opposite sides of the laser material so that each photon is likely to make many transits of the laser, stimulating release of other photons, before escaping. Lasers may operate in the continuous wave mode or in the pulsed mode, in which they store energy and suddenly release a burst of photons. Since the first lasers were reported in the early 1960s, many thousands of different atoms, ions, and molecules, pure or in combinations, have been used. Each generates light at its characteristic wavelengths, which may lie in the energetic X-ray region of the spectrum; the ultraviolet, visible, and infrared regions; or the low-energy microwave region (in which case it becomes a maser).
Applications of lasers increased rapidly. The unique properties of laser beams allow them to be focused into tiny spots, travel long distances without diverging, and be turned on and off rapidly, making them ideal for many uses, including rapid transmission of digital information. In the 1990s lasers were used regularly in scientific research, military weapons, medical diagnosis and surgery, communications, air quality analysis, surveying and seismology, barcode scanners, CD and video disc scanners, printers, welding, etching, and micromachining. Chemists explored the use of lasers to trigger parts of molecules to react while other normally more reactive sites are unaffected, which may allow inexpensive commercial production of molecules that are difficult or impossible to synthesize with other processes. George W. Bush's election to the presidency in 2000 revived Ronald Reagan's dream of a Strategic Defense Initiative (popularly dubbed "Star Wars"), renewing the debate about the technical feasibility using airborne or space-based laser systems to shoot down enemy missiles. Finally, intense laser beams were also being used to heat molecules to the extremely high temperatures needed to initiate nuclear fusion. If achieved, controlled nuclear fusion could create virtually limitless energy, reducing or eliminating dependence on nonrenewable fossil fuels and nuclear fission. A more likely outcome is suggested by a report in March 2001 that petroleum industry researchers had begun exploring the use of high-energy lasers to explore and drill for more oil.
Bibliography
Bromberg, Joan L. The Laser in America, 1950–1970. Cambridge, Mass.: MIT Press, 1991.
Perin, Monica. "Drilling with Lasers." Houston Business Journal (March 2, 2001).
Townes, Charles H. How the Laser Happened. New York: Oxford University Press, 1999.
