(Note—It is suggested that the article Radiation, introduction and subtitle Electromagnetic Radiation be read before reading the technical sections of this article.)

The laser makes use of some of the properties of the atom to generate or amplify light waves. Atoms absorb and emit energy in the form of photons (small bundles or particles of energy). An atom that has absorbed a photon is said to be excited (raised in energy). In returning to its normal, or ground, state, the atom gives up one or more photons. Usually, excited atoms return to the ground state in a spontaneous, random manner. However, an excited atom can be stimulated to give up a photon if the atom is hit by an outside photon having exactly the same energy as the photon the atom would have given up spontaneously. The outside photon and the photon given up move away from the atom and travel in the same direction and in coherent waves.

Any given atom can absorb and emit only photons with particular amounts of energy (particular wavelengths). Which wavelengths can be absorbed and emitted depends on which kind of atom is being used.

Usually most of the atoms in a substance are in the ground state. In a laser, most of the atoms are excited at one time, and are then made to emit their photons in an orderly way. When the photons are emitted, an intense, directional, and coherent beam is generated by the laser. The particular properties of the beam generated depend on the material used for the laser. A solid-state laser uses either a crystal or glass; other kinds of lasers use gases, liquids, or semiconductors.

A characteristic type of solid-state laser is the ruby laser. It contains a rod-shaped crystal of synthetic ruby. This material, like natural ruby gemstones, is composed of aluminum oxide with trace amounts of chromium. The ends of the rod are cut parallel to each other and perpendicular to the sides. One end is heavily silveredthat is, coated with silver or other reflective materialso that it reflects almost all the light that strikes it. The other end is lightly silvered so that some of the light will be reflected and the rest will pass through.

A lamp that can produce an intense flash of white light surrounds the ruby crystal. When the lamp flashes, most of the chromium atoms in the crystal absorb photons. Within a fraction of a second, these excited atoms begin returning to the ground state, and, in the process, emit photons. When these photons strike chromium atoms that are still excited, they stimulate them to emit yet other photons. As this process continues, beams of coherent light are formed. The beam that is parallel to the sides of the ruby is reflected back and forth between the silvered ends until sufficient photons have joined it to make it powerful enough to escape through the lightly silvered end. (The beams that are not parallel to the sides escape from the ruby before they have a chance to build up much intensity.)

The ruby laser emits a red light. Other types of solid-state lasers can be used to generate light of other colors or to generate infrared radiation.

A gas laser contains one or more gases sealed in a glass tube. Two mirrors. one heavily silvered and the other lightly silvered, lie at either end of the tube. The most common type of gas laser uses a mixture of helium and neon. Other gases used include carbon dioxide and argon. In the helium-neon laser, energy is provided by an electrical discharge that excites the helium atoms. When the excited helium atoms collide with the neon atoms, the energy is transferred from the helium to the neon. The neon atoms emit photons that form a laser beam in essentially the same way as in the ruby laser. Helium-neon lasers can produce beams of red light, green light, or infrared radiation, depending on the mirrors used.

The typical liquid laser uses a fluorescent dye in a glass tube. As in the gas laser, two mirrors, one heavily silvered and the other lightly silvered, lie at either end of the tube. Energy to excite the molecules of the dye is provided either by a flash lamp or by an ultraviolet laser. Liquid lasers can be made to produce extremely brief pulses of light; some have produced pulses lasting less than a trillionth of a second. Dye lasers are used to produce beams of visible light of almost any color.

A semiconductor laser is essentially a kind of electronic device called a junction diode. It is made of a semiconductor, typically gallium arsenide, that has been treated to form two types of materialsan n-type material, which has an excess of electrons, and a p-type material, which has a deficiency of electrons. The p-type material contains positively charged vacancies called holes.

When a voltage is applied across the diode, excess electrons of the n-type material combine with holes of the p-type material along the junction between the two types of materials. This process results in the release of energy in the form of photons. These photons, in turn, stimulate other electrons and holes to combine, and a coherent beam is formed along the plane of the junction. The back surface of the diode is generally coated with a highly reflective metal and the front surface is polished to make it partially reflective.

Most semiconductor lasers are used to produce beams of infrared radiation. A major advantage of semiconductor lasers is that they can be made very small. Some types of semiconductor lasers can be made to flash on and off millions of times per second.

In the late 1950's, scientists began seeking ways to devise an optical maser--that is, a maser that would generate or amplify light (what is today called a laser). Preliminary studies were done by Charles H. Townes, inventor of the maser, with Arthur L. Schawlow, and by other scientists, including Gordon Gould, Nikolai G. Basov, and Aleksandr M. Prokhorov. The first successful laser was built in 1960 by Theodore H. Maiman of Hughes Research Laboratories. Maiman's laser contained a single large ruby crystal with two parallel surfaces silvered. The beam was emitted in a series of brief, intermittent pulses. Later in the same year, the first gas laser was operated at Bell Telephone Laboratories by Ali Javan and two collaborators. This laser used a mixture of helium and neon and emitted a continuous beam.

Books about Lasers

Beach, D.P., and others. Applications of Lasers and Laser Systems (Prentice Hall, 1993).

Hawkes, John, and Ian Latimer. Lasers: Theory and Practice (Prentice Hall, 1994).

Hecht, Jeff. Understanding Lasers: an Entry-L2evel Guide (Institute of Electrical and Electronics Engineers, 1994.)

For Younger Readers

Billings, C.W. Lasers: the New Technology of Light (Facts on File, 1992).