Laser
ALTERATIVE NAME/S: Light Amplification by Stimulated Emission of Radiation
DESCRIPTION:
The first laser was developed by the American physicist Maiman* in 1960, and was identical in theory of operation to the maser, invented by Townes* in 1953, except that it operated at frequencies in the optical region of the electro-magnetic spectrum, rather than in the microwave.
The fundamental theoretical concepts behind the maser and laser date back to the late 19th and early 20th centuries, when the early researchers in quantum mechanics discovered that atoms and molecules exist in different energy states, or levels, the lowest of which (unique to a particular atom or molecule) is called the ground state.
This energy is related to the nature and motion of the electrons bound to the atoms, with electromagnetic radiation like heat, radio waves and light consisting of minute, separate packets of energy (called quanta, or photons) whose energy is proportional to the frequency.
Two interactions between radiation and matter had been discovered by 1916:
a) absorption, in which atoms or molecules can absorb a photon whose energy equals the difference between two levels, and rise to a higher energy level (i.e. one of the electrons will jump to an orbit farther away from the nucleus) and enter an "excited" state (i.e. it has excess energy - the energy causing the excitation being known as ‘stimulating’
energy).
b) spontaneous emission, in which, shortly after becoming excited, atoms and molecules spontaneously return to the lower energy level (due to the atom's tendency to seek its lowest energy level), giving up energy in the form of a photon whose energy equals the difference between the two levels.
In 1916 Einstein* discovered a third process, that of stimulated emission, in which, if an excited atom in a higher energy level is struck by a wave of the right frequency, it stimulates radiation by the atom, causing it to jump down to the lower energy level and emit two photons of the same wavelength. (One photon in, two photons out).
Stimulated emission always has the same frequency and phase as the incoming wave, so the wave becomes stronger in passing an excited atom. A wave of the proper frequency, on passing through a medium with a surplus of excited atoms, grows as it stimulates emission and picks up the energy stored in the excited atoms. The wave is amplified (intensified) by stimulated emission.
Although Einstein had outlined a theoretical method for achieving stimulated emission using individual atoms as resonators (rather than using macroscopic-sized objects), up until 1950 no scientist had been able to build a working device based on Einstein's principle.
However, the discovery of double resonance and the invention of optical pumping by the French physicist Kastler* had, by 1950, increased scientists' understanding and ability to manipulate the energy within atoms. Then, in 1951, a professor of physics at the University of Columbia in the United States, Charles H. Townes, conceived of a method of producing stimulated emission by exposing a group of high-energy molecules to electromagnetic waves within a resonator circuit arrangement, which would then provide positive feedback and build up the electromagnetic energy exponentially, similar to the oscillators that generate radio waves in broadcast transmitters.
Working with Herbert Zeiger and James Gordon, Townes then proceeded to build the first working maser (December 1953), in which a beam of ammonia molecules was passed along the axis of a cylindrical cage of alternately-charged metal rods (the focuser). This non-uniform electric field within the focuser deflected the low-energy ammonia molecules, while the high-energy molecules remained within the beam, which was then guided through a small hole into a resonant cavity, or resonator.
Resonance occurs when a wave travels an integral number of wavelengths as it makes a round-trip of the cavity, and the simplest resonant cavity is a half-wavelength long, so a round trip equals one wavelength. Microwaves are measured in centimetres, and microwave cavities typically are on the order of a wavelength across, and enclosed on all sides.
The resonator is essentially a box - usually with dimensions comparable to the wavelength - with reflecting walls, within which the wave can bounce back and forth between the walls, gaining a little more energy at each pass.
When the resonator was exposed to an electrical field of the right frequency (i.e. photon energy equal to the difference between the ground state and excited state of the ammonia molecules), the molecules gave effective amplification through stimulated emission, which then produced the excitation of more molecules from the ground state, and more stimulated emission. The result was an intensified output of radiation, producing a microwave output corresponding to the resonance frequency of ammonia.
Townes and his colleagues announced the creation of the first working maser in 1954, and masers began proliferating in the late 1950s.
However, Townes and others were also considering the prospect of amplifying stimulated emission at wavelengths much shorter than microwaves, those of visible waves. The first detailed proposal for building a laser - which at the time they called an "optical maser" - was published by Townes and Arthur L. Schawlow in an article entitled "Infrared and Optical Masers" in the December 1958 edition of the 'Physical Review'. This paper had a profound impact on American laser development, although the Bell Labs attorneys did not think the idea was worth patenting, and filed a patent application only after Townes insisted, issued in 1960.
The development from maser to laser was far from simple because of the large physical differences between microwaves and visible light. Waves in the microwave region range between 0.1 cm and 100 cm, and so it was easy to design cavity resonators the length of which was equal to the wavelength, but visible wavelengths are far shorter (less than a micrometre), with thousands of times more energy, and so a resonant cavity would be inconveniently, or even impossibly small.
The solution to this problem, as realized by Townes and Schawlow, was an optical instrument known as a Fabry*- Perot* interferometer, which consisted of two flat mirrors mounted parallel to each other (as two ends of the box - known as parallel plates), with the other sides left open.
A wave which travels straight back and forth between the end mirrors will stay in the active medium and be built up by stimulated emission - while a wave travelling in any other direction will soon pass out of the system without much amplification.
Then, to let some light come out of the optical maser, they suggested that one of the end plates be made partially transparent so that it transmitted some of the light falling on it and reflected the rest. The light emerging through the end plate would then be, like the wave between the plates, a highly directional beam travelling along the axis of the system.
Townes and Schawlow's theoretical method of constructing an optical maser would prove to be right, but their proposal of potassium vapour as the medium for the device soon proved inoperable.
Meanwhile, in 1959 Maiman, at the Hughes Research Laboratories in Malibu, California was attempting to use his extensive experience of masers using crystals of synthetic ruby (aluminum oxide doped with chromium atoms) to devise an optical maser.
Maiman used a ruby rod with the ends carefully ground flat and parallel, coated with silver to act as reflectors. When an intense beam of light from a xenon lamp was fed into the rod, the rod emitted a coherent, monochromatic beam of visible light. Maiman's device, which he first demonstrated in May 1960, was named the LASER (acronym for the phrase: light amplification by stimulated emission of radiation).
Laser light differs from that emitted by conventional sources in two important ways. First, laser light is spatially coherent (i.e. the crests and troughs of each light wave in the beam coincide with those of every other light wave, and therefore reinforce each other), whereas all other light sources produce incoherent light in which the waves are radiated
independently of one another.
This means that laser light is highly collimated - i.e. the rays are nearly parallel to one another, and diverge only slightly as they travel. Second, laser light is monochromatic (of a single color), so all light waves in the beam have the same wavelength.
Today, lasers are used in a wide range of applications in communications, medicine, manufacturing, the construction industry, surveying, consumer electronics, scientific instrumentation, and military systems.
Lasers are widely used in industry for cutting and boring metals and other materials, in medicine for surgery, and in communications, scientific research, and holography. They are an integral part of such familiar devices as bar-code scanners used in supermarkets, scanners, laser printers, and compact disc players.
While the first operating laser employed a rod of pink ruby, many rare-earth elements have since been employed, the most widely used being neodymium (discovered in 1885 by Welsbach*), from which power outputs in the form of brilliant flashes of light of thousands of watts can be obtained.
ONELINER: The laser, a powerful and versatile light-operating tool
OTHER CONNECTIONS: None
BIBLIOGRAPHY:
EB
Collier's Encyclopedia
Hecht, Jeff,Laser Pioneers, Academic Press, 1991
Multimedia:
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