Bright Ideas: From Concept to Hardware in the First Lasers
Adapted by Dwight E. Neuenschwander, with permission, from Bright Idea: The First Lasers, an online exhibit of the Center for History of Physics and Niels Bohr Library & Archives at the American Institute of Physics, hereafter called “the Exhibit.” http://www.aip.org/history/exhibits/laser/.
Almost everyone living in a technological society today owns or uses a laser. Compact disc players, supermarket checkout scanners, laser printers, and laser pointers are among the applications we encounter daily. Some specialized laser applications include cauterizing scalpels in surgery, industrial cutters and drills, surveying, artificial guide stars for astronomical observatories, and seismology.
This year, 2010, we celebrate the fiftieth anniversary of the invention of the laser. If you ask people at random, “When were the principles first conceptualized that make lasers possible?”, many guess some date around 1960. That’s correct if you mean the construction of a working laser. But the concept of “stimulated emission” that makes lasers possible was first articulated by Einstein back in 1917! It took four decades for technology and circumstances to catch up with Einstein’s vision.
Einstein’s 1917 paper depended on four facts that were already well known to physicists, but which Einstein put together in an original way. First, the electrons in atoms exist in discrete states with quantized energy levels. Second, electrons make transitions between these states by emitting or absorbing a photon whose energy matches the energy difference be-tween the two levels. Third, Ludwig Boltzmann’s statistical mechanics gave us an expression for the probability that an atom resides in a state of a certain energy when it’s part of matter in thermal equilibrium at a given temperature. Fourth, Max Planck’s statistical physics gave us an expression for the energy distribution in a gas of photons. Einstein’s 1917 paper put these four pieces together.
Meanwhile, scientists and engineers pushed radio techniques to ever shorter wavelengths. In the 1930s some hoped they were on the verge of creating a “death ray” (H.G. Wells’ 1898 novel War of the Worlds, wherein the invading Martians were armed with dreadful death rays that obliterated everything they hit, became well known in the US about this time). That turned out (happily) to be unworkable, but the effort led to something better—radar—thanks to the invention of the magnetron (which later was scaled down to build microwave ovens). By 1940, as World War II began, these ingenious radar devices could generate rays with wavelengths of a centimeter or less. They were swiftly pressed into service to detect enemy airplanes.
After WWII, physicists had reason to boast that radar had played a crucial role in winning the war, and the atomic bomb had promptly ended it. What might the physicists create next? As the Cold War got underway, the US government poured ever larger funds into basic and applied research. Scenting not only military but civilian applications, corporations and entrepreneurs heaped their own money on the pile. Industrial and university laboratories proliferated. It was from this fertile soil that the laser would grow.
The Maser: First Step to the Laser
Already in the 1930s scientists could have built a laser. They had the optical techniques and theoretical knowledge —but nothing pushed these together. The push came around 1950 from an unexpected direction. Short-wavelength radio waves, called microwaves, could make a cluster of atoms vibrate in revealing ways (a technique called microwave spectroscopy). Radar equipment left over from World War II was reworked to provide the radiation. Many of the world’s top physicists were thinking about ways to study systems of molecules by bathing them with this radiation.
Charles Townes of Columbia University had studied molecular physics in the 1930s, and during the war had worked on radar as an electronics engineer. The Office of Naval Research pressed him and other physicists to put their heads together and invent a way to make powerful beams of radiation at ever shorter wavelengths. In 1951 he found a solution. Under the right conditions —say, inside a resonating cavity like the ones used to generate radar waves —the right collection of molecules might generate radiation on their own. He was applying an engineer’s insights to a physicist’s atomic systems. Townes gave the problem to Herbert Zeiger, a postdoctoral student, and James P. Gordon, a graduate student. By 1954 they had the device working. Townes called it a MASER, for “Microwave Amplification by Stimulated Emission of Radiation”. 
Townes had predicted a remarkable and use-ful property for the radiation from the de-vice: it would be at a single frequency, as pure as a note from a tuning fork. And so it was. The high degree of order in such radiation would give the maser, and later the laser, im-portant practical uses.
Townes was not alone in his line of thought. Joseph Weber of the University of Maryland expressed similar ideas independently in 1952. And Robert H. Dicke of Princeton worked toward the same goal along a different path. Neither tried to build a device. In Moscow, A.M. Prokhorov and N.G. Basov were thinking in the same direction, and they built a maser in 1955.
Who Invented the Laser?
Physicists had been working for generations toward controlling ever shorter wavelengths. After radio (meters) and radar/microwave (centimeters, then millimeters), the logical next step would be infrared waves. Masers had been modestly useful, more for scientific research than for military or industrial applications. Only a few scientists thought an infrared maser might be important and pondered how to make one.
Townes thought about the problems intensively. One day in 1957, studying the equations for amplifying radiation, he realized that it would be easier to make it happen with very short waves than with infrared waves. He could leap across the infrared region to the long-familiar techniques for manipulating ordinary light. Townes talked it over with his colleague, friend and brother-in-law Arthur Schawlow. Schawlow found the key—put the atoms you want to stimulate in a long, narrow cavity with mirrors at each end. The waves would shuttle back and forth inside so that there would be more chances for stimulating atoms to radiate. One of the mirrors would be only partly silvered so that some of the rays could leak out. This arrangement (the Fabry-Pérot etalon) was familiar to generations of optics researchers.
The same arrangement meanwhile occurred to Gordon Gould, a graduate student at Columbia University who had discussed the problem with Townes. For his thesis research, Gould had already been working with “pumping” atoms to higher energy states so they would emit light. As Gould elaborated his ideas and speculated about all the things you could do with a concentrated beam of light, he realized that he was onto something far beyond the much-discussed “infrared maser.” In his notebook he confidently named the yet-to-be-invented device a LASER (for Light Amplification by Stimulated Emission of Radiation). Gould, Schawlow and Townes now understood how to build a laser—in principle. To actually build one would require more ideas and a lot of work. Some of the ideas were already in hand. Other physicists in several countries, aiming to build better masers, had worked out various ingenious schemes to pump energy into atoms and molecules in gases and solid crystals. In a way they too were inventors of the laser. So were many others, clear back to Einstein.
In 1957 Townes talked over some ideas about pumping light energy into atoms with Gould. Worried that he might be scooped, Gould wrote down his ideas for the record. He developed many more ideas of how lasers could be built and used, and in April 1959 he filed patent applications with his employer, the high-tech research firm TRG. Nine months earlier Schawlow and Townes had applied for a patent on behalf of Bell Laboratories, which employed Schawlow on staff and Townes as a consultant. This led to a long-running patent suit between Bell Labs and Gould, which lasted until 1987 (see the Exhibit).
The Race to Build the Laser
When Schawlow and Townes pub-lished their ideas in 1958, physicists everywhere realized that an “optical maser” could be built. Teams at half a dozen laboratories set out, each hoping to be the first to succeed. Research groups at Columbia University, TRG Corporation, Westinghouse, IBM, Bell Labs, and Hughes Laboratories were among those whose seminal ideas often ended in useful failures before ultimate success was achieved (see the Exhibit for a fascinating list of approaches). These stories have a wealth of circumstances and personalities, but let’s pick up the story with the investigators at Bell and Hughes Labs.
At Bell Labs, Ali Javan, a former student of Townes, tried gas as the lasing medium. He settled on a combination of helium and neon in a long glass tube. An electric discharge through the gas would energize the helium, and collisions would transfer that energy to the neon. It would have operated in the infrared. Proposed by Javan in mid-1959 and built by Javan, W.R. Bennett Jr., and D.R. Herriott in 1960, it would have been the first continuous wave laser. But they could not get laser action.
Theodore Maiman at Hughes Laboratories made calculations and measurements that convinced him the others were wrong who said it was im-possible to pump much energy into a ruby crystal, which had earlier been suggested as the lasing medium. Even so, one would need an extraordinarily bright energy source. One day Maiman realized the source did not have to shine continuously, as other ruby proponents were trying. A flash lamp would do. Scouring manufacturers’ catalogs, he found a very bright lamp with a helical shape. Just right, he thought, for fitting a ruby inside. He assembled the components with the aid of an assistant, Irnee d’Haenens, and in May 1960 observed pulses of red light. It was the world’s first laser.
Maiman’s laser consisted of a cylindrical synthetic ruby crystal (Al2O3) with 0.05% by weight Cr2O3. The ends of the crystal were flat and silvered (one end half-silvered) to form a resonant cavity for the light. The crystal was “pumped” with light from the flashtube. Photons from the flashtube kick electrons in the chromium ions into a very short-lived excited State 3. These electrons almost instantly (~100 ns) drop down to a State 2 that has a long lifetime against spontaneous emission. Enough atoms are hung up in State 2 to produce a population inversion. When one of the atoms emits, the others are stimulated and the chain reaction ensues. The cycle can be repeated in pulses. The light output was a directional, coherent, monochromatic red light of wavelength 694.3 nm.
Other teams moved quickly when they heard of Maiman’s work. Altogether, by the end of 1960 three quite different types of laser—ruby crystals, calcium fluoride crystals, and gas laser—had been demonstrated (see the Exhibit for a wealth of detail).
In 1962, the first visible light, continuously operating helium-neon laser was built by A.D. White and J.D. Rigden at Bell Labs [7, 9]. Their laser operated at a wavelength of 630 nm. It was the first optical oscillator that met the requirements of the demands of optical communication. The YAG laser, consisting of a yttrium-aluminum- garnet crystal doped with neodymium, was developed in 1964 by J.E. Guesses, H.M. Marcos, and L.G. Van Utter, also of Bell Labs [8, 9]. The YAG was the first high-power laser (hundreds of watts, compared to milliwatts from the He-Ne laser), and brought lasers into machining and drilling applications of heavy industry.
What’s It Good For?
Fifty years after the first laser, there are few people in modern society who have not been affected by the invention. The answers to the question “What’s it good for?” are legion. Lasers have revolutionized communication; improved commerce, industry, and entertainment; offer numerous instan-ces of pain-free surgery; and have become one of the most powerful tools for advancing basic and applied science.
The next time you play a CD or use a laser printer, think about the long road from 1917 to 1960 and these devices we should appreciate and not take for granted. See the AIP Exhibit for intriguing details of laser history and the wide variety of its applications.
The author expresses his gratitude to Greg Good, Director of the Center for History of Physics, American Institute of Physics, for permission to adapt Bright Idea: The First Lasers for this article.
 Bright Idea: The First Lasers, http://aip.org/history/exhibits/laser/, Center for History of Physics and Niels Bohr Library & Archives, AIP.
 Portions of this article on Einstein’s 1917 paper are also adapted herein from D.E. Neuenschwander, “Lasers in 1917: The Stimulated Emission of Radiation,” Radiations, Spring 2004, pp. 18–21.
 A. Einstein, “Zur Quantentheorie der Strahlung,” Phys. Zeit. 18, 121–128 (1917).
 Government largesse also raised serious concerns about who was setting the agenda and values for research. They were eloquently expressed by Melba N. Phillips. “Dangers Confronting American Science,” Science 116, 439–443 (Oct. 24, 1952).
 J.P. Gordon, H.J. Zeiger, and C.W. Townes, “Molecular Microwave Oscillator and a New Hyperfine Structure in the Microwave Spectrum of NH3,” Phys. Rev. 95, 282–284 (July 1, 1954); A.L. Schawlow and C.H. Townes, “Infrared and Optical Maser,” Phys. Rev. 112, 1940–1949 (Dec. 1958).
 T.H. Maiman, “Stimulated Optical Radiation in Ruby,” Nature 187, 493–494 (Aug. 6, 1960).
 E.g., A. Javan, W.R. Bennett Jr., and D.R. Herriott, “Population Inversion and Continuous Optical Maser Oscillation in a Gas Discharge Containing a He-Ne Mixture,” Phys. Rev. Lett. 6, 106–110 (Feb. 1, 1961).
 A.D. White and J.D. Rigden, “Continuous Gas Maser Operation in the Visible,” Proc. IRE 50, 1697 (July 1962).
 J.E. Guesses, H.M. Marcos, and L.G. Van Utter, “Laser Oscillations in Nd-Doped Yttrium Aluminum, Yttrium Gallium and Gadoliniim Garnets,” Appl. Phys. Lett. 4, 182–184 (May 15, 1964).
 For a review of laser history and exhaustive references to original papers, see A History of Engineering & Science in the Bell System: Electronics Technology (1925–1975), F.M. Smitts, Ed., AT&T Bell Laboratories (1985).