Sidebar : Beyond the Ruby Laser
After nine months of intensive study, physicist Theodore Maiman was hoping for a flash of brilliance.
It was spring 1960, and Maiman had been working with an assistant, Irnee D’Haenens, at the Hughes Research Laboratories in Malibu, Calif., to see if he could generate a new type of light by blasting a tiny pink ruby crystal with radiation from a powerful photographic flash.
Using off-the-shelf parts, Maiman was racing against six other research teams, all vying to be the first to produce an intense, pencil-thin beam of visible-light waves perfectly matched in energy and in the alignment of their peaks and troughs. Other scientists had already proclaimed that pink ruby couldn’t generate such radiation. But Maiman was convinced otherwise.
On May 16, 1960, he and D’Haenens watched their oscilloscope as they increased the voltage to a flashlamp coiled around a small ruby rod. A sharp upturn in intensity, followed by a sharp drop, revealed that the ruby was indeed shooting out brilliant, coherent pulses of light.
Exhilarated, Maiman, D’Haenens and several colleagues decided to repeat the experiment and examine the beam as it struck a white cardboard screen. D’Haenens was color-blind and at first couldn’t see the color of the crystal’s light. But after the voltage to the flashlamp was cranked up, the light pulses from the ruby rod became so intense that everyone else’s eyes, with normal sensitivity to red, were too dazzled to register the signal.
Only D’Haenens could see the brilliant, horseshoe-shaped red glow that indicated the team had created a powerful beam — achieving light amplification, D’Haenens recalled in a 1985 interview for the American Institute of Physics.
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It was indeed a new vision. D’Haenens had witnessed the birth of the laser.
Fifty years later, the laser’s importance in daily life may be second only to that of the computer. From welding detached retinas to optically transporting telephone calls around the world, from the heart of every CD player to the treatment of life-threatening diseases, the laser has insinuated itself into nearly every technological aspect of modern society.
Microbiologists routinely use low-power lasers as tweezers to gently nudge bacteria, cells and even DNA. Physicians send laser light through flexible cables to kill cancer cells, pulverize kidney stones and destroy other unwanted growths in the human body. On a more cosmic scale, laser light beamed into space enables ground-based telescopes to produce crystal-clear images of the heavens.
Like many key 20th century discoveries in the physical sciences, the laser traces back to Albert Einstein. Although he had no conception of a laserlike device in 1916, he had an abiding interest in the interaction between light and matter. In those days, many scientists studying light were concerned with two processes, spontaneous absorption and spontaneous emission.
Spontaneous absorption happens when light of just the right energy shines on atoms. The atoms’ outermost electrons absorb the energy (in the form of photons — particles of light), spontaneously jumping to the next higher energy level. In the absence of some external source of energy, however, electrons are like couch potatoes — they will fall back to the lowest possible energy, spitting out a photon in the process. So about as quickly as an electron absorbs a photon, it spontaneously emits it. The sun’s visible surface, the filament of a lightbulb and the wick of a burning candle all shine because of spontaneous emission.
But Einstein showed that to be consistent with quantum theory and thermodynamics, another type of emission must also exist — stimulated emission, which laid the groundwork for the laser. If light striking an atom can excite electrons, then it can also force already excited electrons to radiate light and drop back down to a lower energy level, Einstein reasoned.
Imagine a bunch of atoms whose electrons are in an excited state because they have absorbed a photon. Tickle those atoms with a second pulse of light that has exactly the same energy as the original light absorbed by the electrons. That second pulse, Einstein showed, prompts the electrons to emit photons identical to each other in energy and momentum. For each incoming photon that tickles an atom, there are now two outgoing ones.
“A splendid light has dawned on me about the absorption and emission of radiation,” Einstein wrote to his friend Michele Besso in 1916.
Over the next two decades, several physicists flirted with the idea of using stimulated emission to produce a high-intensity, coherent beam of radiation. Each emitted photon would tickle other still-excited electrons, creating a flood of photons with the same wavelength traveling in the same direction, like a battalion marching in unison. But no one knew how to put that theory into practice.
It wasn’t until the 1950s that stimulated emission was harnessed in the development of the maser — the laser’s microwave cousin.
Emergence of the maser
Studies of microwave radiation and its interaction with molecules got a big boost from the military. During World War II, physicists developed radar that used ordinary radio waves to detect enemy aircraft. Because radar was pushed to shorter and shorter wavelengths for ease of use, by the time the war ended the military had a surplus of sophisticated microwave equipment that scientists were happy to accept. One of these scientists, a young physicist named Charles Townes, had joined Columbia University and had become fascinated with how molecules absorb and emit energy. At that time the military, mindful of the many scientific payoffs during the war, was pouring money into physics research with relatively few strings attached. “Military funding wasn’t so targeted as it is now,” Townes recalls.
Townes had become obsessed with the study of short-wavelength microwaves known as millimeter waves because they interacted more strongly with atoms and molecules than longer wavelengths did. He was convinced that such radiation, if it could be generated at high intensities, would lead to better probes of atomic and molecular structure. But no one knew how to produce a stable, intense source of the millimeter waves. And pressure was mounting because Townes had been appointed chair of a Navy committee on millimeter-wave research and had no advances to report.
A breakthrough came on April 26, 1951. The Navy committee had convened in Washington, D.C., and Townes, a father of young children, was used to waking early. Careful not to disturb his roommate at the Franklin Park Hotel, collaborator and future brother-in-law Arthur Schawlow, Townes crept out of the room and sat on a bench in an adjoining park. Red and white azaleas were in full bloom, Townes recalls, but his full attention was on the millimeter-wave puzzle.
As he was familiar with Einstein’s theory of radiation, Townes knew that a source of photons — including microwaves — could stimulate atoms or molecules to emit light at exactly the same frequency, thereby boosting the intensity of the outgoing signal. But there was a major stumbling block. He needed to find a way to keep more electrons at higher energy levels than at lower ones.
A group of atoms in thermal equilibrium (having reached the same temperature as its surroundings) tends to have more atoms with unexcited electrons than excited ones. So any temporary boost in the signal from stimulated emission would soon be soaked up by electrons at the lowest energy levels. Instead of a net gain in a microwave signal, there would be a net loss.
“I cannot reassemble exactly the sequence of thought that pushed me past that conundrum, but the key revelation came in a rush,” Townes wrote in his 1999 memoir. “The second law of thermodynamics assumes thermal equilibrium; but that doesn’t really have to apply! There is a way to twist nature a bit.”
If a device could be built to keep a collection of atoms or molecules out of equilibrium — with more of them in the higher energy state than the lower one — then Einstein’s stimulated emission could lead to a true amplification of an incoming signal.
Townes’ idea was too undeveloped to talk about at the Navy committee meeting. But soon after returning to Columbia, he pursued the notion at full tilt. He focused his efforts on molecules of ammonia gas made with deuterium, a heavy isotope of hydrogen.
Townes’ strategy was twofold. First, using a changing electric field, he would separate ammonia molecules in the higher energy state from those in the lower energy state. Then he would trap the higher-energy molecules in a cavity designed to keep the microwave radiation they emitted bouncing back and forth through the gas. That radiation would stimulate even more electrons to emit microwaves and generate a larger and larger amplification of the original microwave signal.
Townes recruited two young researchers at Columbia, Herb Zeiger and Jim Gordon, to develop the device. The work took three years, and not everyone in Columbia’s physics department was patient. One day in 1953, one of the university’s physics Nobel laureates, I.I. Rabi, and department chair Polykarp Kusch (who became a Nobel winner two years later) paid Townes a visit. They told him to stop wasting his time.
Townes listened, but ignored the advice of the two heavyweights. “Luckily, I had tenure,” he says.
Besides, he and his students had reason to be optimistic: Almost immediately, they had gotten indications of stimulated emission. And in early April 1954, Gordon burst into a seminar that Townes was holding to announce that amplification had been achieved. Gordon and Townes had developed the first device demonstrating “microwave amplification by stimulated emission of radiation” — the maser (SNL: 2/5/55, p. 83).
Unbeknownst to Townes, several other researchers had begun contemplating similar ideas about a maser. At the University of Maryland in College Park, Joe Weber had published a short paper proposing to use stimulated emission as an amplifier of radiation. And in 1954, Aleksandr Prokhorov and Nikolai Basov of the Lebedev Physical Institute in Moscow wrote an article about using a beam of alkali halide molecules to generate a microwave oscillator.
Steps to the laser
While most other researchers marveled at the concentrated beam produced by the maser and worked to refine its design, Townes leapfrogged to much shorter wavelengths — the infrared and visible-light portions of the electromagnetic spectrum.
“I wanted to develop an infrared [version of the maser] because I saw there were new ways to probe atoms and molecules at infrared wavelengths,” Townes says. “When I sat down and tried to understand how we could get down to these wavelengths, writing down the equations and examining my notes,” he says, “I realized, ‘Hey, it looks like we can go right down to even shorter wavelengths — light waves.’”
Because of the shorter wavelengths, an optical version of the maser posed new design challenges. Some physicists even claimed, based on their understanding of quantum theory, that it could never be done.
But, as Townes pointed out, scientists were familiar with the interaction of light and atoms at infrared and visible-light wavelengths.
With Gordon Gould of Columbia, Townes discussed an experimental arrangement for a visible-light version of the maser. Instead of the maser’s microwave cavity, a system of reflecting mirrors would pass a source of light back and forth through a carefully chosen material to stimulate excited atoms and amplify the radiation.
Gould realized that such a design, for which he coined the term laser (for light amplification by the stimulated emission of radiation), could create sharply focused beams of high intensity that would carry much more energy than the beam produced by a maser. Keenly aware of potential applications, Gould had his notes, which date from 1957, notarized at a local candy store. Later those notes would be part of a 30-year patent war, in which Gould would finally get recognition for his ideas.
In the meantime, Townes and his colleague Schawlow, who had moved to Bell Laboratories in Murray Hill, N.J., detailed their own concept and design in a landmark 1958 paper titled “Infrared and Optical Masers” (SNL: 2/7/59, p. 83).
After reading the paper, several teams joined Townes and Schawlow in the race to be the first to construct the device. Each group attempted to use a different material, or source of atoms, to amplify visible light. “You rarely get a case like this, when there’s sort of a starting gun and everyone tears off at the same time,” notes physical sciences historian Spencer Weart, who is affiliated with the American Institute of Physics.
At Bell Labs, Schawlow investigated a solid material as the lasing medium, while colleagues Ali Javan, William R. Bennett Jr. and Donald Herriott were examining neon gas. Gould, who had left Columbia to join a private research company, TRG, had submitted a proposal to the military to use a metal vapor in a laser.
In September 1959, at a quantum electronics conference in New York’s Catskill Mountains, it was clear that other teams had joined the competition, including Maiman and the Soviet researchers Basov and Prokhorov. At that conference Schawlow reported his analysis that pink ruby would not make a good lasing medium for visible light.
Schawlow’s claim was one reason that Maiman’s success in May 1960 surprised so many (SNL: 6/23/60, p. 53). Some scientists, many of whom had barely heard of Maiman, at first refused to believe the California-based researcher had scooped everyone on the East Coast, who had garnered most of the money and equipment for building a laser.
“It’s like some horse coming up from the outside in the home stretch,” Weart says. “They didn’t even know he was in the race.”
From the beginning, Maiman had adopted a strategy different from his competitors, aiming to develop a pulsed laser rather than a device that would emit a steady beam of amplified light, which allowed him to use more basic equipment. His device was small and deceptively simple-looking: a rod-shaped ruby, its ends silvered to reflect light, sitting inside a coiled flashlamp.
When the lamp flashed at the right energy, its photons stimulated chromium ions in the ruby to emit identical visible-light photons. Those photons, reflected back into the ruby, in turn stimulated the production of even more identical photons, until a luminous stream of clones never before achieved in the laboratory burst through a half-silvered mirror at one end of the device.
The physicists knew the laser was much more than a visible-light analog to the maser. It could probe and manipulate much tinier subatomic structures than the microwave device ever could (SNL: 1/20/62, p. 42).
Winner and losers
Maiman had extraordinarily bad luck publishing his findings. Although he quickly submitted an article to Physical Review Letters, it was just as quickly rejected by editor Samuel Goudsmit, a well-known theoretical physicist, who mistakenly believed that Maiman’s device was merely an unimportant variant of the maser. A scant, four-paragraph description of Maiman’s work did appear in Nature in August 1960.
That July the Hughes Corp. publicized Maiman’s invention with a press conference in New York City, but the public relations photographer didn’t think Maiman’s laser looked substantial enough. He convinced Maiman to pose with a bigger flashlamp and ruby rod than he had actually used. Without a published paper to analyze, many researchers relied on that misleading publicity photo, still being distributed today, to replicate Maiman’s discovery.
By late summer, Bell Labs researchers did manage to build their own ruby laser. Once again, public relations folks got into the act, convincing the scientists to haul their apparatus, which was considerably larger than Maiman’s, up to an old radar tower at Bell’s main headquarters in Murray Hill and beam laser pulses to another Bell tower in Crawford Hill, N.J., some 40 kilometers away (SNL: 10/15/60, p. 245). That publicity stunt garnered some press, and many reporters didn’t seem to realize that the Bell device wasn’t the first laser.
It probably didn’t help matters that the Bell research paper on the ruby laser, which Goudsmit did agree to publish in Physical Review Letters, didn’t give Maiman credit for building the first laser.
In December 1960, another team of Bell scientists, including Javan, Bennett and Herriott, did achieve a new milestone, succeeding in making the first gas-based laser (which generated a steady, rather than pulsed beam), using helium and neon as the lasing material. Over the years, more sophisticated versions of steady-beam lasers would transform electronic communications and a host of other technologies.
In the end, the first physics Nobel prize for the laser went to Townes, Basov and Prokhorov in 1964 for their theoretical and experimental work in developing the device (SNL: 11/7/64, p. 295). Schawlow shared the 1981 physics Nobel for his contributions to laser spectroscopy (SN: 10/24/81, p. 261).
In all, the Nobel Prize has been awarded to more than a dozen researchers for laser-related studies. Maiman never did receive a Nobel, although he was inducted into the National Inventors Hall of Fame and won several international awards. Gould, too, was passed over for a Nobel, but his court battle eventually won him millions in patent fees.
“I think probably everyone but Townes thinks they didn’t quite get their fair share” of credit, Weart says. “It’s like an inheritance; everybody thinks they ought to have gotten a little larger share, but there’s only 100 percent to go around.”
Looking back, it’s obvious that the whole world benefited from the laser — though not quite as the public first imagined it.
Some of the reporters who covered the unveiling of Maiman’s invention at the July 1960 press briefing hyped the laser as a death ray. As lasers became more powerful, researchers would jokingly refer to them as “one-Gillette” or “eight-Gillette” devices, depending on how many razor blades a beam could pierce. And in 1964, the James Bond blockbuster film Goldfinger featured a laser that sliced through a metal table and threatened to slice through Bond as well.
Regardless of the potential for destruction that garnered initial publicity, the laser has made major inroads in the fields of medicine, communications and industry. And biologists and physicists continue to use lasers in the pursuit of basic science.
Given Einstein’s role in developing the theoretical underpinnings of the laser, it may be only fitting that one of its applications is ultraprecise lunar laser ranging to test another of Einstein’s theories — general relativity. If gravity is weaker than he calculated, it may show up as variations in the Earth-moon distance.
In saluting Einstein and the laser’s other scientific fathers on its 50th birthday, says historian Weart, it’s perhaps also appropriate to acknowledge another important player — light itself.
“We should give credit to light for being such an amazing phenomenon,” he says. It’s the nature of photons “that allows lasers to use these wavelengths in such wonderful ways.”
Sidebar | Beyond the ruby laserBy swapping out the lasing material, scientists can build lasers that emit radiation at different wavelengths. Some examples are shown below.
Solid-state laser: The first ruby laser was an example of a solid-state laser. In this case, the ruby crystal emitted wavelengths of red light at 694 nanometers. Solid-state lasers today are often made from a glass or crystal that is doped with a rare earth element. One such laser, made from neodymium-doped yttrium aluminum garnet crystals, can emit infrared light with a wavelength of 1,064 nanometers.
Semiconductor lasers: Small chips of semiconductors take the place of the ruby rod in these lasers. Two outer semiconductor layers are separated by a middle layer, and radiation is generated when oppositely charged particles meet in that middle layer. Often employing gallium arsenide or gallium phosphide as the lasing material, these devices operate in the near-infrared and red light region of the electromagnetic spectrum. Their small size and low power requirements make them ideal for data transmission and for spectroscopy. These lasers are also found in CD players and laser pointers.
Dye laser: Using organic dye, typically in a liquid solution, these lasers generally operate from the ultraviolet to the near-infrared. Rhodamine 6G is a widely used dye because it is one of the most highly fluorescing materials. Though most of the original work on generating short laser pulses relied on these lasers, they are used primarily for spectroscopy today.
Gas laser: Relying on an electric current discharged through gas to produce light, various versions of this laser type can operate in vastly different radiation regimes. Helium-neon lasers, for example, produce red light at 632.8 nanometers — but can also be made to emit green light. The first maser (microwave version of the laser) used ammonia gas to produce radiation at wavelengths around 1.25 centimeters. Carbon dioxide–based lasers generate radiation around 10.6 micrometers, while argon-ion lasers can produce light with wavelengths as short as 351 nanometers. Excimer lasers, which combine an inert gas and a reactive gas as the lasing medium, produce ultraviolet light, typically between 157 and 351 nanometers, and are used for delicate surgeries.
Free-electron laser: Here, the lasing medium is a beam of electrons that has been accelerated to near light-speed. The beam passes through an undulating magnetic field that causes photons to be emitted in a coherent way. Such lasers are the strongest in terms of power and have the widest frequency range; different types can produce radiation that spans the far-infrared, visible light, ultraviolet and X-ray ranges. Wavelengths down to 6.5 nanometers have been achieved. These devices can be used in isotope separation, plasma heating and particle acceleration. Unfortunately, their setup is large and expensive.