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Home , Alexander Prokhorov, Willis Lamb

THE LIGHT THAT SHIN

by CHARLES H. TOWNES

A Nobel laureate recounts ON JULY 21, 1969, astronauts Neil Armstrong and Edwin “Buzz” Aldrin set up an array of small reflectors on the invention of the the moon, facing them toward Earth. At the same time, two teams of astrophysicists, one at the University of ’s Lick Observatory and the other at the University of Texas’s and the birth of quantum McDonald Observatory, were preparing small instruments on two big telescopes. Ten days later, the Lick team pointed electronics. its telescope at the precise location of the reflectors on the moon and sent a small pulse of power into the hardware they had added to it. A few days after that, the McDonald team went through the same steps. In the heart of each telescope, a narrow beam of extraordinarily pure red light emerged from a synthetic crystal, pierced the sky, and entered the near vacuum of space. The two rays were still only a thousand yards wide after traveling 240,000 miles to illumi- nate the moon-based reflectors. Slightly more than a second after each light beam hit its target, the crews in California and Texas detected its faint reflection. The brief time inter- val between launch and detection of these light pulses per- mitted calculation of the distance to the moon to within an inch—a measurement of unprecedented precision. The ruby crystal for each light source was the heart of a laser (an acronym for light amplification by stimulated emis- sion of radiation), which is a device first demonstrated in 1960, just nine years earlier. A laser beam reflected from the Adapted by Michael Riordan from How moon to measure its distance is only one dramatic illustra- The Laser Happened: Adventures of a tion of the spectacular quality of laser light. There are many Scientist, by Charles H. Townes. Reprinted by permission of Oxford other more mundane, practical uses such as in surveying University Press. land and grading roads, as well as myriad everyday uses—

20 SUMMER/FALL 2000 NES STRAIGHT

ranging from compact disc play- ers to grocery-store checkout devices. The smallest are so tiny one cannot see them without a microscope. Thousands can be built on semiconductor chips. The biggest lasers consume as

much electricity as a small P4219F Photo Researchers Will & Deni McIntyre, town. About 45 miles from my office in Berkeley is the Lasers are commonly used in eye Lawrence Livermore National Laboratory, which has some surgery to correct defective vision. of the world’s most powerful lasers. One set of them, collec- tively called NOVA, produces ten individual laser beams that converge to a spot the size of a pinhead and generate temper- atures of many millions of degrees. Such intensely concen- trated energies are essential for experiments that can show how to create conditions for nuclear fusion. The Livermore team hopes thereby to find a way to generate elec- tricity efficiently, with little pollution or radioactive wastes. For several years after the laser’s invention, colleagues liked to tease me about it, saying, “That’s a great idea, but it’s a solution looking for a problem.” The truth is, none of us who worked on the first lasers ever imagined how many uses there might eventually be. But that was not our motiva- tion. In fact, many of today’s practical technologies have re- sulted from basic scientific research done years to decades before. The people involved, motivated mainly by curiosity, often have little idea as to where their research will eventu- ally lead.

BEAM LINE 21 IKE THE TRANSISTOR, principles, that if can be the laser and its progenitor the absorbed by atoms and boost them Lmaser (an acronym for micro- to higher energy states, then light can wave amplification by stimulated also prod an atom to give up its emission of radiation) resulted from energy and drop down to a lower the application of quantum me- level. One hits the atom, and chanics to electronics after World two come out. When this happens, War II. Together with other advances, the emitted photon takes off in they helped spawn a new discipline precisely the same direction as the in applied known since the light that stimulated the energy loss, late 1950s as “quantum electronics.” with the two waves exactly in step An early small laser made of semicon- Since early in the twentieth cen- (or in the same “phase”). This “stim- ducting material (right) compared with a tury, physicists such as , ulated emission” results in coherent U.S. dime. , and amplification, or amplification of a learned how molecules and atoms wave at exactly the same frequency absorb and emit light—or any other and phase. electromagnetic radiation—on the Both absorption and stimulated basis of the newly discovered laws of emission can occur simultaneously. quantum . When atoms or As a light wave comes along, it can molecules absorb light, one might thus excite some atoms that are in say that parts of them wiggle back lower energy states into higher states and forth or twirl with new, added and, at the same time, induce some energy. requires of those in higher states to fall back that they store energy in very spe- down to lower states. If there are cific ways, with precise, discrete lev- more atoms in the upper states than els of energy. An atom or a molecule in the lower states, more light is can exist in either its ground (lowest) emitted than absorbed. In short, the energy state or any of a set of higher light gets stronger. It comes out (quantized) levels, but not at energies brighter than it went in. between those levels. Therefore they The reason why light is usually only absorb light of certain wave- absorbed in materials is that sub- lengths, and no others, because the stances almost always have more wavelength of light determines the atoms and molecules sitting in lower energy of its individual photons (see energy states than in higher ones: box on right). As atoms or molecules more photons are absorbed than drop from higher back to lower en- emitted. Thus we do not expect to ergy levels, they emit photons of the shine a light through a piece of glass same energies or wavelengths as and see it come out the other side those they can absorb. This process brighter than it went in. Yet this is is usually spontaneous, and this kind precisely what happens with lasers. of light is normally emitted when The trick in making a laser is to these atoms or molecules glow, as in produce a material in which the a fluorescent light bulb or neon lamp, energies of the atoms or molecules radiating in nearly all directions. present have been put in a very Einstein was the first to recognize abnormal condition, with more of clearly, from basic thermodynamic them in excited states than in the

22 SUMMER/FALL 2000 How Lasers Work

ECAUSE OF QUANTUM (a) mechanics, atoms and molecules Bcan exist only in discrete states with very specific values of their total energy. They change from one state to another by absorbing or emitting photons whose ener- gies correspond to the difference between two such energy levels. This process, which generally occurs by an jumping (b) between two adjacent quantum states, is illustrated in the accompanying drawings.

(c)

(d)

Stimulated emission of photons, the basis of laser operation, differs from the usual ab- sorption and . When (e) an atom or molecule in the “ground” state absorbs a photon, it is raised to a higher energy state (top). This excited state may then radiate spontaneously, emitting a pho- ton of the same energy and reverting back to the ground state (middle). But an excited atom or molecule can also be stimulated to emit a photon when struck by an approach- (a) Before the cascade begins, the atoms in the crystal are in their ground state. ing photon (bottom). In this case, there is (b) Light pumped in and absorbed by these atoms raises most of them to the excited now a second photon in addition to the state. (c) Although some of the spontaneously emitted photons pass out of the crys- stimulating photon; it has precisely the tal, the cascade begins when an excited atom emits a photon parallel to the axis of same wavelength and travels exactly in the crystal. This photon stimulates another atom to contribute a second photon, and phase with the first. (d) the process continues as the cascading photons are reflected back and forth be- Lasers involve many atoms or mole- tween the parallel ends of the crystal. (e) Because the right-hand end is only partially cules acting in concert. The set of drawings reflecting, the beam eventually passes out this end when its intensity becomes great (right) illustrates laser action in an optical- enough. This beam can be very powerful. quality crystal, producing a cascade of pho- tons emitted in one direction.

BEAM LINE 23 ground, or lower, states. A wave of to find a way to generate waves electromagnetic energy of the proper shorter than those produced by the frequency traveling through such a klystrons and magnetrons developed None of us who worked peculiar substance will pick up rather for radar in World War II. One day I than lose energy. The increase in the suddenly had an idea—use molecules on the first lasers ever number of photons associated with and the stimulated emission from this wave represents amplification— them. With graduate student Jim light amplification by stimulated Gordon and postdoc Herb Zeiger, I imagined how many uses emission of radiation. decided to experiment first with am- If this amplification is not very monia (NH3) molecules, which emit large on a single pass of the wave radiation at a wavelength of about there might eventually be. through the material, there are ways 1 centimeter. After a couple of years to beef it up. For example, two par- of effort, the idea worked. We chris- allel mirrors—between which the tened this device the “.” It Many of today’s practical light is reflected back and forth, with proved so interesting that for a while excited molecules (or atoms) in the I put off my original goal of trying to technologies have resulted middle—can build up the wave. Get- generate even shorter wavelengths. ting the highly directional laser beam In 1954, shortly after Gordon and out of the device is just a matter of I built our second maser and showed from basic scientific one mirror being made partially that the frequency of its transparent, so that when the inter- radiation was indeed remarkably nally reflecting beam gets strong pure, I visited Denmark and saw research done years enough, a substantial amount of its Niels Bohr. As we were walking power shoots right on through one along the street together, he asked me what I was doing. I described the to decades before. end of the device. The way this manipulation of maser and its amazing performance. physical laws came about, with the “But that is not possible!” he ex- many false starts and blind alleys on claimed. I assured him it was. the way to its realization, is the sub- Similarly, at a cocktail party in ject of my book, How the Laser Hap- Princeton, New Jersey, the Hungar- pened. Briefly summarized in this ar- ian mathematician John von Neu- ticle, it also describes my odyssey as mann asked what I was working on. a scientist and its unpredictable and I told him about the maser and the perhaps natural path to the maser purity of its frequency. and laser. This story is interwoven “That can’t be right!” he declared. with the way the field of quantum But it was, I replied, telling him it electronics grew, rapidly and strik- had already been demonstrated. ingly, owing to a variety of important Such protests were not offhand contributors, their cooperation and opinions about obscure aspects of competitiveness. physics; they came from the marrow of these men’s bones. Their objec- URING THE LATE 1940s tions were founded on principle—the and early 1950s, I was exam- Heisenberg . Dining molecules with micro- A central tenet of quantum mechan- waves at —do- ics, this principle is among the core ing microwave spectroscopy. I tried achievements that occurred during

24 SUMMER/FALL 2000 a phenomenal burst of creativity in accurately measured or tracked. The the first few decades of the twentieth precision arises from factors that century. As its name implies, it de- mollify the apparent demands of the scribes the impossibility of achiev- uncertainty principle. ing absolute knowledge of all the as- I am not sure that I ever did con- pects of a system’s condition. There vince Bohr. On that sidewalk in Den- is a price to be paid in attempting mark, he told me emphatically that to measure or define one aspect of if molecules zip through the maser a specific particle to very great ex- so quickly, their emission lines must actness. One must surrender knowl- be broad. After I persisted, howev- edge of, or control over, some other er, he relented. feature. “Oh, well, yes,” he said; “Maybe A corollary of this principle, on you are right.” But my impression NOVA, the world’s largest and most pow- which the maser’s doubters stum- was that he was just trying to be po- erful laser, at the Lawrence Livermore bled, is that one cannot measure an lite to a younger . National Laboratory in California. Its 10 object’s frequency (or energy) to great After our first chat at that Prince- laser beams can deliver 15 trillion watts accuracy in an arbitrarily short time ton party, von Neumann wandered of light in a pulse lasting 3 billionths of a second. With a modified single beam, it interval. Measurements made over a off and had another drink. In about has produced 1,250 trillion watts for half finite period of time automatically fifteen minutes, he was back. a trillionth of a second. (Courtesy impose uncertainty on the observed “Yes, you’re right,” he snapped. Lawrence Livermore National frequency. Clearly, he had seen the point. Laboratory) To many physicists steeped in the uncertainty principle, the maser’s performance, at first blush, made no sense at all. Molecules race through its cavity, spending so little time there—about one ten-thousandth of a second—that it seemed impossible for the frequency of the output mi- crowave radiation to be so narrowly defined. Yet that is exactly what hap- pens in the maser. There is good reason that the un- certainty principle does not apply so simply here. The maser (and by anal- ogy, the laser) does not tell you any- thing about the energy or frequen- cy of any specific molecule. When a molecule (or atom) is stimulated to radiate, it must produce exactly the same frequency as the stimulating radiation. In addition, this radiation represents the average of a large num- ber of molecules (or atoms) work- ing together. Each individual mole- cule remains anonymous, not

BEAM LINE 25 The development

of the laser followed

no script except

He seemed very interested, and he to hew to the nature the total power required—even to asked me about the possibility of do- amplify visible light—is not neces- ing something similar at shorter sarily prohibitive. wavelengths using semiconductors. Not only were there no clear Only later did I learn from his of scientists groping penalties in such a leapfrog to very posthumous papers, in a September short wavelengths or high frequencies, 1953 letter he had written to Edward this approach offered big advantages. Teller, that he had already proposed to understand, In the near-infrared and visible re- producing a cascade of stimulated in- gions, we already had plenty of ex- frared radiation in semiconductors perience and equipment. By contrast, by exciting with intense to explore, and wavelengths near 0.1 mm and tech- neutron bombardment. His idea was niques to handle them were rela- almost a laser, but he had not tively unknown. It was time to take thought of employing a reflecting to create. a big step. cavity nor of using the coherent prop- Still, there remained another ma- erties of stimulated radiation. jor concern: the resonant cavity. To contain enough atoms or molecules, N THE LATE SUMMER of the cavity would have to be much 1957, I felt it was high time I drive any specific resonant frequency longer than the wavelength of the Imoved on to the original goal that inside a cavity. radiation—probably thousands of fostered the maser idea: oscillators As I played with the variety of times longer. This meant, I feared, that work at wavelengths apprecia- possible molecular and atomic tran- that no cavity could be very selective bly shorter than 1 millimeter, beyond sitions, and the methods of excit- for one and only one frequency. The what standard electronics could ing them, what is well-known today great size of the cavity, compared achieve. For some time I had been suddenly became clear to me: it is to a single wavelength, meant that thinking off and on about this goal, just as easy, and probably easier, to many closely spaced but slightly dif- hoping that a great idea would pop go right on down to really short ferent wavelengths would resonate. into my head. But since nothing had wavelengths—to the near-infrared or By great good fortune, I got help spontaneously occurred to me, I de- even optical regions—as to go down and another good idea before I pro- cided I had to take time for concen- one smaller step at a time. This was ceeded any further. I had recently be- trated thought. a revelation, like stepping through gun a consulting job at Bell Labs, A major problem to overcome was a door into a room I did not suspect where my brother-in-law and former that the rate of energy radiation from existed. postdoc Arthur Schawlow was work- a molecule increases as the fourth The Doppler effect does indeed in- ing. I naturally told him that I had power of the frequency. Thus to keep creasingly smear out the frequency been thinking about such optical molecules or atoms excited in a of response of an atom (or molecule) , and he was very interested regime to amplify at a wavelength of, as one goes to shorter wavelengths, because he had also been thinking in say, 0.1 millimeter instead of 1 cen- but there is a compensating factor that very direction. timeter requires an increase in pump- that comes into play. The number of We talked over the cavity prob- ing power by many orders of mag- atoms required to amplify a wave by lem, and Art came up with the so- nitude. Another problem was that for a given amount does not increase, lution. I had been considering a gas molecules or atoms, Doppler ef- because atoms give up their quanta rather well-enclosed cavity, with mir- fects increasingly broaden the emis- more readily at higher frequencies. rors on the ends and holes in the sion spectrum as the wavelength gets And while the power needed to keep sides only large enough to provide smaller. That means there is less am- a certain number of atoms in excited a way to pump energy into the gas plification per molecule available to states increases with the frequency, and to kill some of the directions in

26 SUMMER/FALL 2000 which the waves might bounce. He suggested instead that we use just two plates, two simple mirrors, and leave off the sides altogether. Such arrangements of parallel mirrors were already used at the time in optics; they are called Fabry–Perot interfer- ometers. Art recognized that without the sides, many oscillating modes that depend on internal reflections would eliminate themselves. Any wave hit- ting either end mirror at an angle would eventually walk itself out of the cavity—and so not build up en- ergy. The only modes that could sur- vive and oscillate, then, would be waves reflected exactly straight back and forth between the two mirrors. Labs patent lawyers told us that they James Gordon, Nikolai Basov, Herbert More detailed studies showed that had done their job protecting its Zeiger, , and author the size of the mirrors and the dis- ideas. The Physical Review published Charles Townes (left to right) attending the first international conference on tance between them could even be it in the December 15, 1958, issue. quantum electronics in 1959. chosen so that only one frequency In September 1959 the first scien- would be likely to oscillate. To be tific meeting on quantum electronics sure, any wavelength that fit an ex- and resonance phenomena occurred act number of times between the at the Shawanga Lodge in New York’s mirrors could resonate in such a cav- Catskill Mountains. In retrospect, it ity, just as a piano string produces not represented the formal birth of the just one pure frequency but also maser and its related physics as a dis- many higher harmonics. In a well- tinct subdiscipline. At that meet- designed system, however, only one ing Schawlow delivered a general talk frequency will fall squarely at the on optical masers. transition energy of the medium Listening with interest was within it. Mathematically and phys- from the Hughes ically, it was “neat.” Research Laboratories in Culver City, Art and I agreed to write a paper California. He had been working jointly on optical masers. It seemed with ruby masers and says he was al- clear that we could actually build ready thinking about using ruby as one, but it would take time. We spent the medium for a laser. Ted listened nine months working on and off in closely to the rundown on the pos- our spare time to write the paper. We sibility of a solid-state ruby laser, needed to clear up the engineering about which Art was not too hope- and some specific details, such as ful. “It may well be that more suit- what material to use, and to clarify able solid materials can be found,” the theory. By August 1958 the man- he noted, “and we are looking for uscript was complete, and the Bell them.”

BEAM LINE 27 Maiman felt that Schawlow was tists recently hired after their uni- far too pessimstic and left the meet- versity research in the field of mi- ing intent on building a solid-state crowave and radio spectroscopy— ruby laser. In subsequent months Ted students of , Polykarp made striking measurements on Kusch, and myself, who worked to- ruby, showing that its lowest energy gether in the Columbia Radiation level could be partially emptied by Laboratory, and of Nicholas Bloem- excitation with intense light. He bergen at Harvard. The whole field then pushed on toward still brighter of quantum electronics grew out of excitation sources. On May 16, 1960, this approach to physics. he fired up a flash lamp wrapped The development of the maser around a ruby crystal about 1.5 cm and laser followed no script except long and produced the first operating to hew to the nature of scientists laser. groping to understand, explore and The evidence that it worked was create. As a striking example of how somewhat indirect. The Hughes important technology applied to hu- group did not set it up to shine a spot man interests can grow out of basic of light on the wall. No such flash of university research, the laser’s de- a beam had been seen, which left velopment fits a pattern that could room for doubt about just what they not have been predicted in advance. had obtained. But the instruments What research planner, wanting a had shown a substantial change in more intense light source, would the fluorescence spectrum of the have started by studying molecules ruby; it became much narrower, a with ? What industrial- clear sign that emission had been ist, looking for new cutting and weld- stimulated, and the radiation peaked ing devices, or what doctor, wanting at just the wavelengths that were a new surgical tool as the laser has most favored for such action. A short become, would have urged the study while later, both the Hughes re- of microwave spectroscopy? The searchers and Schawlow at Bell Labs whole field of quantum electronics independently demonstrated power- is truly a textbook example of broad- ful flashes of directed light that made ly applicable technology growing un- spots on the wall—clear intuitive expectedly out of basic research. proof that a laser is indeed working.

T IS NOTEWORTHY that al- most all lasers were first built in Iindustrial labs. Part of the reason for industry’s success is that once its importance becomes apparent, in- dustrial laboratories can devote more resources and concentrated effort to a problem than can a university. When the goal is clear, industry can be effective. But the first lasers were invented and built by young scien-

28 SUMMER/FALL 2000