While “energy beams” and “heat rays” have been a staple of fiction for centuries, most famously appearing in H.G. Wells’s 1897 novel The War of the Worlds, the practical history of the laser begins in the year 1900 with German physicist Max Planck’s discovery of quantum theory. While studying the electromagnetic radiation given off by heated bodies, Planck discovered that said radiation is not emitted continuously as predicted by classical physics but rather in discrete “packets” of a given frequency and energy – which Planck dubbed quanta. In 1911, Danish physicist Niels Bohr proposed an atomic model to explain the emission of quanta. According to Bohr, the electrons orbiting an atom’s nucleus can only occupy a series of discrete energy levels. When an atom is excited – for example, by heating or electrification – this causes electrons to jump to a higher energy level. These excited states, however, are unstable, and when the electrons inevitably drop back down to their original minimum-energy levels or ground states, the excess energy is emitted in the form of light quanta or photons – the energy and frequency of which is dictated by the electron configuration of the atom. As every kind of atom has a different electron configuration, this explains why different elements give off different wavelengths of light – their emission spectra – when heated. Six years earlier, Albert Einstein had shown that this process could also work in reverse and that an atom could absorb a photon and release an electron – a phenomenon known as the photoelectric effect. Interestingly, it was for this discovery and not his more famous theories of relativity that Einstein was awarded the 1921 Nobel Prize for Physics. In a 1917 paper, Einstein expanded on these ideas to show that, under certain circumstances, a photon can interact with with an excited electron in an atom, by causing it to drop an energy level and emit another photon, whose frequency was identical to that of the original photon. This process, known as stimulated emission, is fundamental to the operation of every laser.
At the time, however, Einstein’s theoretical scenario was considered impossible to test. This is because the atomic energy levels in any given substance typically follow the Maxwell-Boltzmann distribution, meaning that very few atoms were in a suitably excited state at any given time. This, in turn, meant that stimulated emissions, if they did occur, would be too rare to be reliably detected. In 1928, German physicist Rudolf Ladenburg was studying the emission and absorption of light in gas discharge tubes when he detected light emissions similar to those predicted by Einstein. However, Ladenburg, like most of his contemporaries, was too blinded by the orthodoxy of the Maxwell-Boltzmann distribution to recognize the significance of his observations. And so, Einstein’s ideas about stimulated emission continued to languish in obscurity for another two decades.
The next major milestone in the development of the laser was the 1940 invention of the cavity magnetron by British physicists John Randall and Harry Boot of the University of Birmingham. The magnetron, which could generate centimetre-wavelength microwaves, allowed for the construction of compact, extremely precise airborne radar sets, and was instrumental in securing Allied air supremacy during the Second World War. After the war, the ready availability of surplus radar equipment inspired a flurry of research and technical developments, including the discovery of magnetic resonance – now used in medical MRI machines; the invention of the microwave oven; and the creation of the laser’s earliest ancestor.
In 1953, American physicists Charles Townes and of Columbia University succeeded in using microwaves to achieve Einstein’s stimulated emission on a practical scale. To accomplish this, Townes bombarded ammonia molecules with microwaves inside a resonant cavity. This caused ammonia molecules to release microwave photons, which then stimulated other molecules to release identical photons and so on, creating a cascade effect that released a focused beam of microwaves all at the same frequency and phase. Townes dubbed his creation the maser, short for Microwave Amplification via the Stimulated Emission of Radiation. The key to the maser’s operation was a phenomenon known as population inversion, in which a large enough proportion of the atoms in a substance are pushed away from Boltzmann equilibrium to allow stimulated emission to predominate. The theoretical possibility of this method had already been predicted by Soviet physicist Valentin Fabrikant in his 1940 doctoral thesis, but his findings were largely ignored.
At the same time as Townes, Soviet physicists Aleksandr Prokhorov and Nikolay Basov of Moscow’s P.N. Lebedev Physical Institute independently demonstrated the principle of the maser. For their groundbreaking discovery, all three would share the 1964 Nobel Prize for Physics.
Townes used his maser to perform microwave spectroscopy, measuring how different substances absorbed or transmitted various frequencies of microwaves to probe their inner structure. As shorter wavelengths yielded more accurate results, Townes began to wonder whether the frequency range of the maser could be extended into the infrared or even visible light range. Together with his brother-in-law Arthur Schawlow and graduate student Gordon Gould, Townes further developed the theory of the “optical maser,” publishing a seminal paper on the subject in the December 15, 1958 issue of the journal Physical Review. Meanwhile, in 1959 Gould wrote and submitted a patent application, in which he replaced the word “microwave” with “light” to coin the acronym Light Amplification via the Stimulated Emission of Radiation – or “laser” for short. However, Gould’s application was rejected in favour of a patent filed by Townes and Schawlow the following year. As we shall later see, this resulted in a fierce legal battle lasting nearly 30 years.
Townes’ 1958 paper touched off an international race to build the first functioning laser. During their early research on the topic, Townes, Schawlow, and Gould had realized that the resonant-cavity design Townes had used in his original maser would not work for visible light. However, Schawlow proposed that the same effect could be achieved using two parallel mirrors – one semi-silvered and one fully silvered. This arrangement would cause light to bounce back and forth between the two mirrors, stimulating the release of ever more photons. Once the light was intense enough, it would push through the semi-silvered mirror, emerging as a coherent, monochromatic beam. But while this design worked in theory, there were still two major obstacles to overcome: finding a material that could exhibit stimulated emission in the visible light range, and a method for stimulating said emissions.
The first to achieve this breakthrough was Dr. Theodore H. Maiman, an engineer at the Hughes Research Laboratories in Malibu, California. Maiman had already created an ultracompact maser for the U.S. Army Signal Corps using synthetic ruby – aluminium oxide containing traces of chromium. (Incidentally, the gemstone sapphire is also a variety of aluminium oxide – or corundum in geological terms – its colour deriving from iron rather than chromium impurities). Following this success, Maiman convinced Hughes to fund further research into optical lasers to the tune of $50,000.
To build his laser, Maiman turned once again to synthetic ruby. While other researchers had dismissed this material as unusable, Maiman’s intimate familiarity with its absorption and emission properties convinced him that it would work – if only the right stimulation or “pumping” mechanism could be found. While browsing through a scientific equipment catalogue, Maiman found his answer: a powerful spiral Xenon flash lamp of the kind used by photographers. Maiman wrapped one of these lamps around a cylindrical rod of synthetic ruby, then placed a pair of mirrors – one fully-silvered, and one half-silvered – at either end, just as Arthur Schawlow had suggested. On May 16, 1960, he observed the first flashes of 695 nanometre wavelength red light streaming from the end of the ruby rod; he had created the world’s first practical laser.
Excited by his discovery, Maiman rushed to publish a paper titled Optical Maser Action in Ruby in the Physical Review, but to his shock it was immediately rejected. This rejection has become part of laser history lore, and is often blamed on a conservative and unimaginative scientific establishment failing – or refusing – to recognize the significance of Maiman’s discovery. After all, as Maiman’s assistant Irnee D’Haenens quipped at the time, for many years the laser was seen as a ‘solution looking for a problem’ with few apparent applications outside of scientific research. The truth, however, is far less exciting. According to Simon Pasternack, an editor at the Physical Review at the time, he rejected Maiman’s paper simply because he had already published a very similar paper earlier that year. In any case, a shortened version of Maiman’s paper was readily accepted by the journal Nature, and his discoveries soon came to the attention of the scientific community.
But while Maiman’s ruby laser was an important technical breakthrough, it was somewhat limited in its capabilities. To understand why, it is necessary to understand in greater detail just how lasers actually work. As we’ve covered earlier in this video, lasers work by using photons to stimulate electrons in atoms to jump from one energy level to another and back, causing the atoms to release photons of the same energy as the original. In the simplest systems such as Townes’ original ammonia maser, there are just two energy levels: the ground state and the excited state. Maiman’s laser, however, had three energy levels: the ground state, the excited state, and a metastable state between the two. When the ruby was stimulated or “pumped” by the flash lamp, the electrons in the atoms jumped first to the excited state then spontaneously dropped to the metastable state, where they remained for a considerable length of time. This longevity was key to the laser’s operation, as it allowed a sufficient population of excited atoms to accumulate to achieve a population inversion – a state in which the photons created by stimulated emission outweighed those absorbed by ground-state atoms. The problem, however, was that once an atom released a photon, it immediately dropped back into the ground state, which soon became oversaturated and prevented population inversion from taking place. For this reason, Maiman’s laser could only produce light in short pulses. Producing a continuous beam required the development of a four-level laser with two metastable states.
Thankfully, however, Maiman’s discovery opened the floodgates on laser research, and new discoveries in the field came hard and fast. Just seven months later, Ali Javan, William Bennet, Jr., and Donald Herriott at Bell Labs in New Jersey invented the first gas laser, which used a mixture of helium and neon as the emission material and could produce a continuous laser beam. In 1962, Robert N. Hall at the General Electric Research and Development Center in Schenectady, New York developed the first solid-state semiconductor laser – the ancestor of the cheap laser pointers we – and our cats – know and love today; while in 1964 Kumar Patel at Bell Labs created the carbon dioxide laser, which could easily be built in the megawatt range – powerful enough to cut and weld steel.
And despite Irnee D’Haenens’ 1960 assertion, lasers soon found a wide range of applications, with their ability to produce extremely straight, focused beams of uniform frequency proving highly useful multiple fields. Helium-neon lasers were used to draw straight lines on construction sites, measure ranges on the battlefield, and create the first 3D holograms; carbon dioxide lasers to cut and weld metal and other materials in factories; and ruby lasers to measure the distance between the earth and the moon, the beam being bounced off retroreflectors left on the surface by the Apollo astronauts. In the 1970s, doctors began using finely-adjustable dye lasers to treat skin diseases like melanoma and perform optical surgery without cutting into the eyeball, while the development of ultra-pure optical glass fibres allowed telephone signals and other data to be transmitted around the world by pulses of laser light – and for more on these latter two developments, please check out our previous videos Alexander Graham Bell’s Forgotten Greatest Invention as well as Changing Views with a Weird Soviet Turntable Procedure – The Story of Radial Keratotomy on our sister channel Highlight History.
One of the first consumer products to incorporate a laser was the laser printer, introduced in 1971, followed closely thereafter by the laser barcode scanner in 1974. 1978 saw the debut of the LaserDisc, the first home optical storage medium which, like its descendants the CD and DVD, used a beam of laser light to decode the data etched into its surface. Today, lasers are used in all manner of consumer products, making them one of the cornerstone technologies of the Twentieth – and the Twenty-First – Century.
Given the key role lasers play in our everyday lives, it is perhaps unsurprising that credit for its invention has been the subject of some controversy. As we’ve previously covered, in 1960 Gordon Gould’s patent application for the laser was rejected in favour of a rival patent filed by his collaborators Charles Townes and Arthur Schawlow. This infuriated Gould, who launched a concerted legal battle have the primacy of his ideas recognized. His primary piece of evidence was a notebook entry on basic laser design dated and notarized November 1957 – predating even Townes and Schawlow’s seminal 1958 paper in the Physical Review. In 1973, the U.S. Court of Customs and Patent Appeals ruled that the patent awarded to Townes and Schawlow in 1960 was too general and did not cover the specifics of laser design in great enough detail. And in 1988, after nearly three decades of fighting, Gould finally received full patent rights to the laser, the royalties from which made him a millionaire.
And that, dear viewers, is a brief history of the laser, a device which many people thought was impossible and even more believed was useless, but which ended up being one of the most important and versatile technologies of modern times. But while fibre optic telecommunications, laser eye surgery, and precision manufacturing are all fine and dandy, if all those scientists and engineers could finally get around to building a working lightsaber, then I’ll truly believe we are living in the future.
Expand for ReferencesLaser, Encyclopedia Britannica, https://www.britannica.com/technology/laser/History
History of Laser Technology, Universal Laser Systems, https://www.ulsinc.com/learn/history-of-lasers
Laser History, Laser Fest, https://laserfest.org/lasers/history/early.cfm
This Month in Physics History – December 1958: Invention of the Laser, APS News, December 2003, https://www.aps.org/publications/apsnews/200312/history.cfm
Townes, Charles, The First Laser, A Century of Nature, https://press.uchicago.edu/Misc/Chicago/284158_townes.html
Sella, Andrea, Maiman’s Laser, Chemistry World, December 1, 2014, https://ift.tt/bjpP5Ss
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