by Hanna Kurlanda-Witek
Charles Hard Townes
Nobel Prize in Physics 1964 together with Nikolai G. Basov and Aleksandr M. Prokhorov "for fundamental work in the field of quantum electronics, which has led to the construction of oscillators and amplifiers based on the maser-laser principle".
The maser, and its’ optical variant, the laser, were developed in the 1950s and since that time have become part of our daily lives. Whether we are listening to a CD, checking the price of a product at a supermarket or following the spot of red light at a lecture, we often take for granted that what is now a reputable and profitable industry emerged from basic scientific research. Charles Hard Townes was at the forefront of this remarkable invention, which has since contributed to the science behind at least twelve Nobel prizes.
Townes was born on July 28, 1915, in Greenville, South Carolina. A keen discoverer by nature with an enthusiasm for bird watching, he aspired to become a biologist, but as his elder brother studied biology, he decided to pursue a different scientific field and turned to physics. In 1935, Townes graduated with highest honours from Furman University in his hometown with a Bachelor of Science in physics and a Bachelor of Arts in modern languages. Afterwards, he received a Master of Arts degree in physics from Duke University, and, only two years later, obtained a PhD from the California Institute of Technology (Caltech), at the age of twenty four. Townes was unsuccessful at securing an academic position at Caltech and moved across the country to New Jersey, where he joined the laboratory at Bell Telephone Company. Here, Townes ventured into the field of microwave spectroscopy and helped develop radar-guided bombing systems during World War II. The aim of this work was to produce microwaves with an increasingly shorter wavelength, as the shorter the wavelengths the higher the resolution of the radar, facilitating the detection of small objects. Townes worked with microwaves measuring 10 centimetres and eventually with 1.25-centimeter microwaves. At this point, he became interested in how molecules absorbed microwaves at a defined wavelength and how this occurrence could be used to identify various molecules. After the war, Townes continued his work on microwave spectroscopy at Columbia University, where he became director of the Columbia Radiation Laboratory in 1950, and chair of the Physics Department in 1952.
The new discipline of scientific research that Townes and his colleagues were developing became known as quantum electronics, where particle-focused processes discovered at the beginning of the twentieth century, known as quantum mechanics, became implemented in the field of electronics. In 1916, Albert Einstein proposed the theory of stimulated emission, the process where a photon strikes an electron in an atom that is in a higher (excited) state, thus releasing a new photon, and causing the energy level of the atom to drop to a lower (ground) state. Thus, two photons are released, with the same wavelength and frequency. Because electrons are unable to travel from one energy level to another but are confined to discrete energy levels (quanta), the emitted photon must have the same energy as the difference of the energy between the higher and lower states. The resulting light production is characteristic of the particular molecule and is visualised as an emission spectrum. Each chemical element possesses its’ own unique spectrum of various colours, which is a result of the particular frequencies of the photons. Moreover, when electromagnetic radiation interacts with a molecule made up of different atoms, not only does this cause the atom’s electrons to move, but it results in rotation and vibration of the whole molecule, which is also discrete. This occurrence determines the defined wavelength of the molecule. Studies of how atoms and molecules behaved when energised with electromagnetic waves paved the way for the new field of spectroscopy. It was precisely these experiments of labelling molecules that pointed Townes in the direction of building the first maser. The medium that sparked his interest was ammonia gas, which emitted a microwave of 1.25 cm in length.
Generally, in accordance with thermal equilibrium, more molecules are in a ground state than in an excited state, which is why, for example, light is absorbed by objects, as more photons are absorbed in atoms than emitted. The occurrence where the number of molecules in a higher state exceed those in a lower state is known as a population inversion. As Townes himself explained in his book, “How the Laser Happened”, this is as if one would shine a light through a piece of glass and the light would become stronger when passing through the glass. After Einstein presented the theories of stimulated emission in his paper, Quantentheorie der Strahlung (“On the Quantum Theory of Radiation”), for decades the theory was regarded as more of a curiosity and scientists failed to see the far-reaching implications of stimulated emission. It wasn’t until 1951, when Edward Purcell and Robert Pound conducted their famous research on nuclear magnetic resonance in a lithium fluoride crystal, that population inversion began to draw attention.
Townes and his PhD student James Gordon, and postdoc Herbert Zeiger used the idea of stimulated emission through ammonia to generate the amplification of microwaves; hence the term maser is an acronym of microwave amplification by stimulated emission of radiation. Townes sought to construct a type of oscillator, which could produce electromagnetic waves of a determined frequency. Yet bombarding ammonia molecules with microwaves was not enough to produce a coherent beam of radiation of a constant wavelength. The ammonia gas had to be placed in a resonant cavity, a hollow cylinder with reflective walls, which would uphold the stimulated emission by causing the emitted photons to deflect from the walls of the cavity. This escalation of energy would trigger other atoms to release photons and result in a growing cascade of photons, leading to microwave amplification.
Despite having unravelled most of the theoretical foundations, the project went through many hurdles, and at one point was almost suspended by Townes’ supervisors as it seemed too costly and was slow to produce results. Eventually, the first maser was built at Columbia University in early 1954 and was announced by Townes at a meeting of the American Physical Society. The paper, “The Maser – New Type of Microwave Amplifier, Frequency Standard and Spectrometer”, authored by Gordon, Zeiger, and Townes, was published in 1955 in Physical Review. Unbeknownst to the group, a team of Soviet scientists, led by Nikolai Basov and Aleksandr Prokhorov were conducting very similar research at the Lebedev Physical Institute in Moscow. It wasn’t until a meeting of the Faraday Society in Cambridge, England in 1955, where Prokhorov presented the theoretical groundwork for the maser, that Townes realised they were working on analogous projects. After some explanation, it appeared that Townes and his colleagues had produced the first operating maser, whereas Prokhorov’s and Basov’s team were first to describe the theory behind the maser. What initially emerged as a rivalry with a Cold War backdrop, eventually became a friendship. Townes visited the Lebedev Institute while on a sabbatical from Columbia University in 1955. All three scientists continued to work in the field of quantum electronics.
At first, Townes was sceptical of whether a light amplifier could be constructed, but by 1957 he and his brother-in-law Arthur Schawlow took time out of their busy schedules to work on the theoretical background of the laser, at that time referred to by Townes as the “optical maser”. Townes and Schawlow were developing the idea at Bell Laboratories, where Schawlow worked and Townes agreed to work part-time as a consultant. The task to devise a laser was not an easy one. Not enough research had been carried out to characterise wavelengths at frequencies higher than microwaves. The higher frequencies also signified higher transition energies, which would lead to a higher loss of energy and so less stimulated emission. Townes realised that it would be easier to work with infrared and visible light. The frequency of visible light is more than a thousand times higher than microwaves, however, there was a large number of studies carried out in the field of optical spectroscopy. Townes and Schawlow were able to demonstrate that although the power needed to run such a device would be high due to the high frequency, it was still feasible. After some consideration of other elements, potassium vapour was chosen as a medium for the laser, which possessed simple spectra, however, when it was used in experiments by Schawlow much later on, it was found to be very reactive to work with. Another problem the pair encountered was how to build a resonant cavity that would have a practical design, yet would be large enough to hold enough atoms for stimulated emission to occur. With the maser, the principle of the cavity design was that the holes of the cavity had to be as large as the microwaves, which were centimetres in length, but in this case, the wavelengths were less than a micrometer in length. Additionally, the cavity itself could not be too large, as that would greatly limit its’ selectivity; waves of many similar frequencies would oscillate in the resonant cavity. A breakthrough in the design of the resonant cavity came when Schawlow proposed to use a Fabry-Perot interferometer, essentially two parallel mirrors, a device which he had used in his doctoral work. By determining the size and distance of the mirrors, as well as the angle at which they would be positioned, it was possible to significantly narrow the number of wavelengths, thus producing a pure monochromatic beam of light. At the time, neither Townes or Schawlow had the time or resources to conduct experiments on this new technique, hence they submitted a paper of the theoretical description of the laser to Physical Review Letters, which was published in December, 1958. They also insisted on Bell Laboratories securing a patent. Almost a year later, the first international conference on quantum electronics took place in High View, New York, bringing together the pioneers of the field; Townes, Schawlow, Gordon, Zeiger, as well as Prokhorov and Basov. Among those attending was Theodore Maiman from Hughes Research Laboratories in California, who had been working on a laser design using a synthetic ruby crystal as a medium. In the end, it was Maiman who produced the first functioning laser, in May, 1960. Maiman used an intense pulse of light to excite the ruby rod, a technique known as optical pumping. His landmark paper, published in Nature in August, 1960 was hailed as “the most important per word of any of the wonderful papers of Nature over the last century” by Townes himself. By that time, Townes had temporarily left his academic post and was working as director of research at the Institute for Defense Analyses in Washington, D.C. In 1961, Townes accepted a professorship at the Massachusetts Institute of Technology (MIT), where he remained for the next six years. During his tenure at MIT, in 1964, Townes, jointly with Prokhorov and Basov, was awarded the Nobel prize “for fundamental work in the field of quantum electronics, which has led to the construction of oscillators and amplifiers based on the maser-laser principle".
The technology of both the maser and the laser became more sophisticated, and eventually laboratory prototypes found applications in various industries. One extraordinary feat was measuring the distance between the Earth and the Moon during the lunar landing in 1969. Townes was chairman of NASA’s Apollo Science and Technology Advisory Committee, and from his front-row seat at NASA’s Space Centre in Houston watched Neil Armstrong and Edwin “Buzz” Aldrin secure mirrors onto the Moon’s surface. Astrophysicists in research labs in California and Texas used telescopic lasers to shine on the mirrors and then observed the reflection of light. By this time an esteemed professor of physics, Townes craved to be able to conduct research in new fields, and this was currently the burgeoning discipline of molecular astrophysics. In 1967, Townes made the unlikely decision to join the physics department at the University of California at Berkeley. It was here that Townes helped to generate a series of interesting discoveries, such as finding ammonia and water molecules in the galaxy Sagittarius B, and later in the Orion Nebula. Peculiarly, Townes realised that his earlier discovery had already existed in nature, when at the Hat Creek Radio Observatory in California, it was found that masers occur naturally in space when light emitted by stars excites water molecules, which induce them to release microwaves. Townes mentioned this property of the interstellar medium during his lecture at the 21st meeting in Lindau in 1971.
Subsequently, Townes and his colleagues worked on the construction of a scanning Fabry-Perot spectrometer, which ultimately led to the discovery of a black hole in the Milky Way, a very dense concentration of gas moving at up to 400 000 miles per hour. Townes also contributed to the construction of a unique infrared spatial interferometer with integrated infrared CO2 lasers, which greatly advanced the resolution of telescopic images, and improved the observation of dust clouds and stars. This work was presented at the 50th Lindau Nobel Laureate Meeting in 2000.
Townes passed away on January 27th, 2015, six months before his 100th birthday, yet up until the summer of 2014, he was still working daily at his laboratory, despite needing a wheelchair and oxygen. Many remember him as a grounded and simultaneously far-seeing scientist, but also an exceptional educator, who supervised the doctoral theses of 68 graduate students. He was also well-known for his charitable work, and for his views on the convergence of faith and science. As with most famous scientists, there are a number of books and articles on the life and research of Charles Hard Townes, but not many Nobel prize winners have their biography written as a storybook for children. “First, You Explore. The Story of the Young Charles Townes” portrays Townes as an amateur scientist, growing up on a farm, and encourages very young readers to be curious of the world around them, which is what motivated Townes to dedicate his life to science and discovery.
Berkowitz, J. (2012) The Stardust Revolution: the New Story of our Origin in the Stars, Prometheus Books, New York NY
Brandt, S. (2008) The Harvest of a Century: Discoveries in Modern Physics in 100 Episodes, Oxford University Press Canada
Haynie R., Cook, T. (2014) First, You Explore. The Story of the Young Charles Townes. Young Palmetto Books
Hecht, J. (2005) Beam: The Race to Make the Laser. Oxford University Press, New York, NY
Shampo M.A., Kyle R.A. and Steensma D.P. (Sept 2011) Charles Townes-Nobel Laureate for Maser-Laser Work Stampvignette on Medical Science, Mayo Clinic Proceedings; 86(9):e48
Townes, C.H. (Dec 1964) Production of coherent radiation by atoms and molecules, Nobel lecture.
Townes, C.H. (1999) How the Laser Happened. Adventures of a Scientist. Oxford University Press
Townes, C.H. (2003) The first laser. From: A Century of Nature: Twnety-One Discoveries that Changed Science and the World, L. Garwin and T. Lincoln (eds), the University of Chicago Press
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