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Remembering the late Charles H Townes

April 2015 Number 103

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Remembering the late Charles H. Townes



Top photo: Charles H Townes (left) and Mario Bertolotti. Bottom photo: The author presenting Townes with a medal from the Italian Optics and Photonics Society in 2004. In the background is Sergio Martellucci.

The international scientific community was ready to celebrate the International Year of Light 2015 (IYL2015) when Charles H Townes passed away, 27 January 2015, at the age of 99.

Townes is remembered for having invented and built the first maser [1] in 1954, skilfully embodying in a practical device the stimulated emission posited by Einstein [2] in 1916. We all know that stimulated emission is minority partner in chaotic thermal emission of radiation; Townes found the way to assign it the principal role, simply by selecting an inverted population [3] of molecules in a resonant cavity. Thirty eight years had been needed to realise how stimulated emission could be used to generate coherent radiation in the microwave range with unprecedented monochromaticity and low noise temperature. Townes was able to call on his expertise in both microwave spectroscopy [4] and quantum mechanics to design a device that was able to amplify and generate microwave radiation using a technique radically different from the usual techniques and that could not originate as a simple improvement of the electronic techniques already known and employed in existing devices.

He had in fact achieved important results even in the preceding years. An expert in microwaves because of his work on radar-related projects for the military, he realized [5] that a narrow microwave absorption line (for example the one of ammonia at 23.8 GHz) could be used to stabilize a microwave oscillator, introducing the basic principle of atomic clocks already in 1945.

A man of culture, Townes was accustomed to taking a broad view of problems, and after building the ammonia maser, he began to speculate on the possibility of extending to the infrared and visible part of the electromagnetic spectrum the same basic principles that he had used in masers: inversion of population, resonant cavity, feedback.

He had the good fortune to be able to involve his brother-in-law Arthur L Schawlow in this research. A vital person with a deep understanding of science, Schawlow proved to be an excellent partner in discussing how an optical maser could be realized and in suggesting possible geometries and materials. Four years after the building of the first maser, Townes and Schawlow wrote a seminal paper [6] on possible ways to extend the maser principle to shorter wavelengths. The paper was published in Physical Review in 1958 and started the race to the building of a laser.

Townes’ inclination to work with simple gaseous systems in order to have complete knowledge and control of all the parameters probably prevented him from building the first laser, which came, to the great surprise of many researchers, through the work of Theodor Maiman [7], a person considered to be a new entry in the laser race.

Townes’ eclectic nature motivated him to look at the deep universe to see if there was a trace of his maser invention there. Beginning of the early 1960s he turned his attention to microwave astrophysics, and it was so that, after having discovered ammonia emission in interstellar space [8] – a few years after the announcement by Professor H. Weaver [9] of the existence of radiation amplified by stimulated emission in clouds of OH molecules and using the same Hat Creek radio telescope used by Weaver – he discovered stimulated emission by water vapour in the Orion nebula [10]. In an after-dinner speech at the Washington maser conference in 1992, he reported that one of his collaborators had announced “It is raining, in Orion!”. Later, using infrared spectroscopy results, he presented the first evidence of a black hole in the centre of our Galaxy [11].

Few men have had the opportunity and ability to be so creative and influential as Townes. With pivotal ideas and experiments he contributed to the quantum electronic and nonlinear optics fields that were growing between 1955 and 1970. I met him on several occasions, but it was with particular pleasure that we met in Erice, Sicily, at the International Centre and Foundation for Scientific Culture, Ettore Majorana, which was created by Antonino Zichichi and is easily the best place, in my opinion, for the free discussion of physics in a friendly and inspiring atmosphere.

In 1964 Townes was awarded the Nobel Prize in Physics with N G Basov and A M Prokhorov, the Russian scientists who independently arrived at the maser concept, starting from a different path but ending proposing the same mechanism of operation [12].

Sergio Martellucci, the Director of the Quantum Optics courses in Erice, and I invited Townes to participate in the 2004 Quantum Optics Courses in Erice in 2004, and on the 40th anniversary of his receiving the Nobel Prize I presented him with a medal in the name of the Italian Optics and Photonics Society (SIOF). It was a nice ceremony in the Dirac Hall of the Ettore Majorana Centre with a hundred “students” [13] attending from all over the world. Wanting to challenge him, I asked whether he thought that one should speak of discovery of the maser rather than invention, being that maser action existed in nature. I do not know how much he appreciated the question, but he was very nice and explained that what makes the maser such a special device is not simply the stimulated emission in an inverted population but the presence of cavity and feedback, which do not exist in nature. We then had dinner and entertainment in his honour, with music and dancing in the kiosk of an ancient convent used by the Ettore Majorana Centre. My wife and I, conversing with Townes’ lovely wife Frances, watched Townes, 89 at the time, dance until midnight, at which time he stopped, not because he was tired but because by law we were forced to shut down the music.

We in the optical community and the scientific community at large are saddened by Townes’ death. He was the last of the laser fathers: Basov, Prokhorov, Schawlow and Maiman, major actors of the starting phase, had already passed away.

Anyone working in areas related to atomic clocks, masers, lasers, nonlinear optics and microwave astrophysics are all indebted to him for the important advancements he brought about in those fields.

Ciao Charles, and be happy where you are now!

Mario Bertolotti (University of Roma La Sapienza, Roma, Italy)

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[1] J P Gordon, H J Zeiger and C H Townes, Phys. Rev. 95, 282 (1954)

[2] A Einstein, Z. Phys. 18, 121 (1917)

[3] More molecules in the upper energy level than in the lower energy level

[4] C H Townes and A L Schawlow, Microwave Spectroscopy, McGraw-Hill, New York 1955

[5] C H Townes suggested the technique in a Bell Report around 1945. Also R V Pound, Rev.Sci. Instrum. 17, 490 (1946) made the same suggestion. The technique was later put into operation by W V Smith, J L Garcia de Quevedo, R L Carter, and W S Bennett, J. Appl. Phys. 18, 1112 (1947); E W Fletcher and S P Cooke, Cruft Lab. Tech. Rep 64, (1950); W D Hershberger and L E Norton, RCA Rev. 9, 38 (1948); C H Townes, A N Holten and F R Merritt, Phys. Rev. 74, 1113 (1948)

[6] C H Townes and A L Schawlow, Phys. Rev. 112, 1940 (1958)

[7] T H Maiman Nature 187, 493 (1960) I have tried to describe these first hectic years in my book Masers and Lasers: An Historic Approach, 2nd ed. to be published by Taylor and Francis soon.

[8] Already in 1955 C H Townes in the IAU Symposium 4, Radio Astronomy, ed. H C van de Hulst (London: Cambridge University Press) p.92 (1957) presented a paper in which he listed a number of molecules that might have radio transitions. Later he discovered the first polyatomic molecule in interstellar space: ammonia (A C Cheung, D M Rank, C H Townes, D D Thornton and W J Welch, Phys. Rev. Lett. 21, 1701 (1968)

[9] H Weaver, D R Williams, H Nannilou Dieter and T W Lum, Nature 208, 29 (1965)

[10] A C Cheung, D M Rank, C H Townes, D D Thornton and W J Welch, Nature 221, 626 (1969)

[11] J C Lacy,F Baas, C H Townes and T R Geballe, ApJ 227, L17 (1979); J C Lacy, C H Townes, T R Geballe and D J Hollenbach. ApJ 241, 132 (1980); J H Lacy, C H Townes and D J Hollenbach, ApJ 262, 120 (1982)

[12] N G Basov and A M Prokhorov, Zh. Eksp. Teor. Fiz. 27, 431 (1954) (in Russian); JETP 27, 431

[13] I wrote with quotation marks “students” because in reality they are PhDs, post-docs and researchers chosen among the best in the field of activity of the school

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International Commission for Optics

Bureau members (2011-2014):

President: Y. Arakawa;

Past-President: D. T. MooreTreasurer: J A Harrington;

Secretary: A M Guzmán, CREOL, The College of Optics and Photonics, University of Central Florida, PO Box 162700, 4000 Central Florida Blvd,Orlando, FL 32816-2700, USA; e-mail

Associate Secretary: G von Bally

Vice-Presidents, elected: J. Harvey, F. Höller, H. Michinel, J. Niemela, R. Ramponi, S-H Park, J. Zakrzewski, M. Zghal

Vice-Presidents, appointed: Y. J. Ding, J. C. Howell, S. Morgan, E. Rosas, P. Urbach, A Wagué, M. J. Yzuel

IUPAP Council Representative: C Cisneros

Editor in chief: A M Guzmán

Editorial committee:
K Baldwin, Australian National University, Australia;
J Dudley, Université de Franche-Comté, France;
William T Rhodes, Florida Atlantic University, USA.