Distinguished Visiting Scientist Program. Prof. Michel Piché Université Laval, Québec

Transcription

1 Institute for Optical Sciences University of Toronto Distinguished Visiting Scientist Program Prof. Michel Piché Université Laval, Québec Lecture-4: The discovery of the laser

2 The discovery of the laser From photons to lasers Michel Piché Département de physique, de génie physique et d'optique Centre d'optique, photonique et laser (COPL) Université Laval, Québec Presented at the University of Toronto on March 9, 2006

3 Four fundamental contributions of Einstein relevant to lasers: - Corpuscular nature of light (1905) - Stimulated emission of light (1916) - Wave-particle duality of light (~ 1909) - Noise and photon statistics ( ) However: The first laser was only operated in 1960 Legal confusion around the invention of the laser

4 Was the invention of the laser a technological challenge? Was it raising conceptual difficulties? Was it possible to make a laser in 1920? When has the link between stimulated emission and generation of coherent light been established? Who really invented the laser?

5 A. Principles of laser oscillation A laser oscillator is made of the following elements: - An active medium where excited atoms provide gain at optical frequencies. - A pumping mechanism that excites atoms to upper levels (e.g. levels having an energy higher than that of the ground state), in such a way as to produce an inversion of population in the active medium. - An optical cavity that provides feedback by circulating the laser beam in the active medium.

6 Commonly used lasers: - Molecular gases: CO 2 (10 μm), KrF (248 nm) - Atomic gases: He-Ne (633 nm), Ar + (514, 488 nm) - Solids: ruby (694 nm), Ti:sapphire ( nm) - Semiconductor junctions: AlGaAs ( nm) - Exotica: nuclear-pumped, JELLO or paint lasers,... Pumping mechanisms: - current in a gas discharge - optical pumping by a flashlamp or a laser - current in a semiconductor structure

7 B. Atomic levels and radiative transitions Atoms can be excited from the ground state to higher-energy levels. Once in an upper level, atoms relax towards states of lower energy. Transitions from upper to lower levels can proceed by spontaneous emission (or fluorescence), or by various collision processes.

8 C. Stimulated emission According to Einstein (1916), there are three mechanisms through which radiative transitions between atomic levels 1 et 2 can take place: 1. Stimulated absorption: leads to losses (and atomic excitation) 2. Spontaneous emission: incoherent, non-directional. 3. Stimulated emission: directional (provides light amplification)

9 Through stimulated emission, one gets: A positive energy transfer from the atomic medium to an optical beam. A light emission in the same direction as that of the incident beam. An emission at the same frequency as that of the incident beam. N.B.: Stimulated emission co-exists with spontaneous emission, whose presence is inevitable. Spontaneous emission constitutes a noise that adds to the stimulated emission signal.

10 D. Inversion of population At thermal equilibrium, the population of excited levels is inferior to that of the ground state. Therefore it is not possible to obtain an inversion of population under conditions of thermodynamic equilibrium. An atomic system with an inversion of population can be viewed as having a negative temperature. A pumping mechanism creates a disequilibrium, such that N 2 > N 1, if E 2 > E 1. Under such a condition stimulated emission is stronger than stimulated absorption

11 To maintain an inversion of population between levels 2 and 1, one has to: - Provide a sustained pumping of level 2. - Choose a transition where level 1 relaxes more rapidly than level 2.

12 E. Optical cavities and laser oscillation To guarantee laser oscillation, a laser cavity terminated by mirrors is used. This cavity can be of two types: standing wave (Fabry-Perot) or traveling wave (ring). At steady state, the field of the optical wave that has made a complete round trip in the cavity must exactly self-reproduce (in amplitude and in phase). Any field configuration verifying this condition is a laser mode. One can distinguish longitudinal and transverse modes. The laser cavity narrows the spectral width of the oscillating mode according to the Schawlow-Townes model.

13 F. Challenges and sequence of events 1. Establish the link between stimulated emission and gain 2. Develop the concept of optical oscillator 3. Design an optical feedback scheme that makes this oscillator operational 4. Understand modal selection 5. Resolve technical problems, such as alignment and detection 6. Find atomic systems suitable for laser action

14 A. Einstein (1916) A. Einstein identifies the three fundamental mechanisms through which radiation produces transitions between atomic levels. Stimulated emission was a totally new concept at the time.

15 R. C. Tolman (1924) R. C. Tolman points out that a medium with an inversion of population would provide an optical gain. W. Kramers came to a similar conclusion.

16 R. W. Ladenburg (1928) R. W. Ladenburg observes negative dispersion (due to stimulated emission) in gaseous discharges

17 M. Goeppert-Mayer (1930) M. Goeppert-Mayer investigates two-photon transitions between atomic levels. Her thesis supervisor was Max Born.

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19 In the 1940 s, V. A. Fabrikant predicts amplification of light in gaseous discharges by means of stimulated emission (a patent was filed in 1950). A. Kastler investigates optical pumping (in the 1950 s) J. Weber proposes to amplify microwaves by stimulated emission (1952).

20 J. von Neumann (1953) John von Neumann sends a letter to E. Teller, where he describes his ideas on light amplification by stimulated emission in semiconductors. He had the idea of using a p-n junction.

21 C. H. Townes (1953) Gordon, Zeiger et Townes report the operation of the first maser. Similar investigations are carried out in USSR by Basov and Prokhorov.

22 Townes and Gordon

23 Basov Prokhorov

24 Prokhorov, Townes and Basov

25 Gordon, Basov, Zeiger, Prokhorov and Townes

26 R. H. Dicke (1954) R. H. Dicke develops the concept of superradiance ("Optical Bomb") He was the inventor of the lock-in amplifier and the founder of Princeton Instruments

27 Proposition of a three-level maser by Basov and Prokhorov (1955). N. Bloembergen proposes the solid-state threelevel maser (1956). R.H. Dicke files a patent application on infrared generation and amplification by stimulated emission. Among other claims, he proposes the use a resonant cavity made of a Fabry-Perot with flat or curved mirrors. The patent is awarded in 1958.

28 N. Bloembergen

29 G. Gould (1957) G. Gould privately develops a number of concepts that have led to the operation of lasers. He does not publish his results, but he attempts to obtain patents.

30 A. Schawlow (1958) Schawlow and Townes publish an historical paper where they establish the basic principles of laser operation. Their paper was most influential. They file a patent application (awarded in 1960).

31 The first "International Quantum Electronics Conference" is held from Sept. 14 au to Sept. 16. In his presentation, Schawlow mentions that ruby is not a good candidate for laser action. Ramsey, Goldenburg and Kleppner develop the hydrogen maser.

32 T. H. Maiman (1960) First observation of laser action by Maiman, with a ruby rod (May 16). His paper is rejected by Physical Review Letters. He publishes a short notice in Nature (Aug. 6). A press conference by Hughes Aircraft has large public impact (July 7).

33 Maiman s first laser

34 Photographs released by Hughes aircraft

35 Collins, Schawlow et al report on the operation of a ruby laser in Physical Review Letters (submitted on August 26 and published on October 1). They investigate relaxation oscillations, beam directivity and spectral narrowing. Later in 1960, P. P. Sorokin and M. J. Stevenson report the operation of an uranium laser (CaF 2 doped with U). This is the first laser with a four-level structure.

36 Sorokin and Stevenson

37 J. C. Polanyi (1960) J. C. Polanyi proposes the chemical laser. He makes a public disclosure at a meeting of the Royal Society of Canada on June 8. His paper is not accepted by Phys. Rev. Lett. He published a paper in 1961 in J. Chem. Phys.

38 A. Javan (1960) Discovery of the HeNe laser by A. Javan (December 12). His paper is published by Physical Review Letters in This is the first gas laser to have been operated.

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40 G. Summary and outcome The discovery of the laser essentially took place in America, and mostly in industry. Many independent initiatives have meet with success. Rules for success: simplicity and well-planified work. Inventors deserve our respect. Peer review failed to appreciate the value of early laser research.

41 It is important to archive original work. This is a case where science caught the attention of the media. Up to recently, the laser industry was still paying royalties for patents held by Gould. The paper by Schawlow and Townes (1958) is fundamental and their formula for the laser linewidth is a permanent legacy. Lasers have become essential instruments in physics, chemistry, engineering, medicine, remote sensing, etc...

42 Major Canadian contributions to laser science Proposal of the chemical laser by J. C. Polanyi (1960) Invention of the TEA-CO 2 laser by J. Beaulieu (1967) Demonstration of transversely-excited excimer lasers by Stoicheff et al (1970's) Demonstration of injection mode locking by P.-A. Bélanger (1973) Demonstration of chirped-pulse amplification by D. Strickland and G. Mourou (1985) Proposal of the attosecond laser by P. B. Corkum ( )

43 Acknowledgements Thanks to Dwayne and to the Institute of Optical Sciences for the opportunity of presenting the lectures Thanks to Ludmila and the staff at the Institute for their support Thanks to Alexandre April for helping in the preparation of the material presented during the lectures Thanks to all who attended the lectures

44 References An excellent overview of laser history, with interviews, can be found in Jeff Hecht, Laser Pioneers, revised edition. Academic Press (1992). Most of the photographs were taken from that book. B. R. Masters, The scientific life of Maria Göppert- Mayer. Optics and Photonics News, Sept. 2000, John von Neumann, Notes on the photon-disequilibrium amplification scheme (JvN), September 16, IEEE J. Quantun Electronics, vol. QE-23, (1987).