Lasers: Physical principles and applications

Po~merTesKng3 (1983) 243-248

LASERS:

PHYSICAL PRINCIPLES APPLICATIONS

AND

R. M. J. COVrERILL

Department of Structural Properties of Materials, The Technical University of Denmark, 2800 Lyngby, Denmark

It is an honour to have been called upon to open this stimulating meeting on the application of lasers to polymer science and technology. My mandate is to provide a brief introduction by describing certain fundamental aspects of the laser, and to discuss some of those physical properties which make it such a useful tool. At the outset, I must make it clear that my qualifications for this task are somewhat marginal, in that my personal interest in the laser field lies at the periphery; I have been directly involved only with the discussions of the possibility of producing lasing action in the X-ray region. It is thus clear that I have had to depend upon the various review articles that have appeared on the subject, and the job was made easier by the availability of several excellent examples. 1-4 The usefulness of lasers stems from their five most salient qualities: (1) their high degree of directionality; (2) their marked monochromaticity; (3) their extremely high intensity; (4) the coherence of the radiation; and in some cases, (5) the extreme brevity of the lasing pulse. These unique characteristics are not available in the more conventional types of light source, which were the only ones available prior to the advent of the laser era in 1960. The word laser itself is acronymic for 'light amplification by stimulated emission of radiation'. In retrospect it is remarkable to contemplate how long it took the physics community to hit upon the idea of making such a splendid device. For the vital elements of the story were already on hand before the end of the second decade of this century. In 1917, Albert Einstein hit upon the idea of stimulated emission, which is to say the sympathetic emission of light from an excited atom by the provoking influence of a passing light photon. Until this inspired piece of insight, only two processes had been recognized as being possible for the transitions between energy levels in an atom: namely, the absorption of a 243 Polymer Testing 0142-9418/83/$03.00 (~) Applied Printed in Northern Ireland

Science

Publishers

Ltd,

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1983.

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photon when an electron jumps from a lower level to a higher level, and the spontaneous emission of a photon for the reverse type of jump. (This does not mean that photon absorption is the only way of producing an excited state, as we shall shortly note.) When a light beam travels through a medium consisting of atoms, it is in general subject to both gain and loss. The former can come from both spontaneous emission and stimulated emission, while the losses occur both through scattering and from photon absorption during excitation processes. Laser action is obtained only if the gain exceeds the loss, and it is to be regarded as remarkable in that the losses usually predominate under normal circumstances. In thermal equilibrium, the principles of statistical mechanics show us that the population of low-energy states exceeds that of the higher lying states. In order to achieve laser action, the reverse situation, known as population inversion, must prevail. This can be achieved if the rate of production of excited states is sufficiently high. One refers to such production as pumping, and a variety of mechanisms are available including (apart from the obvious absorption of light) electric discharge, chemical reaction, and collision of electron beams with the atoms to be excited. The idea for obtaining laser action from population inversion first occurred to Arthur Schawlow and Charles Townes in 1958, and the first working laser was produced by Theodore Maiman in 1960. Maiman's working substance was a ruby rod, the chromium atoms of which were pumped to population inversion with a powerful flashlamp. The ends of the cylindrical rod had been ground to perfect parallelism, and then plated with silver, so as to produce the parallel mirror arrangement that had been pioneered in 1905 by Auguste Perot and Charles Fabry. With the working medium in the inverted state, the lasing action is initiated by the spontaneous decay of some of the atoms, and the stimulated process then takes over as the original photons pass by other excited atoms. As Einstein had realized, the stimulated photon is precisely in step with the stimulating photon, and the light waves rapidly grow in amplitude because of this coherence. In particular, those waves which travel precisely at right angles to the mirrors are reflected again and again so as to traverse the length of the working medium, and the amplitude of the travelling wave grows avalanchefashion. As Nicolaas Bloembergen has remarked, the travelling light wave coherently acquires photons at an ever-accelerating rate, just as financial capital accumulates more revenue at compound interest, in the absence of entries on the debit side of the ledger. The beam is extracted from the device by making one of the end mirrors half-silvered. An important more recent development has been the emergence of continuously emitting lasers. 5 Since the original working laser, it has been discovered that many other types of material can be pumped to inversion, including metal vapours, noble

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gases, water, carbon dioxide, cyanide gas, liquids containing organic dyes, and various rare-earth glasses. In fact, it has subsequently been realized that the man-made examples were not history's first such devices. Credit for the first working laser must go to Nature itself, in that lasing action involving water molecules and hydroxyl radicals has been proceeding in interstellar space since the remote aeons. Let us turn now to the applications of lasers, and see how the five special features enumerated above have been exploited. Their high directionality has tended to be used in relatively large scale applications, such as for alignment in the boring of tunnels, and in the setting up of large pieces of apparatus, such as particle accellerators. The directionality has also been exploited in communication systems. Perhaps the most spectacular example was seen when a laser beam was reflected from the surface of the moon. T h e parallellism of the beam was demonstrated in this case by the fact that it still covered an area only about as large as a football field on arriving at the moon's surface. A related application concerns the measurement of distance to a high degree of accuracy. This, used in connection with satellites, enables one to measure the degree of continental drift, which has obvious geological implications. Until now, nothing has been said about the fact that the excited atoms in the lasing medium might be in motion. Indeed, this motion is inevitable at all temperatures above the absolute zero. In the context of light emission and absorption, the main consequence of this motion is to broaden the spectral line, with a Doppler width given by the expression Av = 2v ~/2KNo In 2 V ~ M c

where v is the line frequency, No is Avogadro's number, K is Boltzmann's constant, M the molecular weight, and T the absolute temperature, as discussed in the recent review by Schawlow. 3 In typical cases, this produces a broadening of about one part in a hundred thousand. Now the Doppler shift will be positive or negative depending upon whether the atom is travelling away from or towards the photon. A special case arises when the atom is either completely stationary, or is moving precisely at right angles to the direction in which the photon is travelling. In this case there will, of course, be no Doppler shift, and Lamb 6 was the first to show that this would produce a centrally located dip in the power output of the spectral line. These principles have been used in Doppler free spectroscopy, and a typical experimental set-up is indicated in Fig. 1, this figure being one due to Schawlow. 4 A laser beam, divided into two parts by a partial mirror, produces two beams travelling in almost opposite directions. One of these beams, which is chopped at an audio frequency, is strong enough to partially saturate the absorption of the molecules in the absorption cell, and this means that the other beam, which is

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R.M.J.

COqTERILL

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The experimental set-up for saturation spectrometry, as described by Schawlow.4

being used to probe the working medium, is less attenuated. When the chopper is blocking the stronger pumping beam, the absorption increases and the transmitted intensity of the probe beam decreases. But any molecule in the working medium which has a net motion along the beam directions cannot simultaneously be in resonance with both of them, so there is a shift, one way or the other, in the frequency. It does not require much fantasy to see that these principles could lead to numerous applications. An example has been the measurement of hyperfine structure, as produced by the interaction between local atomic moments and the net field of the surrounding atoms in the material. This general approach has also been coupled with polarization techniques, in the identification of the various levels in complex atoms and molecules. The aspect of the laser which has won most popular attention has been its high intensity. Indeed, it was this property which won it the ultimate accolade: inclusion in a James Bond movie (Goldfinger, 1963). Lasers have been used to weld steel plates, drill holes through diamonds, and cut almost everything, from tailor's cloth to blood vessels. Detached retinas have been repaired by delicate laser manipulations, and laser beams, guided through flexible optical fibres, have been used to cauterize stomach ulcers. This approach has now been pushed into the microscopic realm, where it promises to make microsurgery

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possible on cells and even on chromosomes. T h e effective field strength in today's really powerful lasers is actually comparable to the fields within individual atoms, and this naturally leads to a marked nonlinearity. Such nonlinear effects are reasonably familiar in everyday circumstances. T h e amateur flute player, for instance, will often accidentally produce an overtone of the desired note simply by blowing too hard. The ramifications of nonlinearity have recently been discussed by Bloembergen. 2 The simultaneous presence of fundamental and harmonic waves obviously makes for complicated situations, and the required modifications to the simple laws of reflection and refraction were derived by Bloembergen and a number of collaborators. 7"8 The dispersion characteristics of a material also become far more complicated in the nonlinear regime, as is indicated in Fig. 2, which is reproduced from the work of Kramer and Bloembergen. 9 Nonlinear effects have rapidly led to a number of novel applications that would have been quite impossible earlier. Indeed, they have given birth to a new era in communications, with a variety of fascinating devices. One particularly exciting spin-off is the production of

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pulses down in the femtosecond region. 1° This brief period of time is well below that of a single atomic vibration in condensed matter, and it opens up the possibility of what could justifiably be called the ultimate snapshot: a picture at the atomic level with all motions frozen in a sort of suspended animation. Finally, there is the question of coherence. This has, of course, already led to the realization of holography, which languished for 12 years, following the original proposal of the holographic principle by Denis G a b o r in 1948, because there was no suitable source of coherent radiation. H o l o g r a p h y is now so c o m m o n that no further c o m m e n t s on the optical version need to added here. T h e r e remains, however, the exciting prospect of holography at the atomic level, if and when it proves possible to produce laser action in the X - r a y region. T h e X - r a y laser is not yet with us, but the prospects for building one have b e e n seriously considered. 11 T h e advantage of being able to carry out holography at the atomic level, as recently discussed in a quite detailed review, 12 could hardly b e exaggerated. T h e notorious phase p r o b l e m of X - r a y crystallography 13"x4 would suddenly b e a thing of the past. T h e a r r a n g e m e n t of atoms in even the most complex biological molecules would be available on a routine basis, and this would revolutionize biophysics and biochemistry. It would be difficult to imagine a greater milestone in the entire field of condensed m a t t e r science.

REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

BLOEMBERGEN,N. (1975). American Scientist, 63, 16. BLOF.MBERGE~,(1982). Science, 216, 1057. SO-IAWLOW,A. L. (1982). Science, 21"/, 9. SCHAWLOW,A. L. (1982). Physics Today, 35, 46. PETERSON,O. G., TUccIo S. A. and SNAVELY,B. B. (1970). Appl. Phys. Left., 17, 245. LAMB,W. E. Jr, (1964). Phys. Rev. A, 134, 1429. BLOEMBERGEN,N. and PERSrL~q,P. S. (1962). Phys. Rev., 128, 606. BLOEMBERGEN,N. and SHE~, Y. R. (1963). Phys. Rev. A, 133, 37. ~ , S. D. and BLOEMBERGEN,N. (1976). Phys. Rev. B, 14, 4654. Announcement in Physics Today, December 1982, p. 19. ~ , G. and WOOD, L. (1975). Physics Today, 28, 40. SOLEM,J. C. and B~.Dw~, G. C. (1982). Science, 2111, 229. POST,B. (1979). Acta Cryst. A, 35, 17. N~DSEN,J. U. and CorrERILL, R. M. J. (1978). Acta Cryst. A 34, 378.