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Lasers in Science Arthur L. Schaw~ow Lasers are nonequilibrium devices. Initially, scientists such as Albert Einstein and Albert Ladenburg, who understood the principle of stimulated emission, did not appreciate its possibil- ities- they were too accustomed to systems near equilibrium. In the 1950s it was understood that systems far from equilibrium could be produced. Since the demonstration of the laser in 1960, it has come to be used everywhere in science. Sometimes it is only an alignment tool, as it was in Burton Richter's 1976 Nobel Prize work, in which a laser was used to align the 2-mile-long linear accelerator. But often, the laser is more central. This paper describes several examples of such central applications of the laser, with an emphasis on spectroscopy the author's field of specialization. Ideal laser light is directional, powerful, monochromatic, and coherent. Some lasers approach these ideals more closely than others, and in any scientific use, it is important to pick the characteristics of significance for that application. For spectros- copy, which makes use of the absorption or emission of light when a material is irradiated by coherent light of the proper frequency, a key issue is the tunability of the light source. Thus, laser spectroscopy really began in the late 1 960s with the development of tunable dye lasers by Schafer et al. (1966) in Germany and Sorokin and Lankard (1966) in the United States. The organic dye molecules have broad emission bands, and tuning is done by introducing a rotatable diffraction grating, which acts as a tunable mirror, into the laser resonator. This paper is adapted from a transcript of Dr. Schaw~ow's presentation at the National Academy of Engineering symposium "Twenty-Five Years of the Laser." 118
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LASERS IN SCIENCE ~ 1 9 A fundamental difficulty in studying the interaction of atoms with radiation comes from the motion of the atoms. The emission or absorption lines are consequently broadened by the Doppler shift of atoms moving with a statistical distribution of velocities in different directions. Thus, much fine structure is lost in a straightforward measurement of the interactions. In the last 15 years several methods of overcoming this problem have been demonstrated. In one method, atoms interact with two light beams coming from opposite directions, and only station- ary atoms interact with both beams at the same frequency. For example, a powerful saturating beam may be turned off and on, bleaching a path for a probe beam of opposite direction. The probe beam is thus modulated, but only when tuned to atoms that are standing still. Hansch and associates (1972) at Stanford University used this method to observe theoretically predicted fine structure in the Balmer spectrum of hydrogen, including the first optical observation of the Lamb shift. Two-photon interactions, using oppositely directed beams, were proposed by Chebotayev and associates in 1970 (Vasilenko et al., 1970) and realized by several groups in 1973. This method has been used at Stanford (Foot et al., 1985) to improve resolution of the IS to 2S transition in hydrogen to one part in 109. Early laser measurements on the Balmer series gave only 1 part in 105, but were improved to 1 part in 107, at which point lifetime limits. The long lifetime of the 2S level would not limit until one reached 1 part in 10~5 or so. This method has also been used in elegant experiments on positronium at Bell Laboratories (Chu and Mills, 1982~. In 1975, using a somewhat different technique, Hansch and Schawlow (1975) proposed using light pressure to slow down the atoms. The radiation should be just below the resonance of the atom at rest, so that atoms moving toward the laser scatter the light and lose momentum in the process. The technique has been used successfully with sodium at the National Bureau of Standards and by Chu and coworkers at Bell Laboratories (Chu et al., 1985) to slow down a cloud of atoms, producing `'optical molasses." Still other variations are producing quite surprising advances in precision measurement techniques. In going from atoms to molecules, the spectra become enor- mously more complicated. With lasers, one can simplify the spectra by "labeling" a particular level. This can be done by pumping atoms out of or into a level or by orienting them. Then, when probes are brought in at two different frequencies, transitions are observed only from the labeled levels. For exam- ple, the complete spectra of diatomic sodium are quite complex, 1 1
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20 ARTHUR L. SCHAWhOW but with the labeling, one can see only two rotational levels for each vibrational level of the molecule. Laser spectroscopy can be very sensitive since one can have repeated scatterings from an atom without destroying it. In my laboratory, as few as 105 sodium atoms/cm3 were detected at room temperature, and as few as 100/cm3 at—30°C (Fairbank et al., 1975~. The mean free path of sodium atoms in this case is greater than the distance from the earth to the moon. An experiment in Heidelberg trapped barium ions in a radio frequency trap, and by continued scattering of light off the atoms for several minutes, a single atom was observed (Neu- hauser et al., 1980~. By holding the laser beam on the material for a long time, one can do precision spectroscopy. Hurst, Payne, and associates at Oak Ridge National Laboratory have also developed methods for detecting single atoms and have shown a number of important applications to nuclear physics (Hurst et al., 19791. They have recently proposed a method for detecting the magnetic monopoles postulated by P. A. M. Dirac many years ago. The method makes use of a transition in helium from the ground state to a metastable state that should be excited by the monopole, if it exists (Hurst et al., 1985~. Very short light pulses produced by lasers have especially important applications in biology. For example, the visual pig- ment rhodopsin is known from Raman spectroscopy (also using lasers) to have a small part attached to a large part. The bond between them flips quickly from cis to bans, two forms of the organic molecule. The molecule is excited electronically and then probed with the short pulses fractions of a picosecond later. Cooperative work at Bell Laboratories and Cornell University (Downer et al., 1984) found that the relaxation time is about 0.7 ps, and that there are differences, depending upon whether the pigment is in water or heavy water. At Lawrence Livermore National Laboratory, high-power lasers are focused onto small spheres of hydrogen to study the potential for fusion, which in principle could provide an almost endless source of energy. The scientific importance of such work is that it permits the study of matter at temperatures and pressures even greater than those at the center of the sun. The apparatus for this research is huge larger than many acceler- ators—consisting of a master laser feeding an array of laser amplifiers. In contrast to the short pulses, lasers can be used to observe phenomena that change very slowly. An example is the mea- surement of the motion of bacteria in a liquid such as water, where the Doppler shift is of the order of kilohertz. To observe
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LASERS ~ N SC! ENCE ~ 21 this motion with conventional interferometers, using distance equivalents to time of about 30 cm/es (1 ft/ns), distance differ- ences of millions of feet would be needed. Photon correlation techniques are used instead, with velocities deduced from com- parison of intensity correlations. In a typical measurement at 30°C, bacterial motion peaked at about 30 ,um/s. Results such as this have helped in the understanding of how bacteria move. A different example of slow-motion measurement is the work at the Johns Hopkins University to measure the sliding and stretching motion of muscles. Again, this velocity is in the range of micrometers per second (Stock, 19761. In geodesy, the relative motion of the earth's poles has been measured by satellite-ranging methods over a period of a year or so. Results agree with long-baseline radiometry to within a few centimeters and show a variation with time of about 600 ms of arc (about 20 m). The cause of this variation is somewhat controversial, but it is apparently not due to earthquakes, although perhaps to meteorological effects, such as air mass movements (Robertson et al., 1985~. Finally, lasers can be applied to a very puzzling experiment in basic science. In 1935, Einstein et al. (1935) raised questions about the completeness of quantum mechanics in describing reality. This has been expressed by the question, "Is the moon there when nobody looks at it?" Most of us are sure it is, but the question is not so simple on the quantum level. Following a theoretical analysis by J. S. Bell (1964), early experiments by Clauser and Shimony (1978) showed quantum theory correct, and recent experiments by Aspect and colleagues in France (Aspect, 1983) have been even more conclusive. In these latter experiments, calcium atoms were excited by photon transitions and then emitted blue photons and green photons. Detectors were placed far apart, and polarization was checked for the two. Since there is no change of angular momentum in this transi- tion, the two photons should have the same polarization (or opposite if circularly polarized). Then, if a rotating polarizer is introduced, the variation with angle is different from classic and quantum predictions. The result appears to be too much corre- lation, as though there were a single probability wave rather than two separate photons. To make it worse, in an experiment proposed by David Bohm (Bohm and Aharanov, 1957), polar- ization was changed by the use of an acousto-optic modulator more rapidly than the time taken for light to go back and forth. The correlation still held. Quantum mechanics says there is no reality to things going back and forth; there is just one quantum system that includes the measuring devices, and the results are 111
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~ 22 ARTHUR L. SCHAWCOW REFERENCES .. created by the measuring process. Of course, this is not trans- mission of a signal faster than light but only a correlation of probability, and many physicists say' "Naturally, it's just what quantum mechanics predicts." Others find it disturbing. A more detailed discussion of the issue is given by Mermin (19851. These are a few examples of the wide range of laser applica- tions in science. Many more could be mentioned. In fact, it is difficult to open any scientific journal today without finding a description of some use of lasers if only for alignment. And it seems certain that still more applications will be developed in the future. Aspect, A. 1983. P. 103 in Atomic Physics 8, I. Lindgren, A. Rosen, and S. Svanberg, eds. New York: Plenum Press. Bell, J. S. 1964. Physics 1:195. Bohm, D., and Y. Aharanov. 1957. Phys. Rev. 108: 1070. Chu, S., and A. P. Mills, Jr. 1982. Phys. Rev. Lett. 48:1333. Chu, S., L. Hollberg, i. E. Bjorkholm, A. Cable, and A. Ashkin. 1985. Phys. Rev. Lett. 55:48. Clauser, J. F., and A. Shimony. 1978. Rep. Prog. Phys. 41:1881. Downer, M. C., M. Islam, C. V. Shank, A. Harootunian, and A. Lewis. 1984. p. 500 in Ultrafast Phenomena, D. H. Auston and K. B. Eisenthal, eds. New York: Springer-Verlag. Einstein, A., B. Podolsky, and N. Rosen. 1935. Phys. Rev. 47:777. Fairbank, W. M., [r., T. W. Hansch, and A. L. Schawlow. 1975. I. Opt. Soc. Am. 65:199. Foot, C. l., B. Couillaud, R. G. Beausoleil, and T. W. Hansch. 1985. Phys. Rev. Lett. 54:1913. Hansch, T. W., and A. L. Schawlow. 1975. Opt. Commun. 13:68. Hansch, T. W., I. S. Shahin, and A. L. Schawlow. 1972. Nature 235:63. Hurst, G. S., M. G. Payne, S. D. Kramer, and }. P. Young. 1979. Rev. Modern Phys. 51:767. Hurst, G. S., H. W. Jones, t. O. Thomson, and R. Wunderlich. 1985. Phys. Rev. A32: 1875. Mermin, N. D. 1985. Phys. Today 38:~38. Neuhauser, W., M. Hohenstatt, P. E. Toschek, and H. G. Dehmelt. 1980. Phys. Rev. A22:1137. Robertson, D. S., W. E. Carter, B. D. Tapley, B. E. Schutz, and R. J. Eanes. 1985. Science 229:1259. Schafer, E. P., W. Schmidt, and ~. Volze. 1966. Appl. Phys. Lett. 9:306. Sorokin, P. P., and J. R. Lankard. 1966. IBM J. Res. Dev. 10:162. Stock, G. B. 1976. Biophys. T 16:535. Vasilenko, L. S., V. P. Chebotayev, and A. V. Shishaev. 1970. JETP Lett. 12: 113.
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