INSTRUMENTATION cause they make use of readily available organic liquids, are the stimulated Raman oscillator and the dye laser. Provided one has a high power pulsed laser (and perhaps the ability to generate the second harmonic of this laser) either of these two types of laser is simple to construct—the Raman oscillator giving a large number of fixed frequencies and the dye laser a more continuous coverage. Continuous operation of neither has been demonstrated. The optical parametric oscillator is the most difficult to construct, primarily because it requires crystals of high optical quality which are not yet readily available. The characteristics of the optical parametric oscillator which make it interesting are continuous tunability, operation in the infrared, and
operation on a cw basis. We have mentioned only three methods of achieving tunable laser action; a number of others exist (4). They include the direct tuning of lasers by the application of electric or magnetic fields, by variation of temperature, or by the application of pressure. An interesting example of the last technique was the demonstration of tunable laser action from 7.5 to 25 microns by the application of hydrostatic pressure up to 14 kbar to a lead selenide diode laser. By using any of the techniques described to generate new wavelengths it is possible to further extend the spectrum covered by mixing various combinations of frequencies in suitable nonlinear media to obtain sum and difference frequencies. As we have seen, a number of techniques exist to achieve tunable laser action. At present, product oriented companies are working on dye lasers and optical parametric oscillators and, in fact, one company is ad-
vertising a pulsed dye laser for sale. In the next year or two many competitive systems should be available, and it is fair to assume that a large number of interesting experiments will result.
References
(1) A good summary of the Stimulated Raman Effect is given by R. W. Terhune and P. D. Maker in "Lasers," Vol. II, ed Albert K. Levine, Marcel Dckker, Inc., New York, 1968. (2) A review of optical parametric oscillator theory is given by S. A. Akhmanov and R. V. Khokhlov in Soviet Physics Uspekhi, 9 (2), 210-222, September-October 1966. (3) For details of dye lasers see "Organic Dye Lasers" by M. R. Kagan, G. I. Farmer, and B. G. Huth, Laser Focus, pp 26-33, Sept., 1968. and "Organic Lasers" by P. P. Sorokin, Scientific American 220, pp 30-40, February, 1969. (4) A more complete paper by the author including a detailed bibliography will appear in the Annals of the New York Academy oj Sciences.
COMMENTARY by Ralph H. Müller
V S T E FIND Dr. Smith's discussion of
' ' Tunable Lasers most provocative, not solely as a promise for new, versatile and useful light sources but for the multitude of electro-optical phenomena which will undoubtedly be investigated in the course of these studies. In the comparatively short time that the laser has been used, it has found important uses in communications radar, holography, surges, time standards, metal cutting and welding, alignment instrumentation, high temperature studies, and in ignition systems. In the forty-one years since its discovery, the Raman effect has been a source of information on molecular structure supplementary and complementary to infrared spectroscopy, but until the advent of laser excitation of Raman spectra, it was rarely used by the analytical chemist, largely because it was too insensitive. Now that excitation by a high power pulsed laser can cause the scattered radiation to become stimulated rather than spontaneous, we have the high power Raman oscillator. This is an accomplishment that transcends mere improvement in detection 78 A ·
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sensitivity—it presents innumerable possibilities. The number of systems which can be used and the multiplicity of frequencies which can be generated would seem to be limited only by the number of substances which the chemist can find on his reagent shelf. It is of major importance that the radiation so produced is coherent and collimated. Dye lasers are likely to be of greatest interest to chemists even though, as the author points out, they are not particularly useful in the infrared. Fluorescence spectrophotometry is a highly developed field and elaborate instruments are available for the accurate delineation of both the absorption (excitation) and fluorescence spectrum. Sensitivities are of the order of parts per million or parts per billion and hundreds of substances have been precisely characterized, many of them of biochemical and medical importance. Is it not reasonable to assume that many of these substances can be excited to the point where laser action ensues? If high levels of output can be attained, it would seem that detection and identification could be achieved by relatively simple
optical methods of abridged spectrophotometry. The physicist and engineer will continue to study and improve tunable lasers and, at each stage of development, the chemist will benefit from the results, but he may do well by considering the implications of Raman and dye laser action. He should not merely regard them solely as high intensity, tunable light sources to be employed in conventional spectrophotometric techniques. In these thoughts, we may do well to keep the meaning of the acronym LASER, constantly in mind— "light amplifications b}' stimulated emission of radiation." To the extent that an array of molecules can, by this optical feedback and ultimate oscillation, become a relatively powerful little broadcasting station and of stable frequency, it becomes relatively simple to establish its identity. To switch for a moment to the region of much lower frequencies—i.e., in the microwave region—the ammonia maser (microwave amplification by stimulated emission of radiation) can be excited
by injecting microwaves into a resonant cavity filled with ammonia. At a fre quency of 24,000 megahertz, amplifica tion at very low noise levels occurs be cause this corresponds to an excitation level of the ammonia molecule. The corresponding wavelength is 1.25 cm or wavenumber ν = 0.8 c m - 1 . At high er amplification, the initiating signal can be turned off because sustained os cillations occur. The maser is then a constant frequency source or "ammonia clock." An accuracy of 1 part in 1010 is characteristic of the ammonia maser. The hydrogen maser has a stability es timated to be 1 part in 10 13 , but for ruggedness, relative simplicity, and wide experience in its use, the cesium beam clock with stability of 1 in 1010 would supercede both in practical ap plications. Atomic clocks have at tained acceptance as international stan dards, replacing celestial measurements, since the October 1967 conference in Paris. The cesium beam clock is the basis of the collision avoidance system (CAS) expected to be operational in the United States for all commercial aircraft in late 1970. (Reader's Digest, July 1969, pp 106-110). This digression on masers or related high stability oscillators has signifi cance in another sense. With stability of this order, would one feel any uncer tainty that one was dealing exclusively with ammonia molecules, hydrogen molecules, or cesium atoms in the re spective examples ? In the case of laser action in a Raman oscillator or dye os cillator with comparable frequency, sta bility could afford quick identification by relatively simple means. These considerations lead us to suggest that it may be profitable to study the analytical possibilities of in ducing laser action in samples as a di rect approach to identification and pos sible quantitative estimation. In the general problem of laser devel opment, there is an extensive back ground of information on physical op tics, much of it of only passing interest today in other respects. In mixtures of gases, much is known about the influ ence of inert gases on the population in metastable states. For example, mer cury vapor excited to the first reso nance level at 4.86 volts (2537 A) is easily thrown into the adjacent metastable state by the admission of nitrogen with an increase of lifetime by a factor of 104. The early studies of R.W. Wood on the step-wise excita tion of the mercury spectrum by suc cessive irradiation with lines from another mercury arc were completely in accord with the known energy levels in this system, but beyond that, afforded several neat tricks to artifically enhance certain transitions. As he once remarked—"It's all very simple, gentlemen, it's done with mirrors."
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VOL. 4 1 , NO. 10, AUGUST 1969
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