A Technical History Of The Laser

A cited and researched article tracing the origin and history of the LASER from radar technology (MASER's)including major advancements over the years.

LASER is an acronym for Light Amplification by the Stimulated Emission of Radiation. The concept consists of an excited state atom encountering a photon of the same energy that corresponds to the DE between the excited and ground states of the atom. When such a photon is encountered, it causes the emission of another photon of the same energy. Albert Einstein first suggested this phenomenon in a 1916 paper proving Plank's law of radiation. The idea, however, was considered odd and the event of photon interaction with an excited state atom rare. Only much later did scientists begin to create inverted populations with more atoms in the excited state than the ground state so that absorption would not dominate the process and stimulated emission could occur.

The precursor to the laser was the maser. The maser amplified electromagnetic radiation of much shorter wavelengths in the microwave range (thus the M instead of L in maser). The impetus for the development of the maser seems to be the interest in microwave radiation following the utility it found in radar technology in World War II. After the war, many scientists who had worked on developing the radar continued their investigations into microwave radiation using much of the military surplus microwave equipment. The first maser was created by Charles H. Townes (published in 1954) who along with James Gordon and Herbert Zeiger succeeded in producing an inverted population by isolating excited ammonia molecules. The excited molecules were aimed into a cavity resonant at the 24-gigahertz frequency of the ammonia transition where stimulated emission occurred. Since there was a physical separation of excited state molecules, however, after emission, the maser action ended. Thus the first maser was incapable of continuous output. In order to achieve continuous output, new systems with more than two energy levels had to be designed. These systems could release stimulated emission without falling to the ground state, thus maintaining a population inversion. Nikolai Basov and Alexander Prokhorov of the USSR first developed this idea. Together, Basov, Prokhorov, and Townes shared the 1964 Nobel Prize in Physics for developing the maser concept.

Soon after masers became a reality, people began to look at the possibility of stimulated emission in other regions of the electromagnetic spectrum. Townes, along with Arthur Schawlow, began investigating the possibility of optical and IR masers. Together they published the first detailed proposal for building an optical maser (later to be renamed a laser) in a December 1958 issue of Physical Review. The obstacles to creating a working laser, however were great. The much smaller wavelengths of visible light and the difficulty of finding an appropriate excitation medium meant that much more accurate measurements needed to be made and that error in setting up the apparatus had a much more crucial effect on the function of a laser. It was not until 1960 that Theodore Maiman created the first working laser. Maiman's laser was a "pink" ruby rod with its ends silvered placed inside a spring-shaped flashlamp. Maiman's laser, however, was only capable of pulsed operation due to its 3 energy level transitions. Soon afterwards, in 1960, Peter Sorokin and Mirek Stevenson developed the first 4 level laser (uranium doped calcium floride) which was capable in theory of continuous output although in solid state, a continuous output could not be achieved. The development of laser technology had begun.

Just before the end of 1960 (published 1961), Ali Javan, William Bennet, and Donald Herriot made the first gas laser using helium and neon. This type of laser (a He-Ne laser) became the dominant laser for the next 20 years until cheap semiconductor lasers took over in the mid-80's. The He-Ne laser is used in such applications as reading UPC product codes, surveying equipment, etc. The He-Ne laser made two crucial steps. One, it was the first laser to emit a continuous beam. And two, the lasing action could be initiated by an electric discharge rather than the intense discharge of photons from a flashlamp. Atomic gas lasers were limited in power, so C. Kumar N. Patel began working with carbon dioxide (published 1964) and carbon monoxide lasers which he mixed with nitrogen, helium and water to fine tune the laser properties. Thus Patel made the first high powered gas lasers. Earl Bell then discovered the ion laser when he placed mercury ions in helium to create lasing action (published 1964). Although the mercury ion laser has never seen much application, it was a direct predecessor to the argon-ion laser developed by William Bridges. The gas ion lasers then led to the development of metal vapor lasers by many people who worked with different metal vapors.

Some further laser developments were as follows: using chemical reactions instead of electric currents to generate a lasing effect, using rapid cooling through expansion to cause excitation, using dyes as a medium to tune the laser across a range of wavelengths, and using p-n junctions in semiconductors or a free electron medium to create lasing effects.

Quickly after their inception the utility of lasers were realized and their conceived uses skyrocketed. After around 1964, however, excitement about lasers began to subside. Although new uses had been conceived for them, some of the projected applications were proving difficult to achieve and many lasers were proving difficult to make. For instance, there was little success in developing a continuous, room-temperature semiconductor laser for computing purposes. Laser power also seemed limited, disillusioning the U.S. government in its potential military applications. Lasers were dubbed "a solution looking for a problem." The first uses of lasers consisted more or less of replacing less efficient light sources, i.e. xenon arc lamps in photocoagulators and mercury arcs in interferometers. Laser research slowly continued and increased its breadth. There were a variety of improvements in laser design which increased laser lifetime, focused beam width, improved continuity of output, regulated and shortened pulse duration, etc. Anthony DeMaria's development of mode-locked neodymium-doped glass lasers with thousands of megawatts peak power and picosecond durations, improved laser performance to standards necessary for high-speed photography and scientific applications such as the study of physical and chemical phenomena.

A series of social factors affected laser development in the U.S. The dentente between the US and USSR brought a decline in military spending and thus a decline in funds for developing laser beam weapons and high powered lasers. However, more mundane military interests in Vietnam led to the development of laser radar, targeting, and reconnaissance systems. Environmental and later energy concerns (the OPEC oil crisis) led to increased funding for laser research investigating things such as air-pollution monitoring and energy applications.

Communications became one of the biggest arenas in which lasers were thought to have application. Increasing telephone use, electronic transmission, relaying of television signals, and the need for communication in space all contributed to the emphasis placed on laser technology toward communications. The initial attraction to lasers for communications came from the fact that the amount of coherent information that an electromagnetic wave can carry if proportional to its frequency. Optical light has frequencies 109 times larger than radio waves and 105 times larger than microwaves. Lasers seemed like an ideal solution to the overcrowding of existing communication technology. The technical complications to using lasers in communications were great, and it took many years for other technologies to be invented which made laser communication practical. The first invention was the discovery in 1970 by Charles Kao and George Hockham that glass fibers could transmit laser light efficiently. Also in 1970, a method was invented to improve the p-n junctions in semiconductors which reduced the current densities needed for semiconductor lasing from 100,000 amperes/cm2 to 8000 amperes/cm2 and then down to 1000 - 3000 amperes/cm2. These two technologies complimented each other and gave a new boost to laser application.

With increasing power and decreasing pulse duration (the developments associated with C.K.N. Patel and Anthony DeMaria) the applications of lasers to chemistry began to increase. Now lasers were tunable to a sufficient level to make them reasonable and advantageous for spectroscopic uses. Over the years, the substitution of lasers for other light sources has improved sensitivity, selectivity, and limits of detection by several orders of magnitude.



In IR spectroscopy, which is used largely for gas analysis and organic structural determination, lasers, with their high intensity and narrow bandwidths, have solved the longstanding problems in IR spectroscopy of poor detector sensitivity and low source intensity. Application of lasers to UV-Vis absorption spectroscopy has been more limited since the narrow bandwidths associated with lasers are unnecessary in most quantitative UV-Vis absorption experiments. Lasers have improved the LOD in very low concentration samples for UV-Vis, however. In fluorescence spectroscopy, the emission is proportional to the excitation intensity - thus, lasers would seem to have an inherent application. In reality, a laser can saturate the sample with light intensity to such an extent that the limiting factor for emission intensity is no longer the excitation intensity, but rather other factors limit the sensitivity and LOD for fluorescence. Lasers are particularly useful, however in producing the narrow bandwidths useful in atomic fluorescence.

Lasers, however, have also led to several new forms of spectroscopy which would be impossible without the laser. Raman spectroscopy was a practical impossibility without the advent of lasers. Raman spectroscopy is based on the Raman effect, a phenomenon involving inelastic (or energy changing) scattering of light. Raman spectroscopy is essentially using visible light to cause vibrational transitions in molecules. The Raman effect is a very weak phenomenon, and only one incident photon in 107 produces a Raman transition. This calls for a very intense source of light that must be monochromatic since the Raman effect produces very small relative energy changes. It is obvious then that lasers were a necessary technology for Raman spectroscopy to be practical.

Lasers have also opened up the realm of nonlinear spectroscopy, or the concerted interaction of two or more photons with a molecule. Multi-photon events of this sort require very intense sources of light and actually were not experimentally demonstrated until the advent of pulsed lasers. Since the process must be concerted, it is necessary for two photons to pass almost simultaneously through a region of space containing one molecule. This kind of spectroscopy is called nonlinear because the usual Beer-Lambert Law does not hold. Some interesting aspects of nonlinear spectroscopy have turned it into a high-information yielding technique. For one, the two-photon selection rule only permits transitions between states of the same parity. The two-photon process can also cancel Doppler shifts by counterpropagating the laser beams. In addition, the two-photon process is highly sensitive to the polarization of the laser beam. By changing the polarization and taking measurements, the types of transitions responsible for the spectrum can be investigated.

Another chemical technique that is the result of the advent of lasers is laser-induced chemistry (a subset of photochemistry). In spectroscopic methods, the laser induces no chemical change in the sample - it simply causes short-lived changes in the electron populations of different energy levels. Recently much effort has been directed to a more dynamic use of lasers in chemistry where the laser light induces a chemical change in the system. The powerful intensity of lasers can be used to overcome energetic barriers to reaction, since it is electronic energies which are involved in the formation and rupture of chemical bonds. The ability to pulse laser radiation, however, creates a means for inducing and monitoring ultrafast photochemical reactions. It is possible to identify short-lived intermediate species in solution with lasers that have ever-decreasing pulse duration. There has even been some progress in monitoring the rotational and steric action as well as electron transfer rates of species in solution using picosecond and femtosecond laser techniques.

The future of lasers is a promising one. Judging from the quick development of lasers in the past and continuing laser research, there does not appear to be a slowing of laser research in the near future. Although lasers have found use in spectroscopic techniques, it is really the new methods that resulted from laser development that indicate the potential fruitfulness of laser research. Laser induced photochemistry, the monitoring of chemical intermediates with picosecond and femtosecond lasers, and nonlinear spectroscopy represent merely the beginning of innovative techniques for lasers in chemistry. As time progresses, there will doubtless be new scientists with new ideas and new experiments which will expand the role of lasers in chemical research.

References

(1) http://www.achilles.net/~jtalbot/history/index.html

(2) Andrews, David L. Lasers in Chemistry. (Springer-Verlag, New York, 1986).

(3) Bertolotti, M. Masers and Lasers: An Historical Approach. (Adam Hilger, Bristol, 1983).

(4) Bromberg, Joan Lisa. The Laser in America, 1950-1970. (MIT Press, Cambridge, MA, 1991).

(5) Evans, D.K. ed. Laser Applications in Physical Chemistry. (Marcel Dekker, New York, 1989).

(6) Evans, Ted R. ed. Applications of Lasers to Chemical Problems. Techniques of Chemistry Volume XVII. (John Wiley & Sons, New York, 1982).

(7) Fox, Marye Anne and Michael Chanon eds. Photoinduced Electron Transfer: Part B - Experimental Techniques and Medium Effects. (Elsevier, New York, 1988).

(8) Gordon, J.P.; Zeiger, H.J.; Townes, C.H. Phys. Rev., 95, 282, 1954.

(9) Hecht, Jeff. Laser Pioneers. (Academic Press, Boston, 1992).

(10) Maiman, T.H. Nature. 187, 493, 1960.

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