Charles Vernon Shank and Erich Peter Ippen shaped laser pulses to times less than one trillionth of a second in a composite dye laser system, opening new regimes of ultrafast electronics and ultrarapid physics and chemistry.
Albert Einstein is known for his theories of relativity; however, his influence in physics was widespread and had surprising results. One of these results was the laser, discovered five years after his death. In 1916, Einstein noted that the known laws of radiation from hot bodies implied that light could be amplified. Einstein showed that radiation must be able to stimulate more of itself when interacting with matter from which the light had spontaneously radiated. Under the proper conditions, that stimulated radiation would amplify as it raced at the speed of light through the material. In addition, he gave an amazing mathematical relation from which to calculate the ratio of the stimulated radiation to the spontaneous radiation.
The light seen from the Sun, a neon sign, or an incandescent light is spontaneously emitted. Visible light is only one part of the electromagnetic spectrum, which includes among its members radio, radar, and television radiation; these are stimulated radiations. Light is an electromagnetic wave with very short wavelength, while radio, radar, and television have long wavelengths. The mathematical relation that Einstein developed said that the ratio of stimulated to spontaneous emission—everything else being the same—depended on the third power of wavelength. Long waves (if they are to radiate in practical circumstances) must radiate by stimulation, while short waves radiate spontaneously. Decrease wavelength by ten, and the spontaneous radiation increases by one thousand. That very strong increase in spontaneous emission at short wavelength explains why a 100-watt lightbulb is much less expensive than a 100-watt radar transmitter.
Spontaneous light emission from common light sources goes in all directions and contains a wide range of jumbled frequencies, while the stimulated electronic emissions are directional and possess a narrow range of well-ordered, or coherent, frequencies, which are easily modulated. In order to “tame” the light to the stimulated emission of an electronic laser, the conditions for light amplification, which Einstein noted, were required.
It was more than fifty years after Einstein developed his formula for light amplification before scientists were able to fashion its instructions to produce a laser. Early in 1960, Theodore Harold Maiman produced the first laser using a ruby crystal; later the same year, Ali Javan, an American physicist, constructed the first gas laser in neon. The discovery of the laser by Maiman opened up broad vistas in optical technology, which required additional discoveries and years to exploit. Most lasers operate as high-frequency oscillators, which generate a coherent beam of light by the process of emission stimulated from atoms or molecules by the beam while it is trapped within the laser material. The coherence of the laser causes the extremely high-frequency oscillations of the laser light to be very regular throughout the space they travel and the time they last, allowing properties of the laser beam, such as its time duration, to be altered radically.
Maiman had produced the red light of the ruby laser by illuminating a 5-centimeter-long, synthetic crystal of ruby with a powerful spiral flashlamp encircling the crystal. The red color of natural ruby is a weak fluorescence produced by a very small number of chromium impurities in an aluminium oxide crystal. The powerful light from the flashlamp pumped the chromium impurities of Maiman’s ruby into the fluorescent state, producing an inverted population of that high-energy state. Silvered reflectors on the ends of the ruby crystal fed the red fluorescence back into the ruby, producing a series of strong, irregular laser pulses lasting millionths of a second. The chromium atoms, which are actually in the ion state inside the ruby, were the source of the stimulated emission, which was trapped and amplified by the simple optical “cavity” formed by the silvered reflectors at the ends of the ruby rod.
The duration of an electronic pulse is measured in a standardized set of units that decrease in duration by stages of one thousand. One thousandth of a second is a millisecond, and one millionth of a second—the typical duration of the ruby laser pulses observed by Maiman—is a microsecond. Normal electronics is quite capable of producing pulses that last one billionth of a second, or a nanosecond; but it was not until 1974 that pulses as short as one trillionth of a second, or one picosecond, were generated by Charles Vernon Shank and Erich Peter Ippen. The molecules used by Shank and Ippen in their ultrafast, subpicosecond laser were those of the strong fluorescent dye, rhodamine 6G. The ultrashort pulses were shaped by use of an absorbing solution of diethyloxadicarbocyanine iodide (DODCI) dissolved in ethylene glycol, a common solvent inserted with the rhodamine 6G inside the laser cavity. Because the rhodamine dye fluoresces so rapidly, it is not possible to produce lasing in the rhodamine molecules by pumping with a normal flashlamp. Instead, the rhodamine was pumped with an extremely powerful argon laser beam in a complicated optical cavity containing five reflectors as well as the dye and absorber.
The short-pulse experiments began in the early 1970’s at the Holmdel Laboratories of Bell Telephone in New Jersey. In 1972, Shank and Ippen began experiments in which both the rhodamine dye and the DODCI absorber were mixed together in one single solution of ethylene glycol, which was injected as a thin stream near the center of the laser cavity, which, of necessity, had grown complex. The laser light was fed out of the cavity through a quartz acousto-optic coupler. The two men hoped to produce electronic light pulses shorter than any science had known.
There are no electronic photodetectors or oscilloscopes that can measure the very short pulses generated in apparatuses such as that of Shank and Ippen. These short pulse durations are measured by the distance they move in space, rather than time. The pulse is split into two parts. Each part is sent along a somewhat different path; then the two parts are recombined in a crystal of potassium dihydrogen phosphate (KDP), which responds strongly only when both beams are present inside the crystal. The beam paths are adjusted so that the crystal glows. Each laser beam consists of quick pulses spread over a short distance in space, and the glow occurs when both beams coincide within the KDP. The glow is picked up by a sensitive photomultiplier and then one path of the two beams is lengthened with respect to the other until the glow ceases. When the glow ceases, the pulses are arriving at the KDP at different times. A 1-picosecond pulse occupies only 0.3 millimeter, the path length difference that Shank and Ippen were seeking as a mark that they had generated picosecond pulses.
In April of 1974, Shank and Ippen reported in Applied Physics Letters that they had adjusted the argon laser pump source to produce about 2.5 watts of its blue laser light, added DODCI into the dye stream until stable pulse operation was observed, and measured the path differences over which the red dye laser pulses overlapped. The shortest distance was 0.15 millimeter. The laser pulses were more than a million times shorter than the original pulses of Maiman’s ruby laser. Shank and Ippen had produced 0.5-picosecond pulses, pulses lasting only half a trillionth of a second.
There were a variety of devices that generated short electronic pulses before Shank and Ippen’s discovery. Photography with electronic spark discharges captured time events as short as one-tenth microsecond long, while high-speed flashlamps and electronics had resolutions of one-tenth nanosecond. Shorter time spans required lasers; mode-locked, flashlamp-pumped, neodymium glass lasers had produced low-repetition-rate, irregular pulses of several picoseconds duration. Shank and Ippen had taken the principle of laser mode-locking and applied it to dye lasers to generate highly reproducible, high-repetition-rate subpicosecond light pulses. In doing so, they opened up a new realm of fast electronics and a new measure of rapidity: femtosecond pulses. A femtosecond is the next unit in the progression, by one one-thousandth, of short-time measures. It lasts only one thousandth of a picosecond and is over in one quadrillionth of a second, which is less than a cycle of visible light. Both Ippen and Shank were to play a strong role in giving technology and science the tool of femtosecond pulses.
In 1981, Shank and his coworkers collided two optical pulses together in a one-hundredth millimeter section of an absorber dye to produce 90-femtosecond-long optical pulses. The pulses had already been narrowed and sent traveling in opposite directions in a ring-shaped dye laser. They then took the 90-femtosecond pulses and sent them through a 15-centimeter section of single mode optical fiber and then onto a pair of optical gratings. Within the optical fiber, the pulses are temporally broadened, but then the blue end of the pulses catches up to the red end within the grating pair to produce compressed pulses only 30 femtoseconds long. These short pulses opened up a wide variety of previously hidden processes to scientists. Vibrational dynamics in molecules, photobiological mechanisms in living matter, and fast electronic properties of semiconductors are some examples of areas in which new science could now be explored. With even shorter pulses—in the range of 10 femtoseconds—entirely new classes of processes could be studied, including vibrations in liquids.
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Alfano, R. R., and S. L. Shapiro. “Ultrafast Phenomena in Liquids and Solids.” Scientific American 228 (June, 1973): 42-60. Excellent article presents the history of fast optical pulses, along with the state-of-the-art of fast laser pulses at a time just before the production of subpicosecond pulses. Gives an informative introduction to the background needed to understand the science and applications of ultrafast optical pulses. -
Bova, Ben. The Beauty of Light. New York: John Wiley & Sons, 1988. Thoroughly enjoyable account of the science and art of light provides an excellent summary of the fascinating properties of light; how these properties are used in art, science and industry, and technology; and how their beauty affects humans. Chapter 15 discusses lasers in an authoritative manner, without equations, and in language general readers can understand. -
Diels, Jean-Claude, and Wolfgang Rudolph. Ultrashort Laser Pulse Phenomena. 2d ed. San Diego, Calif.: Academic Press, 2006. Describes how ultrashort lasers are applied variously, from medical imaging to quantum physics. Though an introductory tutorial guide, it is intended for senior undergraduates and more advanced students and professionals. -
Hitz, C. Breck, James J. Ewing, and Jeff Hecht. Introduction to Laser Technology. 3d ed. New York: IEEE Press, 2001. Introductory chapter provides an overview of the applications of commercial lasers worldwide. Intended for those who are familiar with the principles of electro-optical technology. -
Ippen, Erich P., and Charles V. Shank. “Subpicosecond Spectroscopy.” Physics Today 20 (May, 1978): 41-48. Ippen and Shank discuss the background, discovery, and applications of subpicosecond pulses in an understandable manner, with illustrations, diagrams, and four equations. This is the authoritative article on the discovery of subpicosecond pulses for the general user. -
Kaiser, W., ed. Ultrashort Laser Pulses: Generation and Applications. 2d ed. New York: Springer-Verlag, 1993. Technical monograph is the definitive reference on ultrashort optical laser pulses. The chapters are by various authorities, and although most are very readable, they are intended for readers with some science background. The introductory chapter, written by Shank, contains illustrations.
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