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6 Optical Physics Optical physics encompasses the physics of electromagnetic radia- tion and the interaction of matter and light. It includes the generation and detection of light, linear and nonlinear optical processes, and spectroscopy. The distinction between optical physics, applied phys- ics, and optical engineering is blurred, for devices and applications are close companions to basic research in this area of physics. The first two sections of this chapter deal with lasers and laser spectroscopy topics that have transformed optical science. The last two sections are devoted to quantum optics and coherence, and to femtosecond optics. Nonlinear optics, a major area of modern optics, encompasses so many different streams of research and applications that we have not attempted to describe it separately. Nonlinear optics plays a role in most of the topics discussed in this chapter. LASERSTHE REVOLUTION CONTINUES From checkout scanners at supermarkets to laser disk recordings, lasers have become commonplace, but the scientific revolution they precipitated is continuing, propelled not only by the discovery of more and more applications but by the steady development of new lasers and new laser techniques. The development of tunable lasers that can operate throughout the visible and into the infrared and ultraviolet ranges has had a major 110

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OPTICAL PHYSICS 111 impact on basic science during the past decade; instances of the advances are scattered throughout this report. Dye lasers are the most ubiquitous of these tunable light sources. Continuous-wave dye lasers achieve a stability and resolution far exceeding those of traditional light sources improvements by factors of hundreds to hundreds of thou- sands are typical. (See Figure 6.1.) Pulsed dye lasers provide such intense radiation that nonlinear processes such as frequency doubling and multiphoton absorption are now widely employed. It is possible to use several lasers in one experiment, providing innumerable new strategies for studying atomic and molecular phenomena. Many new laser sources have come into use during the past decade, from color-center and semiconductor lasers in the infrared to excimer lasers in the ultraviolet. The semiconductor diode laser is already a key component of a major new industry- fiber-optic communications- as discussed in Chapter 8. Notwithstanding these advances, other sources are urgently needed. We possess no efficient optical lasers, and many wavelength regions outside of the visible are difficult to achieve or are inaccessible. There is wide interest in ultraviolet lasers, and the x-ray region continues to be a tantalizing goal. Major advances in lasers have come from research in atomic, molecular, and optical (AMO) physics, and the level of activity and excitement continues to be high. Current developments include superstable optical lasers, hollow-cathode lasers that operate in the ultraviolet using sputtered metal ions, and pulsed-gas ultraviolet lasers. One of the most dramatic developments in laser technologies during the past decade has been the construction of gigantic neodymium glass lasers powerful enough to ignite thermonuclear fusion reactions. These devices stand as triumphs of optical engineering: they have achieved energy densities far greater than anything previously produced by man. One final class of lasers must be mentioned the free-electron laser. First demonstrated as an infrared laser, these devices are now being engineered for wavelengths from the far infrared to the vacuum ultraviolet. Free-electron lasers generate coherent radiation by stimu- lated emission from relativistic electrons traveling through a periodi- cally varying magnetic field. They are attractive because their wave- length can be varied simply by changing the energy of the electrons and because high power and high efficiency appear to be possible. Synchrotron radiation provides an alternative to laser light as a source for ultraviolet radiation. Because of their high brightness at short-ultraviolet and x-ray wavelengths, these sources are being used increasingly, particularly in condensed-matter physics and surface science.

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1 1 2 A TOMIC, MOLECULAR, AND OPTICA ~ PHYSICS Shelf for Electronics A. . ,'Tunoble Cavity ,=_ Fost Servo Electronics ~4. 0 `,Ring Dye Loser ~~J ~ isolated Reference Covity In I ntens ty ~ : flocking ~ IStobls Amplifier and Coo - LOO Chopper ' Erg\ ~ Lost L' To O3 ~ c 1~ ,__Beom Splitter for Opticol Heterodyne 10detector Loser \~ ~ MEL e \~ ~i ~ jPhot' ~Spotiol Filter For Lombdomeler - I, i; ~~ ~~ -Gil ...~_ ~: ~ '"~3_ FIGURE 6.1 Superstable Tunable Lasers. This tableful of optical components is a tunable-dye-laser system that is so stable that the frequency of the light can be adjusted much like the signal of a radio-frequency or microwave generator. The jitter in the frequency of the laser is only 100 Hz. (The frequency of light is about 5 x 10'4 Hz.) Highly stabilized lasers can be used to create and study new types of atomic and molecular species and to carry out ultrahigh-precision spectroscopy. They are also useful for applications such as stopping atoms and studying relativity and for gravity-wave detection. Although this highly stabilized system is at the benchtop stage in a research laboratory, industry has been very effective in making advanced laser and optical technologies available rapidly. (Courtesy of the Joint Institute for Laboratory Astrophysics.)

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OPTICAL PHYSICS 1 13 This summary is by no means complete one could mention numer- ous new solid-state and gas lasers, advances in the design of optical resonators, and in new pumping methodsbut it should give some idea of the level of activity and the rapid progress in laser design. Laser-based ultraviolet and x-ray sources are also being developed. For instance, intense, highly monochromatic tunable vacuum ultravi- olet radiation has recently been generated by scattering laser light from metastable excited atoms. Another method uses an infrared pulse of a few Joules of energy from a neodymium-glass laser. When the light is focused on a heavy-metal target it creates a highly ionized plasma that emits a substantial fraction of the incoming laser energy as an intense short burst of soft x-ray radiation, emerging from a pointlike origin and covering a continuous spectrum. The method is disarmingly simple. (See Figure 1.3.) Every one of these lasers and light sources has an interesting scientific and technological history. To illustrate the role of AMO physics in the development of lasers, we have chosen one device the excimer laser to describe in some detail. Excimers and Excimer Lasers Excimers are diatomic molecular systems for which the electroni- cally excited state is tightly bound but the ground state is a very loosely bound, essentially unbound, van der Waals molecule. The emission spectrum for excimers is characteristic of transitions from bound molecules to free atoms; such molecular transitions are ideal for high-power gas lasers. Most excimer systems involve a rare-gas halide molecule (a molecule composed of a rare-gas atom and a halide atom, for instance, xenon-fluoride). The application of rare-gas halide excimer molecules to efficient, high-power lasers is a success story for high technology that has its roots in fundamental AMO physics. Rare-gas halide lasers employ an electron beam or electrical dis- charge to deposit energy into rare-gas mixtures with a halogen- containing fuel. The electronically excited state of the rare-gas halide molecules is formed efficiently owing to the unique properties of rare-gas atoms and molecules. The stored energy in the rare-gas halide excimer molecules is then extracted by laser action. Within a S-year period following the discovery of the rare-gas halide emission spectra, small commercial lasers were available for labora- tory use and large devices were under construction for military and national energy-related goals. This rapid development required wide collaboration within the AMO community, including specialists in the

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1 1 4 A TOMI C, MOLEC ULAR, A ND OPTI CA ~ PH YSI CS interactions of electrons, ions, ground and excited state atoms and molecules; the optical properties of the laser medium; and the hard- ware associated with electrical deposition of energy into high pressures of rare gases. The rapid development of rare-gas excimer lasers illustrates the value of maintaining a reservoir of trained personnel in AMO physics. The development of rare-gas halide excimer lasers provides an excellent example of a situation where detailed kinetic data at the state-to-state level were vital. Such information will also be vital to the design of other energy-storage gaseous systems. Thus there is an urgent need for the knowledge needed to develop kinetic models at the state-to-state level for reactive systems involving atoms and small molecules. A final point about molecular excimers is their role in stimulating theoretical work in bound-free emission spectroscopy. Models have been developed that permit bound-free spectra to be accurately simu- lated for several rare-gas halide excimers. Further advances would be of value not only because of the intrinsic interest of these molecules but because of the possibility of discovering new excimer systems. LASER SPECTROSCOPY Much of what we know about the structure of matter comes from spectroscopy. During the past decade both the techniques and uses of spectroscopy have advanced so rapidly that the term has acquired new meaning. Spectroscopy has truly undergone a revolution. New spectroscopic techniques have achieved a precision and sensi- tivity enormously greater than the classical techniques of absorption and fluorescence; they have opened new areas in atomic and molecular physics. Unusual species such as Rydberg atoms and molecules can be created routinely, and familiar species can be viewed from new perspectives. For example, the ability to excite a single electronic molecular state with known quantum numbers has had a large impact on molecular physics, as described in Chapter 5 in the section on The New Spectroscopy. Beyond this, a new optical technology has emerged, combining atomic, molecular, and optical science and leading to innovations such as optical frequency standards, new light conjuga- tors, four-wave mixers, and far-infrared detectors. The collective enterprise has come to bee called laser spectroscopy. This term, however, is something of a misnomer, for laser spectroscopy extends far beyond the conventional idea of spectroscopy.

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OPTICA ~ PHYSICS 1 15 Ultraprecise Laser Spectroscopy A unique feature of laser light is its spectral purity. Conventional monochromatic light sources typically achieve a spread in frequencies of 1 part in 105. Commercial dye lasers now routely achieve 1 part in 108. In advanced laboratories, dye lasers have been operated with a stability and spectral purity greater than 1 part in 10~2. The art of wavelength measurement has also made impressive advances. For instance, automated digital wavemeters make it possible to determine wavelengths to 1 part in 107 or 108 in a split second. Photodiodes have been developed that can observe beats in the signals of two different lasers at frequencies as high as several terahertz (1 terahertz = 10'2 cycles per second). As a result, lasers operating at quite different frequencies can be compared with high precision. In fact, the possibility of directly measuring optical frequencies in terms of the cesium microwave frequency standard has recently been dem- onstrated. Essentially, this creates a new optical technology in which the frequency of light is directly measured, much as is done with radio-frequency and microwave signals. Ultrasensitive Spectroscopy Lasers make it possible to observe extraordinarily weak absorption of light using a variety of simple techniques. For instance, by placing a small sample of a gas such as ordinary air inside the resonator of a dye laser, strong yellow absorption bands of water vapor and molec- ular oxygen appear in the laser's light. These bands are barely perceptible by classical techniques, even with absorption paths of many miles. Photoacoustic detection is an ultrasensitive technique for observing the absorption of light in gases, liquids, or solids. A laser beam is modulated at an audiofrequency, and a microphone detects sound waves generated by the periodic small heating of the sample. Another ultrasensitive method is optogalvanic spectroscopy. Modulated laser light enters a gas discharge; when the laser frequency is tuned to a resonance between two energy levels, the discharge current or voltage displays a modulated signal, even when both levels correspond to excited states. The technique requires only a discharge tube, a laser, and an oscilloscope. Among its many applications, optogalvanic spec- troscopy permits studies of sputtered metal atoms and transient species, such as ions and molecular radicals.

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1 16 ATOMIC, MOLECULAR, AND OPTICAL PHYSICS The ultimate in sensitivity can be reached with a related technique: resonant photoionization by intense laser light. A single gaseous atom of almost any element or isotope can, in principle, be selectively excited and detected, even in the presence of large numbers of atoms of different species. Potential applications range from trace analysis and the detection of impurities in semiconductor materials to the search for rare unstable isotopes. (The presence of these isotopes in mineral deposits has been proposed as a telltale indicator for solar neutrino reactions or of the double beta-decay process that would occur if the neutrino had a rest mass.) Doppler-Free Laser Spectroscopy In the past, spectral resolution in a gas was limited by the Doppler effect, the frequency broadening due to the motion of the atoms. Laser spectroscopy provides several methods for eliminating the first-order Doppler broadening, permitting observation of the much narrower natural width of the spectral line. Some of these methods are relatively simple, well suited to observing rare or short-lived species. Others have played roles in stabilizing the frequency of lasers to atomic or molecular transitions. One method, saturation spectroscopy, employs a strong laser beam to label a group of atoms and a counterpropagating beam to pick up a signal from those atoms that have no Doppler shift. Saturation spec- troscopy has been applied to problems ranging from the finest details of molecular structure to collisional effects and precise measurements of fundamental wavelengths in atomic hydrogen. Another method of Doppler-free spectroscopy employs two counterpropagating laser beams to induce a two-photon transition: the first-order Doppler shift essentially disappears for all the atoms or molecules. Using this method, the "forbidden" l S-2S transition in hydrogen has been observed. This measurement represents an important spectroscopic advance because its intrinsic linewidth is more than a million times narrower than for normal optical transitions. It provides the opportu- nity to measure the Lamb shift in the ground state, and the wavelength can be directly related to the Rydberg constant and the electron-proton mass ratio. The l S-2S transition in positronium has also been measured by two-photon spectroscopy. As discussed in Chapter 4 in the section on Elementary Atomic Physics, laser spectroscopy of positronium opens a most useful new line of research in the physics of elementary systems.

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OPTICAL PHYSICS 1 17 Laser Cooling Although Doppler-free methods remove the first-order Doppler shift, the second-order Doppler shift remains. The effect is small the frequency is typically shifted by 1 part in 10~- but it can result in serious errors in high-precision measurements. The second-order Dop- pler shift is proportional to the kinetic energy of the particles, and the only way to reduce it is to reduce the particles' motion. This has been accomplished by using the momentum of laser light in various inge- nious experiments to "cool" atoms or ions, that is, to slow them or even bring them to rest. Ions held in an electromagnetic trap have been cooled to the millikelvin range by absorbing laser light that is tuned slightly below resonance. (See Figure 6.2.) Recently an atomic beam of sodium was cooled, actually brought to rest, by laser light. A prime motivation for laser cooling is to create better frequency standards, either at microwave frequencies or at optical frequencies. Optical frequency standards have been proposed as candidates for the next generation of atomic clocks. Coherent Optical Transients One area of laser spectroscopy, coherent optical transients, exploits the temporal coherence of laser light. Gaseous and solid atomic systems can be coherently excited, producing a new and unusual class of nonlinear optical phenomena. Effects such as optical free-induction decay the coherent emission from atoms excited by a single-laser pulse and photon echoes the delayed burst of coherent radiation following excitation by two successive laser pulses can be applied to study dynamic interactions of atoms in their local environment. Optical free induction provides new ways to study elastic collisions of atoms or molecules that are not in a single eigenstate as in traditional scattering experiments but in a superposition of ground and excited states. Cross sections and other parameters can be determined from measurements of the decay. The close impacts with a perturber that annihilates the superposition state can be visualized classically in terms of separate scattering trajectories, one for each state, resembling state selection in a Stern-Gerlach experiment. Distant impacts or small-angle diffractive scattering where the superposition is largely preserved require a quantum description.

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1 18 A TOMIC, MOLECULAR, AND OPTICAL PHYSICS lon~Clo~ I_ a_ in_ ~ ..,~ ^'#~.~"~ FIGURE 6.2 Trapped Ions. Ions can be trapped in high vacuum using static and oscillating electric fields and viewed by laser light. The experiments can be so sensitive that single ions can be observed under close to ideal conditions of isolation. In this experiment barium ions are formed in the center of the donut-shaped electrode by bombarding barium vapor with electrons. The ions are observed by their fluorescence under laser light. The photograph at bottom left shows the laser light scattered by a small cloud of trapped ions. In the blown-up photograph at bottom right, the light scattered by one barium ion can be discerned in the circled region. Laser light can also be used to

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OPTICA ~ PHYSICS 1 19 Ultranarrow Optical Transitions Optical free-induction decay of impurity ions in certain solids (the praseodymium ion in lanthanum triduoride is one example) can display extremely narrow linewidths, 1 kilohertz or less. This is 106 times narrower than typical linewidths in solids. These optical transitions are analogs of the Mossbauer eject: the optically excited impurity ion suffers no recoil effect because its momentum is transferred to the lattice as a whole. Furthermore, at cryogenic temperatures there is virtually no second-order Doppler broadening. These systems are prime candidates for studying the interactions that broaden optical transitions, and possibly for establishing secondary optical frequency standards. The method, which is made possible by the use of a dye laser with a 100-Hz linewidth, has been applied to study the optical Bloch equations, the starting point for many theories in quantum optics. It has been found that in intense laser fields the optical Bloch equations must be modified because the radiation inhibits the line- broadening ejects of nuclear magnetic interactions. The phenomenon is now understood in terms of a microscopic theory of nuclear magnetic interactions. Coherent Raman Spectroscopy The term coherent Raman spectroscopy describes a class of nonlinear optical techniques that are used to study and measure Raman-active modes of molecules. The major techniques are coherent anti-Stokes Raman spectroscopy (CARS) and stimulated Raman spec- troscopy (SRS). In contrast to ordinary Raman light-scattering methods, the signals from CARS and SRS come in the form of strong and highly directional laser beams. As a result, these methods offer tremendous discrimina- tion against undesirable background fluorescence and luminescence. Coherent Raman spectroscopy works in environments where the cool the ions, reducing the energy-level shifts due to the second-order Doppler effect. Trapped-ion methods are being applied to ultrahigh-resolution optical spectroscopy and to the creation of new types of atomic clocks. The methods are also employed to study collisions and chemical reactions, including reactions at very low temperature, and to study collective motion in charged plasmas. Further discussion is in Chapter 5 in the section on Molecular Dynamics and in Chapter 8 in the section on Precision Measure- ment Techniques. (Courtesy of the University of Hamburg, Federal Republic of Germany.)

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120 ATOMIC, MOLECULAR, AND OPTICAL PHYSICS background light level or the need for high resolving power make conventional methods impractical. In combustion diagnosis, the method provides a means of nonintrusively mapping the temperature of a gas and the concentration of its species. CARS studies have been performed in hostile environments such as the combustion chamber of internal combustion engines, in gas-turbine combustors, and even in jet engine exhausts. Other applications for the coherent Raman techniques include stud- ies of the energy-level distributions that result from optical photodis- sociation, trace detection of pollutants in gas and liquid phase, the spectroscopy of biological molecules, plasma diagnostics, and applica- tions in the study of hydrodynamic flow. QUANTUM OPTICS AND COHERENCE The concept of the photon grew out of Einstein's preoccupation with the statistical nature of light, but it was not until the advent of the laser that the statistics of electromagnetic radiation began to be studied methodically. Light can be observed only in its interactions with matter, however; and so the study of light inevitably encompasses the dynamics of atoms in the radiation field. These subjects collectively form the main body of research in quantum optics and coherence. During the past decade these studies have provided new insights into the statistical nature of radiation and the dynamics of lasers and other quantum systems. In addition, they have opened the way to new methods of measurement and to new quantum devices. Photon Antibunching One can observe the arrival of photons at two separate detectors and study the probability that the photons arrive in coincidence. (The light beam is split by a semireflecting mirror, and each half is detected by a separate phototube. The correlations are found from measurements of the coincidence rate.) These are called second-order correlation exper- iments, since they are sensitive to the product of two intensities. The famous Hanbury-Brown and Twiss experiment determined the diame- ter of a star from measurements of the second-order correlations in its light. Second-order corrections are usually positive; photons tend to bunch together. It has now been discovered, however, that it is possible to prepare light so that the photons, instead of coming in clumps, are antibunched. More precisely, if a phototube receives a

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OPTICAL PHYSICS 121 photon, for a short period thereafter it is less likely than otherwise that it will detect a second one. Antibunching can be observed in light coming from a single atom. (It would not suffice to attenuate a conventional light source, or laser light, for that matter, for that would change only the average arrival rate of photons, not the statistics of the radiation field. Antibunching occurs only when the photons are produced by a nonclassical sources This has been achieved in practice by using as the light source a single atom that is coherently excited and radiates spontaneously. Other processes, such as harmonic generation or parametric amplification, should also exhibit the antibunching. The novelty of antibunching, however, lies not so much in the realization that one atom can radiate only a single photon at a time but that the statistical properties of the light are different from any that have previously been observed. Antibunching provides an example of light that is fundamentally different from any light previously studied. Closely related to antibunching is the production of light for which the photon number fluctuations are smaller than random. This has also been observed: it offers the interesting possibility of allowing optical communication with less noise than with a coherent laser beam. Optical Bistability An optically bistable system has two stable output states for a given input level of light. Typically, it consists of a nonlinear medium within an optical resonator. Optical bistability was first observed using sodium vapor in a Fabry-Perot etalon, and now there is an expanding class of optically bistable devices. Often the devices are constructed of tiny semiconductor chips whose faces are polished to form a resonator. Optical bistability provides a new arena for the study of nonlinear systems. Many of the dynamical phenomena that have been studied in lasers, for instance fluctuations and regenerative pulsations, can be observed under far better controlled conditions using optically bistable devices. The transition from ordered to chaotic motion is of particular interest. Such transitions have been studied in hydrodynamic, acous- tical, and electrical systems; optical bistability allows the research to be carried out under highly controlled conditions at extremely high speed. The ability to gather data at high speed is of particular value, for it offers the opportunity to study turbulent motion in ways never before possible. Much of the interest in optical bistability is due to its potential applications to optical computing. A bistable device can serve as a fast

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122 ATOMIC, MOLECULAR, AND OPTICAL PHYSICS memory element; it can be used as an "optical transistor" to amplify small signals at high speeds; and it can be employed as a discriminator, a pulse shaper, an oscillator, or a general logic element. A room- temperature optically bistable device has been created, 5 Em thick and 10 lam in effective diameter, that turns on in a few picoseconds (I picosecond = 10-'2 second). These devices can be integrated into larger systems, with numerous applications to optical computing and data processing. Systematic interest in chaotic phenomena can be traced back to Poincare's studies early in this century. There has been a great renewal of interest in recent years, and the question of regular versus disor- dered motion is now a central problem in physics, with important ramifications in mathematics and engineering. Two discoveries, in particular, have contributed to the present interest. One was that attempts to predict long-range weather patterns were inherently limited by the onset of turbulence. The problem of chaos thereby assumes enormous economic significance. The second discovery was the real- ization of the ubiquitous nature of period doubling in naturally oscil- latory phenomena, revealing an important route from regular to chaotic motion. Optical bistable devices provide a way to study the transition from regular to chaotic motion in reproducible experiments that can be carried out at very high speed. The simplest bistable optical system comprises two mirrors and a nonlinear medium that is operated in a transient mode using pulsed lasers. The onset to chaos often reveals precursors. In the case of optical bistability these have been discovered to produce short pulses with 100 percent modulation, even in a high-finesse cavity. The technique holds the possibility of new methods for short pulse generation and possibly for optical processing. Squeezed States According to quantum mechanics, two canonical variables, such as the position and the momentum of a particle, cannot both be known with great precision; the product given by the uncertainty in one multiplied by the uncertainty in the other must exceed half of Planck's constant. It follows that one variable can be determined accurately only at the expense of large fluctuations in the value of the other. Squeezed states are quantum states that exploit this property. They have become particularly important for the electromagnetic field, in which two oscillatory quadrature (90-of-phase) components of the

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OPTICAL PHYSICS 123 field play the role of canonical variables. In a squeezed state one component of the field can be relatively free from fluctuations while the other fluctuates appreciably. This has potentially important applica- tions for optical communications and high-precision measurements, provided that it is possible to encode and decode information in just one quadrature component of the light. Much effort has been devoted to exploring theoretically the different conditions under which squeezing can be produced. It has been found that squeezing can occur in parametric processes, harmonic genera- tion, phase conjugation, resonance fluorescence, the free-electron laser, and many other circumstances. The practical problem of encod- ing and decoding information in squeezed light is not without difficul- ties, but if they can be overcome it would be possible to achieve signal-to-noise ratios in an optical communication channel that go beyond the quantum limit for coherent or laser light. The problem of detecting gravitational waves with detectors operat- ing close to the quantum limit, where the signal is hidden by quantum noise, has also generated much interest in squeezing. Our ability to make new kinds of astronomical observations may benefit eventually from the use of squeezed states. Rydberg Atoms and Cavity Quantum Electrodynamics Any neutral atom in which one electron is in a high-lying energy level is known as a Rydberg atom. These atoms have opened a new area in the study of fundamental radiative processes cavity quantum electro- dynamics. The interaction between Rydberg atoms and the electromagnetic radiation field scales as n4, where n is the principal quantum number. For n = 30, for instance, the interaction is 106 times larger than for ~ ordinary atoms. As a result, the rates at which Rydberg atoms absorb and emit radiation are anomalously large. Thermal radiation at room temperature, usually ignored, is intense for these atoms: it shortens their radiative lifetimes, redistributes the atoms among the various quantum states, and can photoionize the atoms at measurable rates. In addition, thermal radiation can shift the energy levels. The effect is somewhat analogous to the Lamb shift, except that its origin is the real energy of the radiation field, not the virtual energy of the vacuum. The blackbody shift is small but measurable. It needs to be taken into account in the design of the next generation of atomic clocks. Rydberg atoms are so sensitive to radiation that they provide a natural medium for detecting infrared, submillimeter, and microwave

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124 ATOMIC, MOLECULAR, AND OPTICAL PHYSICS radiation. A number of schemes have been proposed and realized in a laboratory setting. Rydberg atoms can be used to count photons with an efficiency that comes close to the ideal quantum limit. Since absorption is inherently frequency selective, Rydberg atoms can also serve as tuned receivers. In addition, they can be employed in maser amplifiers. Unlike conventional masers' the number of atoms required is small; maser action with only one atom has been achieved. When an atom is placed in a tuned cavity its radiative behavior is fundamentally altered: the spontaneous radiation rate is enhanced; the lifetime shortened. Rydberg atoms have made it possible to study these effects. If the losses in the cavity are made sufficiently small, a point is reached where the atom no longer decays to the lowest state. The atom and the cavity behave like a pair of coupled oscillators one atomic, the other man-made. Such a device represents a new entry into the field of macroscopic quantum electrodynamics and provides a unique opportunity to study the transition from reversible to irreversible behavior and the origins of noise. Cavities not only enhance the spontaneous radiation rate, they can also inhibit it. Simply put, an atom cannot radiate a long wave into a short cavity. From another point of view, the cavity can be viewed as modifying the spectrum of zero-point fluctuations that induce sponta- neous emission. If the cavity is mistuned, the fluctuations are removed and spontaneous emission is inhibited. This effect has been seen. It is possible to "turn off'' spontaneous emission, leaving an atom in a new type of excited state, a state devoid of radiative damping. The natural linewidth is suppressed, and other radiative interactions such as the Lamb shift are altered. The ability to observe basic radiative processes with Rydberg atoms offers a new arena for studying electrodynamic phenomena. Although quantum electrodynamics is usually regarded as a highly developed theory, the new experiments suggest that there is a wide body of phenomena yet to be discovered. For instance, it has been found that it is possible to measure the number of thermal photons in a cavity by counting the number of atoms that the cavity can excite. The technique provides, in principle, an absolute thermal radiometer. A scheme has been proposed for cooling a radiation field below the temperatures of its surroundings. Undoubtedly other surprises are in store. FEMTOSECOND SPECTROSCOPY A decade ago picosecond optics was in its infancy; today picosecond spectroscopy is a mature field and femtosecond (10-'5 second) optics is

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OPTICAL PHYSICS 125 in its infancy. Pulses as short as 9 femtoseconds have been generated; such a pulse contains only 5 cycles of light. Femtosecond spectroscopy can provide revolutionary insights into the dynamics of molecules and solids, and into chemical reactions, since femtosecond pulses are compared to the characteristic times for all of these. For instance, a molecule typically requires 1o-~4 to 1o-~3 second to vibrate; the time for electrons in a semiconductor to equilibrate after they have been excited can be as short as 1o-~4 second; and proton and electron transfer in molecules can be quicker than 1o-~4 second. The state-to-state dynamical steps in many solid- state and surface processes span an enormous range of frequencies; for the first time Femtosecond pulses of light make it possible to observe these phenomena. Chemical reactions in solutions and in biological systems also take place in the Femtosecond regime. Their study is especially important for chemistry since most chemical reactions organic, inorganic, or biochemicaloccur in solutions. The use of Femtosecond spectroscopy for the direct, real-time observation of ultrafast relaxations and reac- tions in condensed-phase chemistry is expected to open new horizons in chemical research. In complex biomolecules the number of energy- transfer paths can be so large that transport and relaxation processes occur on a subpicosecond time scale. Femtosecond spectroscopy can provide a unique means of identifying and studying these primary biophysical events. Femtosecond optics can also have important applications to fast electronic circuitry and high-speed instrumentation. Femtosecond techniques enable optical pulses to reach a domain inaccessible by electronic techniques. Optical pulses can be used to investigate semi- conductor processes that determine the ultimate speed potential of electronic circuitry. For example, high-speed photoconducting pulse generators and sampling gates have been used to measure the elec- tronic input response of gallium arsenide field-effect transistors. The information obtained in such studies will become increasingly impor- tant for the design of faster and smaller computers. Ultimately, all-optical modulation and switching techniques, utilizing nonlinear interactions between the ultrashort light pulses themselves, have the potential to go beyond electronics and advance signal-processing speeds into the Femtosecond domain.