Encounters in Nonlinear Optics

The following sections are included:

• PREFACE

• TABLE OF CONTENTS

Please refer to full text.

In the introduction to this section, the role of Lee R. Wilcox was mentioned as the one who called our attention to Franken's paper, of which he had received a preprint before its publication. Lee Wilcox had already accepted a teaching position at the American University in Beirut, Lebanon, and he did not stay as a member of our study group. He had told us that he did not consider the United States a safe place to live as the cold war confrontation with the Soviet Union reached its peak, threatening mutually assured destruction. Fortunately, he later returned to the United States before the conflagration in Lebanon started. He had a successful career as a professor of physics at the University of New York in Stony Brook until his premature death in 1995.

I convinced John Armstrong to stay on at Harvard as a postdoctoral fellow. He had just received a Ph.D. degree with George B. Benedek on an experimental investigation of nuclear magnetic resonance at high pressures and was eager to join the emerging activity in optics.

Jacques Ducuing had recently arrived from Paris for advanced graduate studies. He was on leave from the Campagnie Général de Té1égraphie sans Fils in Orsay. I assume that his original intent was to work on microwave masers and was glad he agreed to join our optics team instead.

Peter S. Pershan had done his Ph.D. thesis work with me on cross -relaxation effects between nuclear spin systems in lithium fluoride. He had already stayed on as a postdoc for several years, involved not only with ongoing experiments in magnetic resonance, but also with microwave modulation of light. He was about to become an assistant professor and was keen on participating in the emerging field of nonlinear optics.

I was fortunate to have these three young, extremely capable coworkers, who all continued on to have very successful scientific careers. John Armstrong joined the IBM research laboratories in Yorktown Heights, New York. He rose through the ranks to become vice-president for Research and Development for the IBM Corporation. Jacques Ducuing became a professor of physics at the Ecole Polytechnique in France, and later served as Director-General of the CNRS (Centre Nationale des Récherches Scientifiques) and as Chief Scientist of NATO in Brussels. Peter Pershan stayed on at Harvard University as Gordon McKay Professor of Applied Physics.

In the summer and fall of 1961, we had regularly scheduled weekly group meetings to discuss our progress and to coordinate our efforts. The pace was so hectic that we had informal one-on-one discussions almost daily. I was well aware from lectures I had followed from Professor Leon Rosenfeld in Utrecht during World War II and from his booklet, Electron Physics, published in 1951, that electromagnetism is based on the microscopic Maxwell's equation in vacuum, and that the material is represented by moving charges, which are acted upon with Lorentz -type forces by the fields. The moving charges in turn serve as sources for these fields. I was also familiar with the relationship between the microscopic field of the vacuum and the macroscopic fields in media which occur in Maxwell's equations. I had emphasized these concepts in intermediate courses on electromagnetism I taught at Harvard. I always cautioned students that the engineering convention of introducing electric and magnetic permeabilities for the vacuum obscures the basic physics of the interaction between fields and matter.

Thus, our starting point was that nonlinear polarizations produced by nonlineartities in the constitutive relations served as additional sources in Maxwell's equations. These nonlinearities are expressed in terms of second and third order nonlinear susceptibilities which could be calculated by higher order quantum mechanical perturbation theory.

This procedure leads naturally to sets of coupled electromagnetic wave equations. General solutions were developed. My young associates were much more adept in carrying through the rather lengthy, detailed mathematical calculations than I was. We discovered with hindsight that the propagation of microwaves in guided structures, loaded periodically with nonlinear materials, had led to similar results. We also obtained Manley-Rowe relations and other integrals.

In Sec. VIII of this paper, phase correction schemes for efficient harmonic generation in media with cubic media are discussed. I took out a patent on a method which became known as quasi-phase-matching. It is now widely used in periodically modulated optical wave guide geometries. Our work was supported by the Joint Services Electronics Program from the US. Department of Defense. I never wrote a formal proposal that our funding for magnetic resonance and masers in 1961 would be spent on work in optics. We reported our activity regularly in quarterly progress reports. Preprints of our work were widely distributed as JSEP Cruft Laboratory Technical Report 358 in March 1962.

This paper was later designated as a “Citation Classic”. The entry in Current Contents, Volume 22, No. 6, February 11, 1991, is reproduced here to complement this commentary.

Our work on the preceding paper induced me to frequently consult M. Born and E. Wolf's standard textbook, Principles of Optics. It dawned on me that many sections of this book could be extended and generalized to include the regime of high light intensities where nonlinear responses would become observable. An obvious question which presented itself was concerned with the boundary conditions when an intense laser beam is incident from the vacuum onto a nonlinear crystal. This question led in a straightforward manner to generalizations of the laws of reflection and refraction of light. I became interested in the history of these laws for linear optics and was pleased that the referee and the editors allowed a reference to a paper by Hero of Alexandria on the reflection of light published in the first century AD. We calculated the intensities of the second harmonic wave in reflection and of two refracted second harmonic waves, the so-called free and forced waves. We analyzed the polarization dependence, the occurrence of Brewster's angle in harmonic generation, and the phenomenon of total reflection as well as the conditions for nonlinear response in a plane parallel slab and a prism. Although only straightforward applications of the boundary conditions of the electromagnetic fields were involved, the algebra was often lengthy and rather tedious. Peter Pershan carried out the bulk of these calculations. This paper received much less attention than the preceding one. Its relevance to the investigation of surfaces and surface layers was recognized decades later when second harmonic and sum frequency generation became widely used tools in surface physics and chemistry.

The quantum mechanical calculation of the nonlinear susceptibilities presented in Paper 1.1 applies only to purely parametric, non-dissipative processes. It yields real values for the nonlinear susceptibilities. I realized in the fall of 1962 that the general scheme for calculating complex susceptibilities in magnetic resonance, based on a density matrix description of the material system, would also be applicable in optics. These ideas are worked out explicitly for two- and three-level systems in this paper. It mentions the interference of one- and two-photon transitions between the same initial and final states This gives rise to an imaginary part of χ⁽²⁾, the lowest order nonlinear susceptibility. The imaginary χ⁽³⁾ describes both raman processes and two-photon absorption processes.

I was fortunate to keep my graduate student Ron Shen, who obtained his Ph.D. degree from Harvard University in 1962, for another year as a postdoctoral fellow. His approach to physics was very similar to that of Peter Pershan and myself. He combined experimental and theoretical skills in an efficient manner. We were able to connect the results of quantum mechanical analysis with the experimental results of raman lasers and parametric processes. This paper has not attracted much attention and is not often cited. It appears that too many details are hidden in the general formalism. There are a great number of papers published during the past three decades on two- and three-level systems subjected to electromagnetic fields at two or more frequencies. There are so many variants, depending on the detuning, the damping constants, and the amplitudes, or the “Rabi frequencies”, of the applied fields that different terms are selected from the general expression in each particular situation. If our paper accomplished little else, it certainly contributed to our own understanding of nonlinear phenomena.

Circumstances conspired so that Ron Shen and I were able to collaborate again on some joint papers during the academic year 1964–1965. Ron had been appointed as an assistant professor at the University of California in Berkeley and I had independently made plans to spend a sabbatical leave there. I wanted to work with Charles Kittel on solid state problems in the physics department, and to give a course on nonlinear optics in the electrical engineering department which had a lot of activity in quantum electronics. My course was attended by two of Prof. John R. Whinnery's students, Eric Ippen and Robert V. Shank. These two later became well known for the generation of femtosecond pulses which opened up many new avenues of research. Ron Shen stayed in Berkeley as a professor of physics, and he continues to lead a very productive research group in nonlinear optics. He wrote a well-known textbook, Principles of Nonlinear Optics, published by Wiley in 1984. He and his family have also remained life-long friends of ours.

During the summer of 1962, I was invited to contribute to a special issue of the IEEE Proceedings dedicated to quantum electronics. While the main body of this paper contains a brief summary of the results described in Papers 1.1 and 1.2, Sec. VI, “Nonlinear Dissipative Processes”, introduces a new element. It demonstrates that at the time of this paper's submission in October 1962, I was aware of the fact that nonlinear susceptibilities are, in general, complex quantities, and that the imaginary part of the third order susceptibility χ⁽³⁾ is capable of describing both the two-photon absorption and the stimulated raman process. I had heard about the latter effect before its publication, as indicated in footnote 18. I was program chair of the Third International Conference on Quantum Electronics, to be held in Paris in February 1963, and in this capacity, I received a call from Dr. M. Stitch, who was leader of a laser group at the Hughes Research Laboratories. Members of his group wanted to submit a post-deadline paper on the production of infrared radiation in a Q-switched ruby laser system. In the conversation, Malcolm Stitch told me that it was not yet clear whether it was an enhanced infrared fluorescence or a type of raman effect. I immediately assured him that the material could be presented in Paris. To my mind, it was most likely stimulated raman scattering, which could be described by a negative imaginary part of χ⁽³⁾. The experimental results of E.J. Woodbury and W.K. Ng were indeed interpreted correctly as stimulated raman scattering by other workers at the Hughes Research Laboratories a few weeks later. The pertinent reference, not yet known to me in October 1962, is G. Eckardt, R.W. Hellwarth, F.J. McClung, S.E. Schwarz, D. Weiner, and E.J. Woodbury, Phys. Rev. Lett. 9, 455, 1962. Gisela Eckardt obtained a patent for the stimulated raman laser.

At the aforementioned Paris conference in February 1963, I presented a more detailed overview of the theoretical description of various nonlinear optical phenomena known at that time. I gave my talk, “Optique non-linéaire” in French to pay homage to the host country. My French student, Jacques Ducuing, had seen to it that the language was grammatically correct and he also gave me some pointers on pronunciation. There was simultaneous translation at the conference, however, with papers presented in English, French and Russian, and the audience and translators expected me to speak in English. Therefore, there was considerable confusion and switching about of microphones and earphones at the beginning of my talk. The French text, which is not reproduced here, was published in the conference proceedings (Quantum Electrics, edited by P. Grivet and N. Bloembergen, Dunod, Paris, and Columbia University Press, New York, 1964, pp. 1501–1512).

These proceedings of the Third International Conference on Quantum Electronics clearly demonstrate the nearly complete shift of attention from microwave to optical frequencies. The total activity in quantum electronics was still sufficiently limited that all submitted papers could be accommodated in only two parallel sessions. I decided to accept' all papers because the application of absolute refereeing standards would have further enhanced the dominance of the contributions from the United States in this period of early laser development. The Soviet delegation arrived a day late, when N.G. Basov and A.M. Prokhorov showered me with a dozen post-post deadline papers. I was able to accommodate them on the final day of the conference, when there was originally only one session scheduled.

In July 1963, there was another large international gathering to review the rapid scientific progress spawned by laser-based research. The site was the E. Fermi International School of Physics, held in the Villa Monastero on Lake Como in Varenna, Italy. The director was Charles H. Townes, who had invited me to give a course of five two-hour lectures on nonlinear optics. This paper contains the notes on which these lectures were based and describes the status of this emerging field of scientific endeavor in the summer of 1963. The proceedings of this Course XXXI of the E. Fermi School were published more than a year later. By that time, another summer school, held in Les Houches, France, had already taken place in July 1964. There, I presented a longer lecture course, the contents of which were published in early 1965 in my book Nonlinear Optics. A fourth edition of this book was published by World Scientific, Singapore, in 1996. The rapid developments in all branches of quantum electronics caused the proceedings of the Varenna summer school and many other proceedings of subsequent schools and conferences to have a very short useful life span. A reprint of this paper is included here primarily for its historical interest, providing an overview of the first two years of our activities in nonlinear optics.

This Varenna school was memorable not only for exciting lectures by A. Javan, Willis E. Lamb and many others, but also for the social contacts it provided. During one of the excursions, I became acquainted with Boris Stoicheff. He wanted to take a surprise snapshot of Willis Lamb, but was abruptly rebuffed for this attempted invasion of privacy. On a Sunday morning, Deli, my wife, and I, Benjamin Lax, a conference secretary, and several others had been swimming in the lake which was not yet badly polluted. As we dried off in the sun on the steps of the Villa, Charles and Frances Townes returned from church appropriately dressed for that occasion. We invited them to pose for a picture as a contrast to the bikini-clad women. They agreed, but only after they had quickly changed to more relaxed outfits. Then I spotted Willis Lamb in a blue silk dress suit. He explained he had not gone to church, but had washed his sporty wash-and-wear outfit the night before only to discover that it was not wash-and-wear. He saw the humor of the situation and agreed to provide an accent of contrast for the photo. Unfortunately, the original early Polaroid color photographs have faded, but the Townes, Lax and Bloembergen families still keep them as mementos.

Twelve years later, I had the honor and the pleasure to return to Varenna as director of Course 64 of the E. Fermi International School of Physics on Nonlinear Spectroscopy. Unfortunately, the plumbing of the sunken Roman-tiled bath in the Director's suite, which Townes had shown us in 1962 , had fallen into disrepair in 1975.

This paper was written as my acceptance speech for the Optical Society of America's Ives Medal Award. Although I had earlier received distinctions and medals, they had emphasized my work on magnetic resonance and masers. This was the first time that our large body of work in nonlinear optics received explicit recognition from the physics and optics community in the United States, which made it especially meaningful to me.

The paper reviews the significance of conservation laws of energy, momentum, and angular momentum in nonlinear optics. Attention is drawn to the fact that in periodic structures with period d, momentum need only be conserved module 2π/d. This is the basis for the quasi-periodic phase matching technique already proposed in Paper 1.1. This principle was extended to laminar semiconductors, or periodic quantum well structures, in a paper, “Nonlinear optical properties of periodic laminar structures” (N. Bloembergen and A.J. Sievers, App. Phys. Lett. 17, 483–486, 1970).

This paper is the Nobel lecture, which I presented at the Swedish Academy of Sciences in Stockholm in December 1981. Although the Review of Modern Physics is a readily accessible journal and this paper has also appeared in Science and in special volumes of Nobel lectures, it is included in this reprint volume because these journals are not regularly consulted by the quantum electronics community.

The paper sketches a broad overview of nonlinear optical phenomena and their historic link to nonlinear phenomena in magnetic resonance. One half of the 1981 Nobel Prize for Physics was awarded to Kai Siegbahn for “his contribution to the development of high resolution electron spectroscopy” while the other half was awarded jointly to Arthur L. Schawlow and myself for “their contributions to the development of laser spectroscopy”. While this citation is most suitable for the work of my colleague Art Schawlow, I believe that a citation for “contributions to nonlinear optics” would have been a more appropriate description of my work. Of course, I did not protest the decision by the Nobel Physics Award Committee. My whole family had a wonderful time in Stockholm. We enjoyed the pomp and circumstance as well as the friendly hospitality of the occasion.

This paper was originally presented at a conference held in Scheveningen, the seaside resort near The Hague in the Netherlands in the fall of 1990. It was a celebration of the 300th anniversary of Huyghens' treatise, Traité de Lumière, which was published in Paris in 1690. In a preface, Huyghens mentioned that he had presented most of the material contained in this book more than a decade earlier in a series of lectures before the Académie des Sciences in Paris. This was to establish his claim to priority, which was just as important then as it is now. His contenders were small in number, but included Descartes and Newton. The dual wave-particle character of light was already a burning issue in that period.

In the past five years I have given numerous colloquia and keynote talks with the same title as this paper and the topic is apparently of general interest. Since the book in which this paper was first published is not well known in the quantum electronics community, a reprint is included in this volume.

The paper traces the origin of electric and magnetic nonlinearities back to the nineteenth century. It does not include a reference to the quadratic Kerr effect, “A new relation between electricity and light; Dielectrified media birefringence,” John Kerr, Philosophical Magazine and Journal of Science (fourth series) 50, 332–348, 1875. I originally omitted this reference because, unlike the Pockels effect, the Kerr effect is not a pure electronic nonlinearity. Kerr mentioned explicitly in his papers the long times it takes for material electric dipoles to align themselves in glass, and that the times for molecular reorientation in fluids are still much longer than an optical period. This effect could not be extrapolated to third harmonic generation, or to other third-order nonlinearities involving arbitrary optical frequencies. Nevertheless, a discussion of the Kerr effect and a reference to it is appropriate in the historical context.

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When our group obtained its first ruby laser heads from Trion and Raytheon corporations in the course of 1962, we naturally concentrated our first experimental efforts on a verification of the results obtained in our theoretical paper 1.2. Reflected harmonic generation also permitted the study of absorbing crystals. Here, my graduate students faced less competition from the well-established active laser groups in many industrial research organizations as these concentrated their efforts mostly on the realization of optical parametric devices. Clearly, optical absorbing materials were of lesser interest to them. Thus Jacques Ducuing was the first to observe reflected second harmonics.

While attending a rather boring lecture at some quantum electronics conference in 1963, I doodled the following limerick about this experiment:

There was a young lady who radiated

When, in a mirror, her figure she contemplated

The nonlinear way

Induced her to say

By Jove, I am ultraviolated.

A year later I was on a sabbatical leave at the University of California in Berkeley. Scheduled as a physics colloquium speaker, I asked my good friend and colleague Erwin Hahn whether I could recite this limerick during my talk on nonlinear optics. He told me that the majority of the colleagues and students would probably not appreciate it. I recited; Erwin was right.

This note demonstrates for the first time that second harmonic light can emerge from a nonlinear mirror in directions different from those of the reflected fundamental light rays. The latter follow the law, already formulated by Hero of Alexandria in the first century AD., that the angle of incidence equals the angle of reflection. Two geometries presented which confirm the rules that determine the direction of second harmonic reflected beams. A colored copy of Fig. 1 in this paper was used as a jacket design for Russian translation of my book Nonlinear Optics.

Jacques Ducuing also realized that the early ruby lasers usually operated in many modes simultaneously. This gave rise to large statistical fluctuations in the observed second harmonic signal. We published a full length paper on these effects (not included this volume), “Statistical fluctuations in nonlinear optical processes” (J. Ducuing and Bloembergen, Phys. Rev. 133, A1493–A1502, 1964). Ducuing also made essential contributions to the theory of coupled waves presented in paper 1.1. All this material is described in detail in his Thèse de Doctorat which he defended at the University of Paris before an examining committee presided over by Professor Alfred Kastler. It was the beginning of his very successful scientific career in France. Our families have kept in touch ever since.

The general theory presented in papers 1.3 and 1.4 for the nonlinear susceptibilities in absorbing media show that these quantities are complex. This paper describes the first measurement of the phase of the complex quantity χ⁽²⁾. The real and imaginary are obtained from this phase and the amplitude, which is derived from the second harmonic intensity. The imaginary part is interpreted in paper 1.3 as an interference of one- and two-photon transitions between the same initial and final states.

Richard Chang obtained his Ph.D. degree at Harvard on the work described in this and the following three papers. He joined the faculty at Yale University where he is a Professor of Applied Physics. Again, we have kept in touch with him and his wife up to the present.

Lest the reader get the mistaken impression that all of our experimental work was limited to exploring reflections from absorbing media, one early contribution on the nonlinear susceptibility tensor of a transparent KDP crystal should be mentioned here. The temperature dependence of this tensor in the vicinity of the ferroelectric phase transition is described in “Temperature dependence of optical harmonic generation in KH2PO4 ferro-electrics” (J.P. van der Ziel and N. Bloembergen, Phys. Rev. 135, A 1662–A1669, 1964). Jan van der Ziel, the son of fellow Dutch immigrant A. van der Ziel, an expert in electronic noise processes, went on to a successful research career in quantum electronics. After several decades at the Bell Telephone Research Laboratories, he now holds a Distinguished Chair in Electrical Engineering at the University of Texas in Dallas.

The frequency dependence or dispersion of nonlinear susceptibilities is evident from the general theoretical expressions. Before the advent of dye lasers, which would permit continuous tuning through regions of resonant absorption, it was not so straightforward to demonstrate this nonlinear dispersion. Convincing experimental evidence is presented in this paper based on the use of lasers at several discrete wavelengths in the broad and strong absorption continuum of several semiconductors. In addition to the frequency dependence of the intensity of the reflected second harmonic, Richard Chang also investigated its dependence on the angle of incidence and the direction of polarization of the incident fundamental beam. The Brewster angle for second harmonic generation was demonstrated. These results may be found in a full-length paper, “Experimental verification of the laws for the reflected intensity of second -harmonic light” (R.K. Chang and N. Bloembergen, Phys. Rev. 144, 775–780, 1966), which is not reproduced in this volume.

In contrast to other papers in this section which describe definitive results which have stood up under the test of time, this note has a more tentative nature. It is included here because it anticipates the enormous developments of second harmonic and sum frequency generation as a tool for surface studies which have taken place during the past quarter century. Second harmonic generation from metallic surfaces had first been observed by F. Brown and coworkers (see Refs. 4 and 8 of this note) and had been interpreted as an effect characteristic of the conduction electron plasma. This paper demonstrates that comparable effects occur at surfaces of semiconductors and insulators. A follow-up investigation showed the influence of d.c. electric field discontinuities normal to the surface. (“Nonlinear electroreflectance in silicon and silver,” C.H. Lee, R.K. Chang and N. Bloembergen, Phys. Rev. Lett. 18, 167–170, 1967.) Dr. Sudanji S. Jah had made theoretical studies of related problems while working for his Ph.D. thesis with Professor Felix Bloch at Stanford University. He joined our group as a postdoctoral research fellow. A rather comprehensive discussion of these surface effects resulted in a paper, “Optical second harmonic generation in reflection from media with inversion symmetry” (N. Bloembergen, R.K. Chang, S.S. Jha and C.H. Lee, Phys. Rev. 174, 813–822, 1968). Dr. S.S. Jha returned to his native country, India, where he became a professor at the Tata Institute in Bombay. I enjoyed my visits with him there in 1972 and 1979.

The delicate questions of the treatment of the discontinuity in the normal component of the macroscopic electric field at the surface in terms of the rapid variations on an atomic scale of the microscopic electric field have received much further scrutiny and refinement. The relationship between interface effect and bulk quadrupole effect has also been elucidated. These issues have been reviewed by Y.R. Shen (Annual Reviews Phys. Chem. 40, 327, 1989), and by B. Koopmans, Ph.D. Thesis, University of Groningeu, 1993.

This paper summarizes all work on this subject up to May 1966. I was invited to review the field for a conference in Paris organized by the International Commission on Optics.

Here, the study of reflected second harmonic light is extended to the situation in which the fundamental and/or the second harmonic rays are totally reflected at the interface of the nonlinear crystal. It is shown that phase matching for total reflection is possible. A much more detailed presentation of both theory and experiment may be found in a follow-up publication, “Total reflection phenomena in second harmonic generation of light,” (N. Bloembergen, H.J. Simon and C.H. Lee, Phys. Rev. 181, 1261–1271, 1969). Another paper which extends the theory further is, “Phase-matched critical total reflection and the Goos-Haenchen shift in second-harmonic generation” (H. Shih and N. Bloembergen, Phys. Rev. 3A, 412–420, 1971). My graduate student Chi Lee went on to become a professor of electrical engineering at the University of Maryland, while John Simon became a professor of physics at the University of Toledo, Ohio. Hansen Shih is employed by a naval research station in Florida.

This note reports the first observation of reflected third harmonic light. It is, of course, a straightforward extension in the theoretical sense, but the high intensities required to observe this third order effect must be obtained from picosecond pulses because melting and damage of the surface would occur for longer pulses. Again, this note was followed by a full-length paper, “Third-harmonic generation in absorbing media of cubic or isotropic symmetry” (W.K. Burns and N. Bloembergen, Phys. Rev. 4B, 3437–3450, 1971).

M. Matsuoka was a postdoctoral fellow who returned to Japan to become a professor of physics, first at the University of Kyoto and later at the University of Tokyo. It was a pleasure to visit with him and his wife in both cities on several occasions. My graduate student Bill Burns later joined the Naval Research Laboratory near Washington, D.C.

We had found nonlinear extensions for most of the classical optical phenomena associated with the reflection and refraction of light. It remained to consider a nonlinear analogue for the unusual phenomenon of conical refraction which is discussed in the final chapter of Born and Wolf's standard textbook, Principles of Optics. This note introduces second harmonic conical refraction. A full-length theoretical paper, “Conical refraction in second-harmonic generation” was published subsequently (H. Shih and N. Bloembergen, Phys. Rev. 184, 895–904, 1969).

This note announces the observation of the effect predicted eight years earlier in the preceding paper. The subject matter was of purely academic interest and attracted no attention from the quantum electronics community. My graduate student Jane Schell had no competition and was able to complete a delicate investigation with care. The complete results were published as, “Laser studies of internal conical diffraction III, second harmonic conical refraction in a-iodic acid,” A.J. Schell and N. Bloembergen, Phys. Rev. A, 2592–2602, 1978. This paper, not reprinted here, is the third in a series. The first two parts report a quantitative determination of intensity patterns in conical refraction obtained with a diffraction limited helium-neon laser. To my knowledge, the phenomenon of second harmonic external conical refraction, mentioned in the preceeding note, has not been reported to date.

Jane Schell married Peter P. Sorokin, another former Ph.D. student of mine. She continued her scientific research for many years at the IBM Research Laboratories in Yorktown Heights, New York. Peter Sorokin did his doctoral research in my laboratory on a problem in nuclear magnetic resonance in the mid -fifties. He is the inventor of the dye laser and has made several other pioneering contributions to the field of quantum electronics. He is an IBM Watson Fellow. It was a pleasure to honor him at a celebration on the occasion of the 25th anniversary of the dye laser which was held a few years ago at the IBM research laboratories in Yorktown Heights.

The question of conservation of angular momentum in nonlinear optics had intrigued me in connection with the selection rules of harmonic generation by circularly polarized light in media with various types of axial symmetry. We had conducted a systematic experimental investigation of this problem in a single crystal of NaClO3 with 23 cubic symmetry, which was reported in “Second-harmonic light generation in crystals with natural optical activity” (H.J. Simon and N. Bloembergen, Phys. Rev. 171, 1104–1114, 1968).

It was natural to write a more general review of angular momentum conservation in optical processes for a volume in honor of Nobel laureate Alfred Kastler who had introduced the method of optical angular momentum pumping. Professor Kastler wrote me a nice thank-you note saying that he had enjoyed reading my contribution which had provided him with some additional insight. The paper is reprinted in this volume to make such insights available to other members of the quantum electronics community.

I received an invitation to participate in an international school of quantum chemistry held on Sanibel Island Florida in the winter of 1970–71. The director of the school, Dr. Per-Olov Lowdin, wanted to honor John C. Slater and John H. Van Vleck in the program. This paper is dedicated to the latter, a long-time mentor and colleague, from whom I learned much about magnetism and angular momentum. Van, who often called me “fellow Dutchman”, had a soft spot for the Netherlands in general and Dutch physics in particular. He was proud of his Dutch ancestry, which he could trace to some immigrants on the “Half Moon”, the second ship to sail up the Hudson River in the first decade of the seventeenth century.

The extension of the symmetry considerations for harmonic generation by circularly polarized light to the imaginary part of the nonlinear susceptibilities appeared to be an appropriate subject for my lecture at the above-mentioned school. Professor C.L. Tang from Cornell University spent a sabbatical leave at Harvard at that time. My graduate student Bill Burns was also a co-author of paper 2.8.

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Ron Shen and I had already addressed the connection between parametric and raman-type processes in paper 1.3. Terhune and coworkers were the first to observe antistokes radiation; they interpreted it as a parametric process at the combination frequency equal to twice the laser frequency minus the stokes frequency. This interpretation is basically correct, if two beams separated in frequency by the vibration frequency are incident on the sample. The antistokes light would normally suffer an exponential attenuation in a raman laser medium. This paper shows that the antistokes radiation can build up with an exponential gain in the form of a coupled mode having some antistokes character mixed with the predominant stokes character. The paper predicts the presence of a dark ring in the conical antistokes radiation pattern, which was strikingly confirmed more than a decade later in a carefully controlled study of the generation of antistokes light in hydrogen gas. (See M.D. Duncan, R. Mahon, J. Reintjes and L.L. Tankersly, Optics Letters 11, 803, 1986.) It is also described in an article which my former student John Reintjes contributed for a volume published in honor of my 70th birthday. (J. Reintjes, M.D. Duncan, R. Mahon and L.L. Tankersley, “Spatial effects in transient stimulated raman scattering,” in Resonances edited by M.D. Levenson, E. Mazur, P.S. Pershan and Y.R. Shen, World Scientific, Singapore, 1990, pp. 70–79.) The dust jacket of this book shows a color photograph of the antistokes ring pattern.

The theory of gain for coupled stokes–antistokes modes is more important for the case of stimulated Rayleigh wing scattering. It was used successfully to explain the intensity distribution for this type of near forward scattering by Chiao and coworkers (R.Y. Chiao, P.L. Kelley and E. Garmire, Phys. Rev. Lett. 17, 1158, 1966).

The ruby laser used to study stimulated Raman scattering accidentally had an output in two longitudinal modes with frequencies about one wave number apart which gave rise to a very strong modulation of the index of refraction in CS₂ at that frequency. This resulted in the creation of over eighty side bands as described in this paper.

Pierre Lallemand came from Kastler's laboratory in Paris where he had completed the troisième cycle at the Ecole Normale Supérieure. He would return there to obtain his doctorate after having completed the research for it at Harvard, as Jacques Ducuing had done before him. I have always treasured many durable scientific and social contacts with French colleagues and their families.

In 1965, many laboratories attempted to understand gain anomalies observed in stimulated raman scattering. There was fierce competition and several groups arrived independently at the conclusion that self-focusing was the cause for most of the so-called “anomalies”. This note describes our experimental demonstration of this fact. The references show that this subject was a “hot topic” at that time.

This note is a sequel to the preceding one. It shows how self-focusing may also determine the threshold for other stimulated scattering processes. Al Pine came to us from Berkeley as a postdoctoral fellow. After about a year, he accepted a position at MIT Lincoln Laboratories. He then moved to the University of Maryland, where his current interest is in biophysics.

The key word in the title of this note is the word “controlled”. It refers to the fact that self-focusing has carefully been avoided. There are no “anomalies”. Good agreement between the theoretical and observed Raman gain and line width is obtained as a function of hydrogen gas pressure.

Georges Bret had joined our group as a postdoc after having worked on related problems with Professor Guy Meyer at the University of Paris. He later started a laser company, first in France and then in California. We were saddened by his untimely death a few years ago.

Paraskeva Simova came to us from Bulgaria, on leave from a research laboratory of the Bulgarian Academy of Sciences. She and Pierre Lallemand communicated in a language that nobody else could understand. The usual means of communication between foreign scientists is broken English; their English, however, was further pulverized by Pierre's halting pronunciation with a heavy French accent and Simova's Russian-Bulgarian approximation of English scientific terms. We met Simova again after many years at a conference in Sofia, Bulgaria, in 1983, where she works in a laboratory of the Bulgarian Academy of Sciences.

This effect, which was first suggested by Soviet physicists, is not of practical significance, but its experimental study turned out to be a good Ph.D. thesis topic for my graduate student W.H. Lowdermilk. There was no significant competition and he was the first to obtain convincing experimental evidence for the effect. Lowdermilk went on to a very successful career in the glass laser group at Lawrence Livermore Radiation Laboratory.

This short announcement was followed by a full-length paper, “Stimulated concentration scattering in the binary GaS mixtures Xe-He and SF₆-He” (W.H. Lowdermilk and N. Bloembergen, Phys. Rev. A5, 1423–1443, 1972). The detailed theory of the effect was presented in another paper, “Theory of stimulated concentration scattering” (N. Bloembergen, W.H. Lowdermilk, M. Matsuoka and C.S. Wang, Phys. Rev. A3, 404–412, 1971). Matsuoka and Wang were postdoctoral research fellows. In another note, we investigated the influence of concentration fluctuations on Brillouin and Rayleigh scattering, “Coupling between Rayleigh and Brillouin scattering in a disparate-mass gas mixture” (W.S. Gornall, C.S. Wang, C.C. Yang and N. Bloembergen, Phys. Rev. Lett. 26, 10094–10097, 1971). Bill Gornall was a postdoc who came to us from Canada. C.C. Yang was a graduate student who did Ph.D. thesis work on light scattering in a critical fluid mixture. All three coworkers later had successful careers in industrial research settings, although their activity at Harvard concerned a topic of purely academic interest. I believe that our papers pretty much preempted the field of stimulated concentration scattering and I am not aware of any subsequent work on this topic.

In the late sixties I served on a laser advisory committee for the United Aircraft Research Laboratories which had a very active research program on lasers at that time. I paid regular visits to East Hartford and became very well acquainted with Dr. A.J. DeMaria and many other scientists there. Tony DeMaria had pioneered the generation of picosecond pulses with mode-locked Nd-glass lasers. I wanted to get a picosecond laser facility at Harvard. A graduate student, R.L. Carman, enjoyed building a large installation which was more elaborate than our customary small-scale experiments. Fujio Shimizu had joined us for additional postdoctoral experience after having spent a couple of years with Boris Stoicheff at the University of Toronto. His quiet demeanor failed to hide his great competence in both theoretical and experimental physics. He later became a professor of physics at the University of Tokyo where I visited his laboratory several times. My last visit occurred in 1992. Dr. M.E. Mack was a very active and enthusiastic researcher who helped to transfer this technology from industry to the university.

The first problem we addressed with our picosecond laser was that of transient stimulated raman scattering. Our early results in liquids are reported here. A little later, additional results obtained in gases were reported in another short communication, “Transient stimulated rotation and vibration raman scattering in gases” (M.E. Mack, R.L. Carman, J. Reintjes and N. Bloembergen, App. Phys. Lett. 16, 209–211, 1970). At the same time, we had started a thorough theoretical study of the transient scattering problem. I had attracted Chen-sow Wang as a postdoctoral fellow. He had already studied these problems as a Ph.D. candidate with Professor Keith Bruekner at the University of California in La Jolla. The theoretical results are published in “Theory of stokes pulse shapes in transient stimulated raman scattering,” R.L. Carman, F. Shimizu, C.S. Wang and N. Bloembergen, Phys. Rev. 2A, 60–72, 1970.

This brief theoretical note was prepared for a special issue of an Indian journal dedicated to the memory of C.V. Raman, who died in 1970. John Colles joined our group for a rather brief period between his stay at the Bell Telephone Laboratories and his return to the United Kingdom. John Reintjes was a graduate student who wrote his Ph.D. thesis on “Transient stimulated raman scattering and self-focusing in the picosecond time regime.” He joined the Naval Research Laboratories in Washington DC where he continued to work on pulsed stimulated raman scattering and other problems in nonlinear optics. He is the author of a book, Nonlinear Optical Parametric Processes in Liquids and Gases published in 1984 by Academic Press.

In September 1971, I presented a review paper, “Picosecond nonlinear optics” at a memorable conference held in Esfahan, Iran, organized by Ali Javan. The paper summarizes early developments and uses of picosecond pulse techniques (N. Bloembergen in Fundamentals and Applied Laser Physics, edited by M.S. Feld, A. Javan and N.A. Kurnit, Wiley-Interscience, New York, 1973).

Dye lasers presented another technique which obviously would be very valuable for nonlinear optics research. In 1971, we had started building our own dye lasers, pumped either by a frequency doubled Nd-Glass laser or a nitrogen gas discharge laser. This paper describes the first use of the dye lasers to condensed matter spectroscopy. My work in the Division of Applied Sciences at Harvard University had always been focused primarily on solid state physics problems, while colleagues in the physics and chemistry departments concentrated their efforts on atomic and molecular spectroscopy.

Marc Levenson had obtained his Ph.D. degree at Stanford University working with Arthur L. Schawlow. He is a highly motivated, enthusiastic worker with restless energy. After leaving Harvard, he became a professor of physics at the University of Southern California; he then joined the IBM research laboratories in San Jose and Almaden. More recently, he carried out research at a small private company and at the University of Colorado. With S.S. Kano, he is the author of a well-known textbook, Nonlinear Laser Spectroscopy, Academic Press, San Diego, 1988. I am pleased that this book is based on work he started at Harvard.

Chris Flytzanis joined us after having obtained his Ph.D. degree in Paris. He wrote some extensive theoretical reviews on the calculation of nonlinear susceptibilities. I was a co-author on one of these, “Infrared dispersion of third-order susceptibilities in dielectrics,” N. Bloembergen and C. Flytzanis, Progress in Quantum Electronics, Vol. 4, Part 3, edited by J.H. Sanders and S. Stenholm, Pergamon, New York, 1976, pp. 271–300. Flytzanis returned to the Ecole Polytechnique near Paris, where he succeeded Jacques Ducuing as the director of the group on nonlinear optics.

We developed the techniques for dispersion and polarization spectroscopy in condensed matter, further described in the following references, “Dispersion of nonlinear optical susceptibility tensor in centrosymmetric media,” N. Bloembergen and M.D. Levenson, Phys. Rev. B10, 4446–4463, 1974, and “Dispersion of nonlinear optical susceptibilities of organic liquids and solutions,” N. Bloembergen and M.D. Levenson, J. Chem. Phys. 60, 1323–1327, 1974.

This note announces the first examples of dispersion in a multidimensional frequency space. Independently tunable dye lasers are used with two or three light beams incident on the sample. The intensity generated at the frequency ω₄ − ω₁ + ω₂ − ω₃ shows resonances when ω₁ − ω₃ and ω₂ − ω₃ are resonant with two different raman vibrations, or when ω₁ + ω₂ is near a two-photon transition and ω₁ − ω₃ near a vibrational resonance. The latter situation occurs in CuCl crystal, where two collective excitations are simultaneously enhanced, the Z₃ excitation at the sum frequency ω₁ + ω₂ in the near ultraviolet, and the resonance at w₁ − w₃ of the phonon–polariton in the infrared. Two full-length papers on this subject were published soon afterwards, “Third-order nonlinear optical spectroscopy in CuCl,” S.D. Kramer and N. Bloembergen, Phys. Rev. B14, 4654–4669, 1976, and “Interference between raman resonances in four-wave difference mixing,” Haim Lotem, R.T. Lynch, Jr. and N. Bloembergen, Phys. Rev. B14, 1748–1754, 1976.

Papers 3.9, 3.10, and 3.11 describe early examples of the general features of third-order nonlinear spectroscopy with three incident light beams with independent frequencies, directions of propagation and polarization vectors. At the same time that these developments in condensed matter spectroscopy took place, analogous methods evolved elsewhere in atomic spectroscopy.

S.D. Kramer and R.T. Lynch, Jr. were both graduate students, while Haim Lotem was a postdoctoral research fellow from Israel. After obtaining his Ph.D. degree, Steve Kramer joined the Oak Ridge National Laboratory and Bob Lynch joined the IBM development laboratory near San Jose. Steve Kramer now works at the Institute for Defense Analyses near Washington, D.C.

Haim Lotem returned to Israel, where he joined the Nuclear Research Facility near Beersheba. Soon after the Lotem family arrived in the United States, their son Zohar was born. Since sadly no one of their immediate family from Israel could be present, I was asked to serve as godparent at the circumcision ceremony in their Brooklyn apartment. I reminded Haim that I was not Jewish but he replied that it did not matter. Just before the ceremony, the rabbi, or mohel, had a talk with me about how I knew the Lotems, etc., and soon asked about the origin of my family name. When he found out the Protestant Christian background of my family, he disqualified me from entering the room where the circumcision would take place, much to the consternation of the anxious parents. A qualified replacement was immediately found in the person of my graduate student, Eli Yablonovitch. He had been born in Poland and raised in Canada where he obtained a Bachelor of Science degree at McGill University in Montreal. He was a confirmed bachelor at the time and he stated that the last circumcision ceremony he had attended was his own. He carried out this extracurricular duty with good humor. We have kept in close touch with the Lotem family. They returned to the United States several times on sabbatical leaves.

Eli Yablonovitch and I attended a conference in Israel a few years later. On the El Al overnight flight to Israel, I was woken up by a group of men who were involved in an agitated discussion: they needed a tenth person to form a minion. I informed them that they had woken up a disqualified person and pointed out Eli Yablonovitch to them, who again performed the task without fail.

The terms “three-wave” and “double two-wave mixing” occurring in the title are no longer used. “Three-wave mixing” refers to a four-wave mixing process, based on a third-order linearity, χ⁽³⁾, with one incident frequency used twice. “Double two-wave mixing” refers to a cascade process in which second-order nonlinearity χ⁽²⁾ is used in two separate stages.

Already in the late sixties, a CO₂ laser facility had been built in our laboratory. It was used by my graduate student Jim Wynne to measure χ⁽²⁾ in several III-V semiconductors by second harmonic generation in the far infrared (N. Bloembergen and J.J. Wynne, Phys. Rev. 188, 1211–1220, 1969). Jim Wynne joined the IBM research laboratories, first in Zurich and then in Yorktown Heights. He was a very good squash player and was a member of the Harvard Squash team; he also started two teams for graduate students. I played on the second team as a substitute a few times. The teacher-student relationship in squash was the reverse of that in physics, but both were amicable. Jim Wynne was coauthor on another paper which describes infrared dispersion of the nonlinear response due to cyclotron resonance between Landau levels in n-InSb, (E. Yablonovitch, N. Bloembergen and J.J. Wynne, Phys. Rev. B3, 2060–2062, 1971.) Unfortunately, only one note on nonlinear spectroscopy with CO2 lasers could be included in this volume. It reports for the first time an interference between a third-order and a cascaded lower-order light scattering process. A similar interference was observed in an experiment carried out with a ruby and a dye laser in CuCl (“Interference of third-order light mixing and secondharmonic exciton generation in CuCl,” S.D. Kramer, F.G. Parsons and N. Bloembergen, Phys. Rev. B9, 1853–1856, 1974).

In the mid-seventies there was considerable confusion and controversy about the correct treatment of damping terms in CARS spectroscopy. I maintained that the general density matrix approach described in papers 1.3 and 1.4 should yield unambiguous results. Rob Lynch systematically kept track of all the terms in the calculation of χ⁽²⁾ and χ⁽³⁾, and rearranged them in such a manner that terms arising only in the presence of damping are clearly separated. In the absence of damping, only resonances between the ground state and an excited level occur in the denominators. Damping introduces extra resonances between pairs of excited states. Their numerators vanish if the only damping is the natural width caused by spontaneous emission decay to the ground state. For collisional or other types of damping, their numerators are still small, but non-vanishing. I decided to publish the elaborate algebraic results as an appendix to a paper on line shapes of resonant raman scattering, which was our contribution to a special issue of an Indian journal commemorating the 50th anniversary, or golden jubilee, of the discovery by Raman of the effect named after him.

Our results agreed, except for a few minor algebraic errors, with the independent calculations by J.P. Taran and M. Denariez at ONERA near Paris, the French institute for studies of the upper atmosphere. At scientific meetings, we had a difficult time convincing chemists who carried out the bulk of resonant raman experiments of the correctness of our approach. Taran summarized the situation at the end of one international conference to me in the following words, “Perhaps we won this battle, but we have lost the war. ” This reinforced my resolve to demonstrate the physical reality of the “extra” terms explicitly exhibited in this note.

This note contains the first observation of a resonance of a χ⁽³⁾ response of sodium vapor, when two incident frequencies have a difference corresponding to the fine structure separation in the 3P doublet. The intensity of this resonance increased with increasing pressure of the noble buffer gas. This so-called “collision-induced coherence” signal was initially met by surprise and disbelief from many colleagues. I used the following qualitative reasoning to convince them: There are many pathways, or Feynman diagrams, contributing to a χ⁽³⁾ process. Sometimes three pathway combinations interfere destructively and yield a zero sum. The introduction of damping destroys the phase coherence which led to the destructive interference. Later papers showed explicitly that in the presence of collisional damping, right- and left-handed Feynman diagrams should be considered for the evolution of a state and its conjugate.

My co-authors on this paper were two postdoctoral fellows and a graduate student. Yehaim Prior had obtained his Ph.D. degree at the University of California, Berkeley, working on a magnetic resonance problem with Alex Pines in the chemistry department. He wanted to do some optics work before returning to Israel, and correctly thought that I could judge his potential from his work in magnetic resonance. Prior later became a professor at the Weizmann Institute in Rehovoth, Israel. Our families have kept in touch. Mario Dagenais, a French Canadian, obtained his Ph.D. degree at the University of Rochester, where he worked on photon anti-bunching with Leon Mandel. He became a professor of electrical engineering at the University of Maryland in College Park. Again, we have kept in touch with him and his French wife, who is also a scientist. Andy Bogdan obtained his Ph.D. degree for his role in these pioneering experiments. He has since worked for various industrial research organizations.

This note is an extension of the preceding one and deals with a zero-frequency resonance, distinguishable by the four different directions of the four light waves involved. Both theoretical and experimental investigations of collisional induced coherences in sodium vapor as a function of inert buffer gas pressure were quickly refined and reported in two subsequent short communications, “Quantitative characteristics of pressure induced four-wave mixing signals observed with c.w. laser beams,” N. Bloembergen, A.R. Bogdan and M.C. Downer, Phys. Rev. A24, 623–626, 1981, and “Quantitative characteristics of pressure-induced degenerate frequency resonance in four-wave mixing with c.w. laser beams,” N. Bloembergen, A.R. Bogdan and M.C. Downer, Optics Letters 6, 348–350, 1981.

Two new associates continued the experimental investigations in sodium vapor with variable inert buffer gas pressure, concentrating on the low frequency resonance corresponding to a raman-type resonance between two Zeeman-sublevels of the electronic ground state and the polarization characteristics of a raman-type transition with vanishing Stokes shift between two degenerate Zeeman levels. The latter effect may be considered as a collision-induced Hanle-effect resonance and is the subject of this communication. Previously, two detailed papers on collision-induced resonances between non-degenerate hyperfine and Zeeman sublevels had been published, “High-resolution studies of collision-induced population grating resonances in optical four-wave mixing in sodium vapor,” (N. Bloembergen and L.J. Rothberg, Phys. Rev. A30, 2327–2338, 1984) and “High resolution four-wave light-mixing studies of collision-induced coherence in Na vapor,” (N. Bloembergen and L.J. Rothberg, Phys. Rev. A30, 820–830, 1984). Lewis Rotheberg obtained his Ph.D. degree from Harvard University on the basis of this work. He joined the AT&T Bell Telephone Laboratories, where he works on femtosecond biophysical processes.

Ying-hua Zou, a professor of physics at Peking University in Beijing, China, joined our group as a visiting scholar under an exchange program between the American Physical Society and the Chinese Ministry of Education. He continued the detailed study of the collision-induced Hanle resonances. They were written up in a full-length paper, “Collision-enhanced four-wave light mixing in Na vapor,” (N. Bloembergen and Y.H. Zou, Phys. Rev. A33, 1730–1742, 1986). We visited Y.H. Zou and his wife twice in our travels to China in 1989 and 1994, respectively. I visited his laboratory at the university, and we were also guests in his modest apartment, where the hospitality was inversely proportional to the physical space. In 1994, Professor Zou was our host companion on a five day trip up and down the gorges of the Yang-tse river. He was almost as surprised as we were by the contrast in living conditions between the burgeoning metropolis Beijing and the towns in interior China.

This paper was presented at a conference in Paris dedicated to the memory of Alfred Kastler. It took place during a cold, snowy week in January 1985. Since Kastler himself had recognized the importance of Hanle's work for his development of optical polarization pumping, I felt it appropriate to review a novel variant on the Hanle effect. I consider this to be my last significant contribution to original physics research. In my talk, I sketched how we had arrived at the collision induced Hanle coherences as a degenerate case of resonant raman scattering. In the Hanle limit of zero magnetic field, it would, however, also be understood as an induced polarization grating of the Na atoms, with a life time limited by diffusion through the buffer gas. I had thus arrived, in a round-about way, at optically pumped polarization in Kastler's original language. Thus I felt like Mr. Jourdan in Molière's play “Le Bourgeois Gentilhomme”, who learned from his language instructor that he had been speaking in prose all his life.

The polarization gratings in alkali vapors have become of great practical significance in the cooling of atoms by laser beams. Kastler's successor, Claude Cohen-Tannoudji, and his associates, played a major role in the startling new developments of the past decade which have recently culminated in the observation of Bose–Einstein condensation of atoms in momentum space.

A few years after this conference, in 1987, I had the pleasure of meeting Professor Hanle himself, then almost ninety years of age. He was still keenly interested in the novel variations on his theme. I was glad to give him some of our reprints. The occasion was a dinner at the home of his daughter, who is married to Professor Scharfmann at the University of Giessen, Germany, where I spoke at a physics colloquium. I also knew Hanle's son, Helmut, very well, as he has an executive function in the Alexander von Humboldt foundation. I was a senior Humboldt fellow twice, in 1980 and in 1987, respectively. Both times, my principal host was Herbert Walther, one of the directors of the Max Planck Institute for Quantum Optics in Garching. I have always treasured my regular visits and interactions with many colleagues at the leading institutions on laser spectroscopy in the European continent, with centers of gravity in Munich and Paris, respectively.

This is a major review of the entire field indicated in the title as of the summer of 1969. It includes the stimulated scattering of light by acoustic phonons, magnons, plasmons, and other processes not previously discussed in this volume. It attempts to present a balanced picture of the work of numerous research groups which all made major original contributions. Although such conference proceeding often have a very short useful life, this review is reprinted here as a summary of the historic development of nonlinear optics in condensed matter during its infancy and youth between the years 1961 and 1969.

The timing of the Scottish Summer School coincided with the first landing of two men from the Apollo spaceship on the moon. Most participants at the school stayed up late to watch this unique technological achievement on television.

This review deals with the first five or six years of the stimulated raman effect. I have received more requests for reprints (over one thousand) for this paper than for any other. In an editorial, “Sixty years of the American Journal of Physics (Am. J. Phys. 61, 103, 1993), R.H. Romer states that this paper ranked third in frequency of citation for his journal in the Science Citation Index.

The paper was written by invitation of the editor of the journal, Forrest I. Boley, in 1966. I even received a check for five hundred dollars from him, but shortly thereafter he wrote me a letter suggesting that I withdraw the paper on the basis of criticism he had received. He did not request restitution of the money. I replied that he should send me the critical comments to which he was referring. They were insignificant. I made a few minor corrections in response and insisted on publication. I have not been able to trace the source of this curious incident. Forrest Boley wrote me last year that he does not remember anything unusual. At any rate, the only critical comment I have received since its publication is that the paper did not mention that a US patent for raman lasers was issued to Gisela Eckart. I had given adequate credit to the contributions of the Hughes Research group as a whole, but I did not know about the individual credits within this group.

It is hoped that the paper can still serve as an introduction for new workers entering this ever-active field and that it is not only of historic but also of educational significance.

This paper was prepared for a plenary lecture I was invited to present at the International Conference on Raman Spectroscopy, ICORS X, held in Eugene, Oregon in September 1986. It gives a cursory overview of some of the major developments from 1961 to 1986. It was used to start the first issue of a new journal, International Journal of Modern Physics B, devoted to atomic, molecular, and condensed matter physics.

When I was invited to give a plenary lecture at another ICORS meeting, the fourteenth in the series, held in August 1994 in Hong Kong, I decided to present a review of transient time-resolved raman-type phenomena. My own activity in this area is limited to early work, around 1970, with picosecond pulses. The various developments in time-resolved spectroscopy, especially with femtosecond pulse techniques, are fascinating. Only a few highlights could be mentioned in this brief overview.

An important correction in the middle of the third page of this reprint should be made. The phrase “as caused by the vibrational anharmonicity as the various vibrational eigenstates evolve with slightly different periods” should be replaced by “as caused by the evolution of the rotational wave packet excited by the femtosecond pulse.”

Please refer to full text.

In the summer of 1973, Marc Levenson returned from a conference in Europe, where he had heard about a prediction made several years earlier by V. Chebotayev and associates (see Ref. 1 of this note). He asked me if I believed that this effect of Doppler-free two-photon transitions would exist. I immediately said yes, even before I went to the library to read Chebotayev's paper. Marc and I were both surprised that the effect had not yet been demonstrated experimentally by research groups in atomic physics. It must be remembered, however, that dye lasers in those years were still in a rather primitive stage of development and could not yet be purchased commercially. Marc felt that he could rather easily do the experiment with his dye lasers, which we were using for solid state spectroscopy. I encouraged him to go ahead. The main obstacle for us was to obtain a glass cell with some sodium vapor. Marc got some glass blowing help from the Physics Department. He was able to demonstrate the effect independently and at about the same time that a French team and a group at MIT succeeded. The three papers were published in the same issue of Physical Review Letters.

The simple symmetry of an S–S transition should lead to very simple Zeeman patterns in the presence of a magnetic field. My familiarity with hyperfine splitting and with the Breit–Rabi formula from my magnetic resonance years could be applied directly to this optical transition. We had a suitable magnet available, dating from the magnetic resonance activities. With a little prodding, Marc Levenson and a new graduate student, Mike Salour, obtained the Zeeman patterns which were independent of the angle between the magnetic field and the wave or polarization vectors. This is a unique situation caused by the vanishing orbital angular momentum in both the initial and final state.

Mike Salour continued his Ph.D. research on the use of Ramsey split pulse geometries in optics. He wrote a joint paper with Professor Claude Cohen-Tannoudji in Paris. Before he started his graduate studies, he was already an accomplished professional pilot licensed to fly wide-body jets and to serve as a flight instructor. I asked him why he wanted a physics Ph.D. degree, as he could earn more without it. He clearly wanted to show that he could also be a good scientist. He served as assistant and associate professor in the Electrical Engineering department at MIT. Then he founded a successful opto-electronics corporation. He is the president and chief executive officer of Tacan corporation in Carlsbad, California. He is still a fully licensed pilot. He gave me my first and last flying lesson in a Cessna with dual controls when I visited his headquarters a few years ago. We flew from Carlsbad to La Jolla and back. Mike Salour has shown that he is also a very successful business executive. He told me that the most important thing he learned from me at Harvard was “insistence on quality and excellence.”

These brief forays into atomic spectroscopy were not pursued further in my laboratory, but Marc Levenson and I completed a full-length review on “Doppler-free two-photon spectroscopy,” published in High Resolution Laser Spectroscopy, edited by K. Shimoda, Sprinter-Verlag, Berlin, 1976, pp. 315–369.

This note announces the beginning of a systematic investigation of the intensities and the polarization dependence of two-photon spectral lines within the 4fⁿ configurations of rare-earth ions in crystals. The problem is reviewed in Paper 4.6 where more complete references to the pertinent literature and to the historic context are given.

Mario Dagenais and Richard Neumann were postdoctoral research fellows. Dagenais became more involved with experiments on collision-induced coherence described in the preceding section. Neumann was a postdoctoral visitor for one year. He came to us from the University of Heidelberg and returned to Germany where he obtained an appointment at the University of Karlsruhe. My graduate student Mike Downer really pursued the subject in depth. In my opinion, he could have obtained two Ph.D. degrees, one for the theoretical part and one for the experimental part of his heavy-weight Ph.D. thesis. Since I did not feel competent to judge the theoretical part, I invited Professor B.R. Judd from Johns Hopkins University to serve on the Ph.D. examining committee. His name appears in the title of this note. Professor John H. van Vleck would have been interested in the subject had he lived a few years longer.

Mike Downer became a professor of physics at the University of Texas in Austin where he leads a group in femtosecond nonlinear optics. We have kept in touch over the years.

This paper discusses some of the unexpected anomalies that appeared in the quantitative systematic investigation of two-photon absorption spectra of rare-earth ions. A comprehensive paper on the same subject was published later (M.C. Downer and A. Bivas, Phys Rev. B28, 3677, 1983). A. Bivas joined our group as a postdoctoral fellow from the University of Strassbourg, France. He moved on to an industrial research organization in California.

Downer's work was continued for another year or two after his departure. Professor Rana was on sabbatical leave from the College of the Holy Cross, where he had carried out extensive work on the one-photon absorption and fluorescence of rare-earth ions. The ¹S₀ level, referred to in the title, could not be reached in a one-photon transition. Cesar Cordero was a Ph.D. from the Caycey University in Puerto Rico. He was also the principal author of a companion paper submitted at the same time, “Two-photon transition from ³H₄ to ¹S₀ of Pr³⁺ in LaCl₃,” Phys. Rev. B30, 438–440, 1984. He returned to Puerto Rico where he became a dean of science at the university in Mayaguez.

This note was the subject of an invited paper presented at an International Conference on Luminescence, held in Madison, Wisconsin, in the summer of 1984. It reviews the work of the Harvard group and puts it into a larger historical context.

Please refer to full text.

While solid state masers proved their utility as extremely low noise microwave receivers, I wondered how infrared radiation might be detected in the quantum limit. This led to a variant of a multilevel pumping scheme which could be realizable for energy level systems of transition metal ions. The principle of this proposal is correct, but it never led to a practical device. Helium-cooled infrared semiconductor diode arrays are more practical, especially for broad-band infrared radiation. Parametric up-conversion from the infrared to the visible became another possible scheme after powerful lasers became available. I was told that a group at the Bell Telephone Laboratories snidely dubbed my device “SNIRD”, which stands for Supposedly Noiseless Infrared Detector. At any rate, the note attracted some attention and also stimulated subsequent research at laboratories concerned with night-vision instruments. It provides an early example of optical cascade processes.

My interest was aroused when I heard a presentation by Dr. C.P. Robinson of the Los Alamos National Laboratories on this subject at the Third Conference of the Laser, sponsored by the New York Academy of Sciences in April 1975. The interest of the Los Alamos group had in turn been fueled by Ambartsumian et al.'s pioneering paper announcing that this process could be isotope selective. (R.V. Ambarsumian, V.S. Letokhov, E.A. Ryabov and N.B. Chekalin, J.E.TP. Lett. 20, 273, 1974.) Clearly, this could not be a simple heating effect, but it was equally clear that it was not a nonlinear process involving thirty or more infrared photons simultaneously, as the yield of dissociation products was not proportional to a very high power of the intensity.

This early note distinguishes two stages, which turned out to be basic in further developments. An anharmonic vibrational mode is excited by a multiphoton transition, involving a relatively small number of photons. This step is isotope selective. Subsequently, the molecule is “heated” in the quasi-continuous vibrational phase space by a cascade of one-photon absorption and emission process. This leads to a unimolecular dissociation reaction which preserves isotopic selectivity.

I had been scheduled to give an invited talk, presenting a rather routine review of nonlinear spectroscopy, at a conference on laser spectroscopy held in Les Houches, France, in June 1975. The text for that talk appeared in the proceedings, Laser Spectroscopy, edited by S. Haroche, J.C. Pebay-Peyroula, T.W. Hansch and S. E. Harris, Springer Verlag, Berlin, 1975, pp. 31–38. That talk was, however, never presented. I volunteered to speak instead about an exciting new development: infrared multiphoton dissociation. The session chair and the audience readily approved of this change. My presentation, which is described in this note, infuriated V.S. Letokhov, the young leader of the group who had discovered the isotope selective process. He came to me after my lecture and told me that he had these same ideas already years ago. When I asked him where he had published them, he said that they were “too simple, obvious, and unimportant,” but he would show me his laboratory notes. I told him I could not read Russian and he should not get so excited if the problem was really so unimportant. Rem Khoklov, the leader of the Soviet delegation to the conference, had witnessed this rather heated exchange. He told Letokhov to make amends and the latter offered a formal apology after lunch. For several years, our groups remained fiercely competitive, but from about 1980 on, Letokhov and I have been on good collegial speaking terms. Letokhov's group has probably done the largest amount of work on the subject. He and his associates have written several books about it, including V.S. Letokhov's Nonlinear Laser Chemistry, Springer Verlag, Heidelberg, 1983, and Laser Spectroscopy of Highly Vibrationally Excited Molecules, edited by V.S. Letokhov, Adam Hilger, London, 1989.

In 1975, David Larson of the MIT Lincoln laboratory, was encouraged by our mutual colleague, Paul Kelley, to work on the quantum theory of the anharmonic oscillator transition. We published a note which is not reproduced here, “Excitation of polyatomic molecules by radiation,” D.M. Larsen and N. Bloembergen, Optics Comm. 17, 254–258, 1976.

I had close contact with the important experimental group at Los Alamos headed by C.P. Robinson. I spent several summers there as a consultant. I got aquainted with Cy Cantrell, who was a theoretician in the group. Our next theoretical communication was a presentation by David Larson, Cy Cantrell, and myself at a memorable conference held in Loen, Norway, in June 1976, preceding the larger International Quantum Electronics Conference, held in Amsterdam, Netherlands. At both conferences, many research groups presented papers on multiphoton dissociation processes as it had become a “hot topic”. The interested reader is referred to the conference proceedings Tunable Lasers and Applications, edited by A. Mooradian, T. Jaeger and P. Stokseth, Springer-Verlag, Berlin, 1976.

This note reports quantitative results on the intensity and fluence dependence of the collisionless dissociation of SF₆, which led to a simple description of the excitation in the vibrational quasicontinuum. It was later correctly criticized by others as somewhat oversimplified. Figure 1 in this note, however, illustrates the essence of the physical processes involved. This figure and its variants were widely adopted by most other workers in the field. I remember a conference in Edinburgh, Scotland, in September 1977, where several speakers showed the familiar picture with the words, “I borrowed this diagram from Bloembergen.” When it was my turn to speak in that session, I quipped, “… and I borrowed this slide from myself.”

Further details may be found in the proceedings, Laser-Induced Processes in Molecules, edited by K.L. Kompe and S.D. Smith, Springer-Verlag, Heidelberg, 1979. We followed up on this letter with a full-length publication, “Collisionless multiphoton energy deposition and dissociation of SF₆” (J.B. Black, P. Kolodner, M.J. Shultz, E. Yablonovitch and N. Bloembergen, Phys. Rev. A19, 704–716, 1979).

The experimental program on molecular dissociation by pulsed CO₂ lasers at Harvard was started by Professor Eli Yablonovitch, one of my former graduate students. His development of a system to produce powerful CO₂-laser pulses in the picosecond regime, of much shorter time duration than those available from simple transversely excited CO₂ lasers, played a major role in further studies of collisionless dissociation. Black and Kolodner were his graduate students. Mary Shultz held a Radcliffe postdoctoral fellowship. She later became a professor of chemistry at Tufts University. S. Mukamel was a postdoctoral fellow at the Massachusetts Institute of Technology. He is now a professor of chemistry at the University of Rochester, and is the author of a recent text, Principles of Nonlinear Optical Spectroscopy, Oxford University Press, 1995.

This broad review was the subject of an invited paper at the 31st International Meeting of the Société de Chimie Physique, held in the recently restored historic setting of the Abbaye de Fontevreau, France, in September 1978. An overview is presented of the large variety of multiphoton processes, including non-resonant parametric interactions, cascades of one- or two-photon transitions, or true multiphoton transitions between sharply defined energy states or energy continua.

A few years later, there was another conference dedicated to multiphoton processes in Western France. The location was Benodet in Brittany. I remember joining in a tennis game there with Yuan T. Lee, who was the first to study infrared multiphoton dissociation with molecular beams which rigorously excludes any collisional mechanism.

This note provides another test of the nearly statistical distribution of energy in SF₆ with a high total vibrational energy. E. Mazur joined our group as a postdoctoral research fellow after having obtained his Ph.D. degree at the University of Leiden in a study of molecular transport phenomena. He soon became a member of the Harvard faculty and is now Gordon McKay professor of applied physics. Itamar Burak was a regular visitor to our group for many years. He obtained leaves of absence from Tel Aviv University.

It was already clear in 1977 that the situation in molecules smaller and lighter than SF₆, could not be described by a simple thermodynamic model that worked reasonably well for very large and heavy molecules. This paper presents an overview of our work on smaller molecules with detailed references to investigations carried out between 1977 and 1983. Tom Simpson received his Ph.D. degree in that year as one of my last graduate students. He is a research scientist at Jaycor in the San Diego area.

Much work in the Ph.D. theses of T.B. Simpson and J.G. Black had never been published. Eli Yablonovitch and Itamar Burak had also left Harvard. I decided that this original unpublished work should be tied together in a comprehensive paper. The referee and editors of the Journal of Chemical Physics first suggested that it had the character of a review and should be published elsewhere. I pointed out that it was not an overview, that it was limited to hitherto unpublished original work carried out at Harvard University. A second referee supported our statement. The paper provides a broad picture of the influence of molecular weight on the multiphoton dissociation process. By this time, the subject had, however, lost much of its urgency and glamour.

In the spring of 1984, I was a Fairchild visiting scholar at the California Institute of Technology in Pasadena. My main host was Professor Amnon Yariv in the Applied Physics Laboratory, but I also had a fruitful interaction with Ahmed Zewail in the chemistry department. I knew him well from a series of international conferences, included one he organized in Alexandria, Egypt. We reviewed the holy grail of mode-selective chemistry. With the emerging technique of femto-second laser pulses, the rate of energy redistribution could be followed more precisely. Ahmed Zewail became one of the founders of the new field of endeavor called femtochemistry.

Nonlinear photo-emissive processes had been observed and studied by many different groups in earlier years, as discussed in the first nine references of this note. Our work concentrated on obtaining quantitative reproducible results by the use of temporally and spatially controlled picosecond pulses. In this note, four-photon photo-emissive process is unambiguously identified. Jim Bechtel joined our group as a postdoctoral research fellow after obtaining his Ph.D. degree at the University of Michigan with Peter Franken. He now works for Tacan Corporation, whose CEO is my former student M.M. Salour. Lee Smith was one of my graduate students. After obtaining his Ph.D. degree from Harvard, he joined Lawrence Livermore Laboratory. Now he works for an optoelectrics company in the San Francisco Bay area.

This is a continuation of the work started in the preceding note. It was possible to separate volume and surface contributions of a multiphoton photoelectric emission process.

Pao-lo Liu was a graduate student who is now a professor of electrical engineering at the State University of New York in Buffalo. Richard Yen was a graduate student who really preferred to go into business. With my encouragement, he finished his Ph.D. degree to comply with the wishes of his parents. He was a competent scientist, but he chose to become a successful entrepreneur in electronics technology.

This is our last contribution to the quantitative study of high-order nonlinear photoemission. Precise measurements reveal that thermally enhanced three-photon emission overtakes four-photon emission in a narrow fluence interval below the fluence threshold at which picosecond pulse induced melting of the surface layer occurs. The subject of nonlinear photoelectric emission from solid interfaces has not received much attention in subsequent years.

This paper was presented as a plenary address at the Sixth International Conference on Multiphoton Processes held in Quebec, P.Q., Canada, in June 1993. It recalls the situation at earlier conferences in this series when multiphoton molecular dissociation and multiphoton photo-electric emission held centerstage.

Please refer to full text.

This note demonstrates that the considerations governing electric breakdown in d.c. electric fields can be extended through the microwave region to the far infrared, and perhaps even to visible frequencies. Avalanche ionization must start from a free electron which could be supplied by thermal ionization from a shallow impurity level or by multiphoton excitation. This breakdown mechanism is certainly correct for radiation at 10.6μm wavelength. Very recent studies in alkali halide crystals have shown that heating by multiphoton absorption processes may be more important at visible and shorter wavelengths. Avalanche breakdown studies were reported in detail in E. Yablonovitch's Ph.D. thesis, “Nonlinear Optics with the CO₂ Laser.” He stayed on as an assistant and associate professor at Harvard to conduct original studies on collisionless multiphoton dissociation of molecules. During the eighties, he worked at the industrial research laboratories of Exxon and Bellcore. He carried out research on photon band structures in three-dimensional media with periodic variations in the index of refraction, and on other topics in opto-electronics. He is now professor of electrical engineering at the University of California in Los Angeles. My contacts with Eli are always stimulating.

The optical strength of nonabsorbing dielectrics was a recurring theme at summer studies programs on materials by the Advanced Research Project Agency of the Department of Defense. These sessions were usually held in La Jolla, CA, although we met a few times in Cape Cod, MA. It was generally observed that the damage threshold at surfaces, even when meticulously cleaned, could be lower by a factor four or more than that of the bulk material. It occurred to me that electron avalanche ionization is determined by a concentration of electric field lines and that submicroscopic imperfection would concentrate the field lines equally well for optical fields as for fields at lower frequency.

During the spring of 1973, I spent a sabbatical leave in my native country, the Netherlands. I was the Lorentz guest professor at the University of Leiden, where I presented a special course on nonlinear optics. My host at the Lorentz institute for Theoretical Physics was Professor Peter Mazur. Professor C.J. Gorter was still directing the Kamerlingh Onnes Laboratory, while Professor Johan van der Waals had an active research group at the new Huyghens laboratory. I also interacted with many other colleagues whom I had known since my stay at Leiden as a research fellow in 1947 and 1948.

Active research in nonlinear optics was carried out at the Philips Research Laboratories in Waalre, near Eindhoven by a group which included B. Bolger, D. Polder, C.J.E. Schuurmans, Q.H.F. Vrehen and J.P. Woerdman and several others. At the invitation of G.W. Rathenau, who was one of the directors of the Philips lab, I served as a consultant. During our discussions of the recent literature, the problem of frequency broadening by self-phase modulation came up. It occurred to me that the creation of a plasma following electric breakdown by short intense light pulses would cause rapid changes in the index of refraction. This note was written during my employ of four months at the Philips Laboratories in Waalre.

The topic of optical breakdown was of such general interest that I received an invitation to speak about it at a quantum electronics conference in June 1973 at Dresden, East Germany. I traveled by train from Eindhoven and was rudely awakened by border guards on crossing behind the iron curtain. Dresden was still heavily scarred by ruins from the World War II fire bombing. The train from Dresden to Berlin was pulled by a coal-burning steam locomotive. I crossed at Checkpoint Charlie back to West Berlin. The three-day trip was a powerful reminder of the East–West contrasts which prevailed at that time. Nikolai G. Basov from Moscow attended the conference as well, and he encouraged me to write up my Dresden talk in the form of a review article. This is the result. It was also published in the Soviet Journal of Quantum Electronics.

This paper demonstrates that melting and resolidification can occur on picosecond timescales with heating and cooling rates as fast as 10¹⁴ degrees per second. At the highest cooling rates, resolidification into an amorphous phase on a single crystal substrate takes place.

In November 1978, I presented a plenary talk, “Fundamentals of Laser-Solid Interactions,” at a meeting of the Materials Research Society in Boston. These meetings became an annual event, and for many years thereafter were a major forum for discussions on laser-materials interactions. Our viewpoint was that, at least on timescales longer than ten picoseconds, the experimental data could be interpreted on the basis of rapid heating and cooling with a quasi-equilibrium between electrons and phonon. This was confirmed by a series of quantitative investigations during subsequent years. The proceedings of the annual Boston meetings of the Materials Research Society present a record of these developments.

This is one paper, among several presented at various MRS meetings in Boston by members of our group, which is reproduced in this volume. It demonstrates a variety of observations which are all consistent with the picture of rapid heating and cooling of a surface layer. Even as late as 1982, our viewpoint was contested by J.A. van Vechten of the IBM Watson Research Laboratories in Yorktown Heights. He championed the picture of a change in the electron structure of silicon and other semiconductors. It was later shown that his ideas are pertinent on femtosecond timescales. Intense femtosecond pulses can create a very high density plasma before appreciable energy transfer to lattice phonons takes place.

Dr. Heinz Kurz joined our group as a senior physicist on a leave of absence from the Philips Laboratories in Hamburg. Subsequently, he obtained a fellowship from the von Humboldt foundation and stayed for several years at Harvard until he was appointed as a professor in electrical engineering at the Technical University in Aachen, Germany. He is now the director of a large institute for semiconductor research and technology there.

Jia-min Liu was a graduate student. He later joined the University of California in Los Angeles, where he is now a professor of electrical engineering.

This note shows how picosecond time-resolved pump-probe experiments can provide detailed information about the plasma density and the temperature as a function of pump intensity, pump and probe wavelength and time delay between pump and probe. The abrupt change in the reflectivity on melting at a pump fluence threshold of 0.2 Joule/cm2 is evident.

This work was pursued further when two other foreign visitors joined our group. Louis-Andre Lompre was on leave for one year from the French laboratory CEN/Saclay, while Professor J.M. van Driel had a sabbatical leave from the University of Toronto. A partial list of further publications which are not reprinted in this volume follows:

L.A. Lompre, J.M. Liu, H. Kurz and N. Bloembergen, “Optical heating of electron-hole plasma in silicon by picosecond pulses,” App. Phys. Lett. 43, 168–170, 1983.

J.M. Liu, L.A. Lompre, H. Kurz and N. Bloembergen, “Phenomenology of picosecond heating and evaporation of silicon surfaces coated with SiO₂ layers,” Springer Appl Phys. A34, 25–29, 1984.

L.A. Lompre, J.M. Liu, H. Kurz and N . Bloembergen, “Optical heating of electron-hole plasma in silicon by picosecond pulses,” App. Phys. Lett. 44, 3–5, 1984.

H.M. van Driel, L.A. Lompre and N. Bloembergen, “Picosecond time-resolved reflectivity and transmission at 1.9 and 2.8μm of laser-generated plasmas in silicons and germanium,” App. Phys. Lett. 44, 285–287, 1984.

This note describes our use of the abrupt decrease in second harmonic generation when a layer of piezo-electric crystalline GaAs melts to form an isotropic liquid surface layer. This transformation takes places on a picosecond timescale.

Marco Malvezzi joined our group as a postdoctoral research fellow and stayed for about four years. He and Heinz Kurz collaborated on many other investigations of semiconductors irradiated by laser pulses. Marco Malvezzi is now an associate professor of physics at the University of Pavia. I had the pleasure of visiting with him there in June 1995. We have kept in touch with the Kurz and Malvezzi families over the past decade.

This paper is mainly the work of my colleague Eric Mazur and his graduate students, who introduced femtosecond pulsed lasers at the Gordon McKay laboratory. At the time of this publication, I had become professor emeritus. In the preceding years, Eric Mazur had gradually taken over all laboratory space of my former nonlinear optics group. He removed all obsolete and obsolescent equipment which my associates had used and started several new avenues of research. The work in this note is, however, a natural continuation of the investigation with picosecond pulses reported in the preceding paper. The potential of femtosecond techniques for solid state investigations was already evident to me a decade earlier. On this timescale, the energy absorbed by the electrons from the laser pulse has not yet been shared with lattice phonons. So melting, in the conventional sense, cannot take place in such a short time interval. It was a surprise to find that the second harmonic signal nevertheless diminishes rapidly on this short timescale. The change in electronic structure, originally proposed by J.A. van Vechten, to occur on nanoand picosecond time scales, finally manifests itself.

These femtosecond investigations have been pursued further by Mazur and coworkers to obtain the temporal behavior of the complex dielectric response function, which gives more detailed information about the collapse of the electronic band structure.

The work described in this note was carried out at the Massachusetts Institute of Technology, where Professor Ippen and coworkers had pioneered femtosecond laser techniques. My role had merely been to suggest the use of femtosecond time-resolved spectroscopy to investigate metals in a regime where the electron temperature would be much higher than the lattice temperature. This note reports an early result of femtosecond solid state physics.

This paper was originally prepared as a keynote lecture for the International Conference on Laser Advanced Materials Processing (LAMP ‘92) held in Nagaoka, Japan, in June 1992. The two other speakers at the plenary session were Arthur L. Schawlow and Nikolai G. Basov. I also presented this material at the Second International Conference on Laser Ablation, held in Knoxville, TN, in April 1993. After a similar lecture at the University of Mexico in Mexico City in November 1993, my hosts expressed an interest in publishing the material in the Mexican Journal of Physics, as the earlier conference proceedings were not readily accessible. My lectures on this subject were, of course, illustrated by a large number of slides which could not be incorporated in the printed text.

This brief note is included only to draw attention to a comprehensive study with the same title, published in the Reviews of Modern Physics 59, S1–S201, 1987. Chapter 6, under the heading, “Beam Material Interactions and Lethality,” occupies pages S119–S144, and is too long to be reprinted here. It should be readily accessible in many science libraries.

Kumar Patel and I were co-chairs of the study group sponsored by the American Physical Society. The group consisted of seventeen scientists with expertise in different technical areas. Some came from governmental and industrial research organizations, others from academia. Their political convictions also covered a wide spectrum. The whole report was approved of by all members with no dissenting minority opinions. Although the study group members sometimes debated issues for hours without reaching agreement, I recognized that such an impasse had one of two causes: 1. The issue under consideration could be political in nature. In that case, it was omitted from the report, which was only to address issues of science and technology. 2. The other possibility was that the issue concerned a matter of science, but that the wording in the draft had political overtones. In that case, the text was modified until a politically neutral wording was found which had the approval of all members.

The report was widely distributed and attracted much attention, including political flak, both in the United States and abroad. The Strategic Defense Initiative Organization acknowledged that the report's content accurately presented the state of the art as of September 1986 when it was formally submitted for security clearance.

Objections were raised against the wording of the study group's conclusions. Almost ten years later, the correctness of these conclusions still stands. This report retains its validity and is a source of pride for all members of the study group, although enormous historical changes in the political world have occurred during the past decade.

Although all seventeen members are co-authors of the whole report, the draft for Chapter 6 was prepared by Dr. T.H. Johnson and myself. Tom Johnson was a professor in both physics and English at the US. Military Academy, West Point, New York. He had served on numerous committees concerned with issues of strategic defense. His scientific input and wise counsel was appreciated by all members of the study group. He combined a thorough knowledge of strategic weapons systems with poetic vision. Some of his poems were published in Harvard Magazine at about the same time as our report came out. He was one of the youngest members of the study group. It is tragic that an incurable cancer took his life about five years ago. It appears fitting to end this section on laser-materials interactions with reference to work on which he and I collaborated.

The following sections are included:

• EPILOGUE