Throughout the history of physics, optical physics has played a central role. It was a midwife at the birth of quantum mechanics, when Planck introduced the new physical constant h in his description of cavity radiation. His use of the latest experimental results in finding the formula for the spectrum of that radiation is characteristic of the close interaction between experiment and theory that is still a feature of optical physics. Later milestones in quantum mechanics, such as the study of the photoelectric effect, the spectroscopy of atoms and molecules, and the development of quantum electrodynamics, relied on the results of optical physics, and both expanded its techniques and its conceptual repertoire.
As if in acknowledgement for the contributions of optical physics, quantum mechanics led to the invention of the laser in 1960. Except perhaps for the early development of lenses and mirrors, no technological development has had an impact on optical physics greater than the advent of the laser. The coherence properties of laser light allow for the development of high intensity pulses, either narrow in frequency or short in time, that serve as the starting point for both the scientific research efforts of scientists who use the laser, and the laser’s industrial applications. Indeed, what might be called the modern period of optical physics dates from the laser’s invention just over forty years ago. Canadian researchers have been active players throughout the period, and this special issue of Physics in Canada presents a snapshot of just some of the work being done in optical physics in Canada today.
A major theme of research within the modern period has always been the development of new light sources, either at new frequencies, or at higher and higher intensities, or with shorter and shorter pulses. Often the focus is an effort to make these new sources convenient and usable to a large community of researchers. Donna Strickland’s work at the University of Waterloo is a striking example of this kind of research. The use of optical parametric amplification, through the nonlinear mixing of optical frequencies to lead to coherent radiation in the mid-infrared, offers the opportunity for the construction of lab-sized sources of tunable, high-intensity pulses in the mid-infrared range that is so crucial for molecular spectroscopy. Work at the National Research Council, by Paul Corkum and his colleagues, pushes laser technology pulses towards the attosecond (1 attosecond = 10-18 second) regime. Here research into short pulses and high intensities becomes intimately connected, as the use of strong laser fields allows control over the quantum mechanical motion of the electron, whose wave packet becomes partly ionized from an atom. The part of the wave packet that is ionized can be driven back to the nucleus and made to interfere with the part of the wave packet still remaining there, leading to the generation of a photon. When all this is done “just so,” optical pulses in the attosecond regime result. With control of pulses on the time scale of the motion of electrons in matter now possible, these techniques point to new methods for probing chemical and physical processes in exquisite detail.
The “strong field physics” that is part of this research is heir to the tradition of research in nonlinear optics that has characterized much of optical physics since the invention of the laser. The first nonlinear optical effect observed, shortly after the laser’s invention, was second harmonic generation, where two photons at a fundamental frequency are converted to one photon at the harmonic frequency. The first detected second harmonic light was so weak that the faint trace on the photographic plate was mistaken for a flaw by the editors of The Physical Review Letters, and the evidence for second harmonic generation was “air brushed” out of the very article that announced it!
Times have changed. S.L. Chin at the University of Laval reports on his studies of the effects of powerful femtosecond (1 femtosecond = 10-15 second) laser pulses in optical media. Here the pulses are so intense that the femtosecond pulse transforms itself into a white light pulse, or “supercontinuum,” with higher and higher frequencies being generated. The resulting pulses can exhibit filamentation, and when travelling through air can write a channel of ionized material through which they can propagate long distances. The application of such high intensity pulses in melting and material processing is obvious. They have also been used in remarkable experiments, such as in the control of lightening in laboratory tests!
Intensities of light that are much lower, and only slightly perturb electronic motion in material media, are of interest for microscopy and spectroscopy; two of the oldest continuing traditions of research in optical physics. Xiabin Zhu and Mark Freeman, from the University of Alberta, describe their ultrafast optical studies of magnetism using magneto-optical microscopy, which relies on magnetic effects on the optical properties of condensed matter. They have been able to make “stroboscopic movies” of magnetic materials, studying magnetic dynamics on a picosecond (1 picosecond = 10-12 second) timescale with resolution limited only by the optical wavelength. Spin relaxation and magnetization switching have been observed, and analogs of non-magnetic nonlinear effects, such as magnetization induced second harmonic generation, provide powerful new probes of these important materials.
Another application of optical physics to condensed matter physics is described by John Preston and his colleagues at McMaster University, who are using ultrafast optical pulses to study the cuprate superconductors. The laser pulses are used to generate high energy quasi-particles in the superconductors, and probing these nonequilibrium states via transient inductance and infrared absorption is helping to characterize these important materials and their properties. Both the work on magnetic materials, and that on superconductors, illustrates how optical physics can help bring new tools and techniques to the study of materials not normally associated with optics.
Still on the topic of spectroscopy, at least as very broadly defined, is the work of Georg Rieger and Jeff F. Young, at the University of British Columbia, on semiconductor waveguides patterned with hi-index-contrast, sub-wavelength features. The condensed matter systems studied in investigations such as these are not materials provided by nature, but rather “artificially structured materials,” such as photonic crystals that possess a periodic variation in their local refractive index, which are fabricated for use or indeed just for interest. Here optical physics and nanotechnology meet, with the fabrication of structures possessing novel and perhaps technologically important properties. Optical physics plays two roles here. The first is in the study of these new structures via their spectroscopy. The second is the use of these structures to control light itself: its flow, its generation by spontaneous or stimulated emission, and its self-interaction via nonlinear effects. There is clearly also a meeting here of optical physics and optical engineering.
The article by William Wijngaarden and Baolong Lu of York University, on Bose-Einstein condensation (BEC) of dilute alkali vapours, reminds us that optical physics is contributing to the most significant developments in physics as a whole, with two Nobel prizes recently awarded in this area. One was for the optical cooling and trapping that helped lead to achieving BEC in alkali vapours, and the second was for that achievement itself. Unlike superfluid helium, where the interaction between the atoms is strong and the condensate fraction very small, in the BEC of alkali vapours the bosonic atoms are only weakly interacting, and theory has been able to make huge strides in understanding these novel many-body systems. Recent fundamental developments, such as the Cooper pairing of cooled fermionic atoms, illustrate the wide range of physics that these kinds of systems exhibit. They also serve as a venue for interesting optical effects, such as electromagnetically induced transparency. And new technological efforts, such as building “microtraps on a chip,” promise even more interesting physics and engineering.
The remaining two articles highlight the role that optical physics plays in physics and engineering education. Roger Lessard and his colleagues from the University of Laval review the history of optics education there, beginning with the establishment of the Department of Physics there in 1938, and illustrate the evolution in the curriculum that has occurred as new developments and interests arise. Marc Nantel and his colleagues from Photonics Research Ontario sketch the establishing of three new undergraduate degrees in photonics in Ontario, the Bachelors of Applied Technology-Photonics at Algonquin College and Niagara College, the Honours B.Sc. in Photonics at Wilfred Laurier University, and the Bachelor of Engineering in Photonics Engineering at McMaster University. With photonics being described as “the next multi-trillion-dollar industry,” we can expect an enhanced role for optics education in physics and engineering, and the establishment of other new programs, as the new millennium advances.
In a short collection of articles such as this it is impossible to overview all the work being done in optical physics in Canada. Many exciting research programs have not been reviewed; many universities in which optical physics is a major strength in the department of physics — including even our own! — are not represented. And new interdisciplinary efforts, such as the application of optical physics in quantum information processing and in biomedical research and development, have not been covered. Yet the few articles in this special issue illustrate both the breadth and importance of research in optical physics, and its strength in Canada.
Henry van Driel / John Sipe
University of Toronto
Guest Editors
The comments of readers on this Editorial are more than welcome.
Editorial Board welcomes articles from readers suitable for, and understandable to, any practising or student physicist. Review papers and contributions of general interest are particularly welcome.