"We managed to break the mental fixation"

Interview with research professor Moti Segev, Faculty of Physics "Discovering a new, unexpected or counterintuitive phenomenon - these are the moments that motivate me." says Professor Segev 

At the end of the conversation with research professor Moti Segev, there is no doubt that this is a researcher who loves new experiments - not only mountains and glaciers like the one seen in the picture, but also new ideas and experiments in science. Segev, one of the five distinguished professors (Distinguished Professors) at the Technion, took a short break from his adventurous hobbies (and his research at the Technion) to give the opening lecture at the most important conference on lasers and electro-optics, which took place in early May in Baltimore. Segev, who has won many awards, including the prestigious quantum electronics award (the most important European award in the field of optics and lasers), and the prestigious Max Born award given by the Optical Society of America (OSA), talked with us about his extensive interests and achievements, and contributed some advice to young researchers and students entering this field.

What is the highlight of your career?
The peak was in Victoria, Canada, in 1988, at the end of the OSA conference on nonlinear waves, when it was clear that my work in the field of solitons had resonated. In the beginning, my ideas in this direction received many attacks. Papers have been published claiming that such solitons cannot exist. Others argued that these solitons are unstable and cannot be observed experimentally.
For a long time people simply did not believe, despite the fact that we published several theoretical and experimental articles in leading journals. People tend to mental fixation and walking in a rut. It is very difficult to change ways of thinking, especially when you are attacked by very senior scientists. But some good friends supported me all the way: Amnon Yariv from the California Institute of Technology, Steve Harris from Stanford University, Steve Forrest and Paul Prosnell from Princeton, and at some critical moments also Nobel laureate Nicholas Blumbergen. I also gained three very good friends who walked this difficult path with me: Bruno Crosignani, Greg Selmo and Dimitri Christodoulides. I believe that together we created a revolution in the study of solitons. So if we go back to that conference in Victoria in 1998, when dozens of groups continued our research, we realized that we had won: we had managed to break the mental fixation of a large community in science.

What are solitons?
Solitons are "wave packets" that are trapped within themselves and thus maintain a fixed configuration. The single soliton behaves, also in its relations with other solitons, as if it were a particle. Solitons can attract or repel each other, like two charged bodies, as well as become trapped in a spiral orbit around each other.
Solitons appear in a variety of systems, from liquids to plasma, acoustic waves on DNA molecules, sound waves in superfluids, elementary excitations in string theory, and gravitational phenomena in black holes. Solitons are found in many places and in many fields. Over the past 20 years, optical solitons—beams of light that behave as if they were particles—have become the leading research direction in all of soliton and nonlinear wave science.
Largely following your discoveries.
Indeed, I have been studying optical solitons since 1991, and over the years we have made many surprising discoveries. The most important among them, I think, is the discovery of the non-coherent solitons: solitons made of white light originating from an incandescent bulb. Imagine a simple light bulb, we focus part of its light into a beam 10 microns wide (a beam 10 times thinner than a hair), and direct it into a non-linear medium. The result: the beam remains focused for an unlimited time - as long as it continues to disperse in this medium. Amazing, isn't it?
Until 1996, all solitons, in every theoretical and experimental study in which they were studied, were seen as completely coherent entities; But my intuitions told me that maybe, if we use a non-linear medium that reacts slowly - much slower than the random fluctuations of the incoherent light - we could create solitons originating from the incoherent light. We began to perform experiments, and in 1996 we demonstrated a soliton composed of light with partial spatial coherence: laser light transmitted through a rotating light diffuser. A year later we presented solitons from "white light", which originate from the light of a simple incandescent bulb!
Those who led these experiments are my students Matt Mitchell (Matt Mitchell) currently a senior researcher at Infinera - a company engaged in the development of smart optical communication; Ming-feng Shih, currently a professor of physics at the National Taiwan University; And my postdoctoral fellow Zhigang Chen, now a professor of physics at the University of San Francisco.
In the first two years of the research, we had no theory - everything was based on intuition only. At the end of 1997 we developed the first theories, in collaboration with a very good friend of mine named Demetri Christodoulides, currently a professor at the University of Florida. The discovery of the incoherent solitons ("random solitons") revolutionized research in the field. Since then, such solitons have been discovered in many systems, among others in periodic structures and photonic crystals - research by Hrvoje Buljan, now a professor at the University of Zagreb, Oren Cohen and Guy Bartel when they were my students, and by Jason Fleischer when he was my postdoctoral fellow. Oren and Guy are currently faculty members at the Technion, in the faculties of physics and electrical engineering, and Jason is a professor at Princeton University in the USA. Non-coherent solitons were also discovered in non-local nonlinear media - work by Gaetano Assanto and Claudio Conti from Italy, and by Oren Cohen and Carmel Rothschild, who is currently a faculty member in the Faculty of Mechanical Engineering at the Technion . Many other researchers around the world are currently working on ideas based on incoherent solitons.
An interesting fact is that incoherent solitons have also been discovered in the time domain: pulses that do not expand in optical fibers. This important research was carried out by Antonio Picozzi from France. In recent years, incoherent solitons have also been discovered in non-optical physical systems. In 2006, incoherent solitons composed of spin waves in magnetic layers were observed [Mingzhong Wu and Carl Patton in Colorado]. Several years ago I showed, theoretically, together with Hrvoje Buljan, when he was my postdoctoral student, and Ami Vardi (a professor at Ben Gurion University), that incoherent solitons should also be found in matter waves.
The idea of ​​"random nonlinear waves" is now taking diverse and interesting directions, such as creating spontaneous patterns with weakly correlated waves, as I showed with my student Marin Soljacic (now a professor of physics at MIT), my postdoctoral fellow Detlef Kip (now Professor at the University of Hamburg, Germany), and Dimitri Christodoulides. I believe that in the coming years we will see new discoveries in the field of random nonlinear waves resulting from the interplay between incoherence, dispersion and nonlinearity.

What did you focus on in the opening lecture at the conference in Baltimore?
I talked about a phenomenon called "Anderson localization" of light: how a beam of light propagates in a medium characterized by random and different refractive indices, and when the light becomes localized thanks to that disorder. The idea of ​​propagation and location of waves in a disordered medium was proposed in 1958, when Philip Anderson studied the movement of electrons in a crystal containing disorder.
Anderson won the Nobel Prize for this research in 1977, but many fundamental questions remain unresolved to this day. This field of research entered the world of optics in the mid-XNUMXs by Sajib John and Philip Anderson, who realized that electromagnetic waves are a fertile ground for the study of this universal phenomenon, since by means of light it is possible to isolate the various effects that influence the development of "wave packets" ( wavepackets).
Equally important is the fact that through the light you can actually see how the propagation of the wave changes and/or position. Over the years, many research groups have worked in this field.
Describe your research on the Anderson location - what did you discover and what are you still working on today?
Several years ago we were able to show experimentally the Anderson localization phenomenon in a photonic crystal containing disorder. The importance of this study stems in part from the fact that it is the first experiment to demonstrate the Anderson position in any periodic system that includes disorder. In the following experiments we investigated the influence of nonlinearity on the location effects. This research, which I conducted with my students at the time - Tal Schwartz and Guy Bartel - and with my colleague Shmuel Fishman, led to many follow-up studies all over the world. The idea of ​​placing waves within a system that includes both disorder and nonlinearity raises fundamental questions, which have led to many debates. For example, the question of whether over time the wavepacket will eventually expand, or if the disorder will slow down the dispersion, as happens in a linear medium. The answer to this question is widely disputed in the scientific community. Our 2007 study was the first ever experimental study of location effects in a nonlinear system involving disorder.
In the last two years, I worked with my student, Liad Levy, and my postdoctoral fellow, Michael Rechtsman, on positioning in quasi-periodic crystals - for example, a Baal crystal with pentagonal symmetry.
Quasi-periodic crystals are in themselves a unique phenomenon in nature, and they were discovered by Professor Danny Shechtman from the Technion and explained by my colleagues Professor Dov Levin from the Technion and Professor Paul Steinhardt from Princeton; We all hope that the three will win a Nobel Prize for their discovery.
Liyad, Michael and I examined the propagation of waves in quasi-periodic photonic crystals, and to our great surprise we discovered that under certain circumstances the wave packet can go through a process called
disorder-enhanced transport. In other words, introducing disorder into a quasi-periodic crystal causes the light beam to expand more than it would in pure quasi-periodic crystals. Moreover, this expansion, which is counterintuitive - because disorder actually tends to reduce the expansion of wave beams - is characterized in a way reminiscent of diffusion processes: the average light beam has a Gaussian shape. The first article about this phenomenon recently appeared in the prestigious journal Science.
Finally, in the last few months we discovered another surprise: if the random disorder changes rapidly during the expansion, the light beam expands much faster than it would even in a perfectly homogeneous medium, with no disorder at all. At the same time, my postdoctoral students Alex Szameit and Michael Rechtsman studied amorphous photonic systems. These are disordered systems that have no Bragg diffraction at all, meaning they lack the most basic property of crystals. Despite this, we were able to prove experimentally and theoretically the existence of a "forbidden gap" - angles where light cannot propagate at all. The first article on the subject appeared this week in the journal Physical Review Letters.
I believe this is only the tip of the iceberg. The physics of optical systems characterized by disorder, linear and non-linear, is in its infancy. I expect many surprises in the coming years.
What discoveries can the optics community expect from your research group at the Technion?
We are working on several projects at the same time, probably because I have always liked entering new fields and learning new topics. But if I am asked to point to one discovery, I would say:
sparsity-based sub-wavelength imaging. This is work that I started two years ago with Professor Yonina Elder, an expert in the field of information processing, and my young colleague Oren Cohen, who added some excellent ideas on the subject
sparsity-based super-resolution in time and frequency spaces. Fortunately, we were blessed with some excellent students in this research: Yoav Shechtman, Shanir Gazit and Pavel Sidorenko, and Alex Szameit, an excellent postdoctoral student from Germany, who recently became a faculty member at JENA University.
What does the future hold in the field of non-linear optics?
Non-linear optics is a very broad field, and its potential research spectrum is enormous - among other things in the field of opto-fluidics, that is, light-fluid relations (see bottom frame on p. 16). Another research target is related to non-linear phenomena in metamaterials: artificial materials with electromagnetic properties that are engineered on a scale smaller than the wavelength. Ideas of this kind have already been proposed in many theoretical papers, but the experiments are few. There is no doubt that experiments in this field will lead to exciting ideas that have not been raised by theorists.

What is the best part of a physicist's life?
Discovering a new, unexpected or counterintuitive phenomenon. These are the moments that drive me.
Article: Angela Stark, Director of Communications at the Optical Society of America (OSA).
The interview was published in the association's journal, OPTICS AND PHOTONICS NEWS.

Looking forward to many surprises in the coming years. Professor Segev in the lab

Light and liquid: interrelationships
One of the future research goals in the field of non-linear optics is the combination of non-linear phenomena and opto-fluidics: combined dynamics of light and fluid. So far the field has been dominated by engineers and material scientists. "They did an excellent engineering job, but I have no doubt that the field is still not exhausted. It has been known for many years that light exerts forces on small particles that move when they are immersed in a liquid. The studies that were done focused almost entirely on the particles themselves, but there is no doubt that the particles moving in the liquid create movement in the liquid itself: they have friction, which creates a flow in the liquid that originates from light. At the same time, the light changes the local concentration of the particles in the liquid, and as a result the refractive indices of the "suspension" change - therefore the shape of the light beam also changes. There are three non-linear processes here that work in an integrated manner: light creates a flow in the liquid; The flow affects the concentration of particles everywhere in the liquid; And changing the concentration changes the refractive index of light everywhere in the liquid. As a result, the propagation of light in the liquid changes, and God forbid. This direction may lead to new basic physics, as well as interesting applications, for example control of drug dispersion using light, optical control of chemical processes (such as catalysis) with a precision of tens of microns, and more. This project was launched through a former student of mine, Carmel Rothschild, upon completion of his PhD (Carmel did a post-doc at MIT in the field of solar energy; he is joining the Technion Mechanical Engineering faculty this summer as a new faculty member). The project continued and reached great achievements through my other students, Elad Greenfeld, and Yuval Lemhot, and now Kobi Lomer (new doctoral student) and Dr. Yoni Namirovsky (Sabbathologist from Raphael.) have joined the project.

people
Don't let the fame fool you
Professor Segev, who has "raised" many graduate students, as well as many post-doctoral students, believes in the importance of the personal relationship between supervisor and student. "Don't let the fame fool you," he suggests to students and postdoctoral fellows who are about to choose a supervisor. "You want direct contact with senior researchers on a daily basis, not through a 'chain of command'. Before you choose a research group and a supervisor, talk to the students who are currently active in the group as well as those who were active in it in the past. They taught who the researcher is as a scientist and as a person. This is the man who will be your scientific father, or your scientific mother, so make sure you made the right choice. I was lucky enough to learn from one of the great mentors of the world of optics: my scientific father, Amnon Yariv from the California Institute of Technology. Amnon had a huge impact on my career.”
He also has advice for the young faculty members: to recruit students wisely and carefully, and not to rely only on high grades from the first degree. "It is impossible to draw any conclusion related to creativity from such scores. Let the new students work on small projects for two to three months, working closely with you. Consult with the old students and postdocs, then recruit only students who can think creatively. You don't want skilled people who excel at solving homework but are mentally fixed!"

Published in the Technion magazine