Interview with Shanhui Fan

This interview with Shanhui Fan from Stanford University, USA, was recorded as part of the 2017 international symposium “20 Years Nano-Optics”.

Q: What was your research field around the year 2000?
A: Around year 2000. I was mostly working on photonic crystals. so uh that was probably at the peak of the photonic crystal research. So i was doing a lot of uh theoretical calculation for hana crystal and trying to figure out how to use these kind of structures for what kind of applications.

Q: What conferences did you attend?
A: There was a series of conferences on photonic crystal called pacs. These are uh attended mostly by photonic crystal communities and started in 1997, and so i’ve attended quite a few of those. That was one of the uh probably the focal point of many of the discussions about the field.

Q: What were that big dreams around that time?
A: So that was a time when I guess people were really fascinated by internet , optical communications and information processing. So there were a lot of discussions about trying to use photonic crystal structures for information processing, and even dreaming about maybe computing based on photonic crystals. The idea was to have very small optical components on the single wavelength scale and to integrate them on a trip in a large scale way. So that one can do powerful optical information processing

Q: Were farbrication and theoretical limitations known to people?
A: It’s maybe come little by little as the right way to say it. Certainly there were a lot of challenges on fabrication on getting accurate devices. But also there are fundamental questions about whether, for example, computing entirely based on optics actually make sense or not. So that i think gradually influenced how people do their research in that field at the time.

Q: How closely did you relate the theory of photonic crystals with that of near-field optics?
A: I guess at least my own thought was more like looking at photonic crystal structures almost entirely from a band structure perspective, and it’s only later that gradually it becomes useful for me to also think about the individual scatter picture, and to gradually evolve into not just looking at, you know, periodic structure, but looking at individual antenna structure. That get a lot closer to perhaps what the some linear fourier optics community has been doing, but the fundamental point is that they are in the end all based on maxwell equations. So there are tremendous amount of connection and similarities. But when when i at the time when i was uh just finishing graduate school of course when i adopt one viewpoint and then over years you gradually learn the other viewpoints.

Q: What are your thoughts about metamaterials?
A: In 2000, I attended a conference where John Pendry gave a talk on perfect lens and that left a very strong impression on me. I thought it was even then I thought it was a very interesting idea that you could rethink about electromagnetism, in a very fundamental way. And I think that really conceptually at least was a very exciting time to to think in that way.

Q: Don’t you think that photonic crystals are also metamaterials?
A: Many of these concepts photonic crystal or metal materials of course very closely related. These days, I like to tell my students that there are nothing that’s 100 new and there are nothing that’s 100 old. So there are also, there’s always a connection to existing concept, but these new twists are important because they give you a perspective. They’re allow to think of something that may be different, so that’s how I would say that I wouldn’t make a very strict distinction between photonic crystal, metal material. Because I do think there are a lot of similarities between the two fields and the two concepts. But by emphasizing effective electromagnetic response, like permittivity and permeability, that really give a interesting perspective, that’s not completely transparent from a band structure description.

Q: Where do you see the future of nano-optics? More in basic research or in applications?
A: I think it’s both, and in fact one of the things that has always been exciting about nano optics is the fact that there is a very close connection between fundamental studies and application. Uh many of the early work that we do we did photonic crystal were motivated by application consideration, even the very early work in thinking about control spontaneous emission was motivated by either light emitting diodes or a laser application. So what i would think is that the exciting aspects of it has always been at this interface between fundamental studies and with the prospect of trying to impact practical technology.

Q: What are the remaining grand challenges in theoretical nano-optics?
A: I would say that in terms of grand challenges. Uh optics or electromagnetic magnetics is such a fundamental interaction of the uh of physics, and in fact it’s probably among the two interactions that we have actual access to the other one being gravity. But we can’t really control gravity uh very well. So it’s probably the most controllable fundamental interactio. So in that regard I think there are many many areas that the ability to control electromagnetism will have impact on something, that i’ve been personally very excited about. Uh is in thinking about the energy implication of controlling electromagnetism and also in thinking about the interface between for example uh photonics or electromagnetism and thermodynamics. And I think there are a lot of interesting things and fundamental concepts in thinking about light from a thermodynamic point of view, and trying to understand how nanophotonic structure can influence that kind of understanding. So i think there are still a lot of interesting basic work, that can be done. And many of these work will have practical technological implicationst.

Q: What would you think might be the next breakthrough?
A: The general property of thinking about using light to control energy flow and to impact., for example a wide range of energy conversion technology. I think that actually would be quite interesting area to think about in the future. That’s at least one of the areas that I found it to be particularly exciting. For example, we usually think about light as carrying energy that of course is true but light also carry entropy. And how do we influence these balance of entropy and energy flow in light. We have a very important implication ranging from thinking about solar energy conversion to cooling and to many other energy devices in a most fundamental level. Most of our energy come from the sun which is light and so the ability to think about light as a thermodynamic object I think will play a very significant role in thinking about the many of these technology.

Q: How would you describe nano-optics?
A: Certainly what really I think is interesting to me about nano optics is the ability that you can shape light, you can use subwavelengthstructure to shape light, and that gives you a possibility to control light in a way that was difficult to think about before you.

Interview with Gerd Leuchs

Q: Were you aware of scanning near-field microscopy and single-molecule spectroscopy in their early days?
A: Yes sure I worked at the Max Planck institute for quantum optics， and so we had, we heard about this, we had talks about this, so I was well aware of this.

Q: What were you working on when SNOM was invented in the early 80s?
A: Let me see, at this time I was working on multiphoton excitations in atoms and multiphoton ionization, looking for distributions of photoelectrons, but in neighboring groups， were doing， were closer to the to this field. But at that time， I did not see the immediate connection to it between the SNOM and the quantum optics world partly. Because the SNOM in the early days had lots of losses. There were, it was very inefficient and this kills most of the quantum optics effects. So we were well aware of this but I thought at that time it has just impact on resolution in microscopy, which at that time was not the immediate impact to quantum optics issues.

Q: Can you think of the early dreams at the interface between quantum optics and these fields?
A: So I think nano optics is the challenging side of classical optics in the very small describing the three-dimensional properties of space, where the spatial temporal modes of light to live and quantum optics needs all this, and the only thing that come that is new in quantum optics, is the uh is the excitation in the modes how they are described and correlation between modes. So basically, if you want to do optics at the advanced level, you need the combination of optics and quantum optics or nano optics , and quantum optics, and I think as a challenging thing in the future in this wider field of quantum technology from technologies, where we will now have this flagship at the European level. So I think the challenge in this regime would be the quantum internet that people are talking about, where you need both aspects to in their full power, you need to, you need to couple two level systems, small, small atoms, whatever they are um small quantum dots, and this is, this is, and then you need to couple to them, you need to couple them efficiently, that’s very important in for quantum technologies. So yes, I think this you need both aspects in and in the full in the maximum possible performance.

Q: How did you start your work on doughnut beams and tight focusing?
A: So first of all, there was a gap. Because at some point I decided to leave academia and I went to switzerland to accompany , and after five years. I had the offer to come here to Erlangen, that was in (19)94. And so then was my first co-worker, we were looking at possible things that we could do. And he dug out a paper on sunderluminescence, which was a hot topic at the time, and people had not understood. so, so, so luminescence, if you remember, is sending in a spherical acoustic wave to a focus in water, and in the very center there’s a small bubble, and inside the bubble there was some blue light, and people were wondering how to explain this. And then I noticed that this bubble is much much smaller than the wavelengths. And then I was thinking how can this be, and the so my co-worker thought maybe we can go into the field and help um discover what sonar luminescence is about. But my heart was more on the side of optics and light. And I was thinking how about, how is this in optics and uh I mean in optics we have the resolution limit of given by the wavelengths. And so how does this uh compare and then we were thinking of what nature gives us, and we were thinking about the emission, emission and absorption of an atom. And if you look at spontaneous emission then initially the atom is inside the state. Eventually, it’s in the ground state and the photon is far away and thinking of the word spontaneous emission, many people would have thought that maybe you kind of reverse this because spontaneous mission has this stochastic connotation and stochastic uh statistical events you cannot reverse. Um, but when we thought about it became clear that this is just a quantum mechanical unitary evolution, and that should it should be possible to go backwards. And of course, if you go backwards, all the light energy of a single photon goes into the atom, and eventually, it comes to a point much much to a volume, much much closer than the uh, much much smaller than the wavelengths cubed, and we were thinking oh my god okay. Here’s a possibility to concentrate much much light, much smaller on a volume, much smaller than a wavelength cube, and so then we were thinking of what kinds of experiments we could do, and then in the early days, we were thinking ah this will be difficult um. But then we found that even if you don’t have an atom, but you take guidance from this reversed spontaneous emission that tells you that you should send in a dipole wave with the proper vectorial properties of light, and then we did the calculation. We discovered that focusing a dipole wave gives the focus which is as small as you can get and this you can never achieve. Even if you focus in only, so ideally, you have to come from the full solid angle, but even if you come from one half space with a lens, um linear polarization wouldn’t do this job. You have to have a different polarization, so how this is, how we got into this, and apparently we were the first people to think in this direction, and then we also showed this experiment, so that was basically a spin-off from this initial thought how about reversing spontaneous emission, and only when we had this first funding from max planck, when our institute was 10-year trek, then I decided to take the challenge and start the real project.

Q: When did you hear the word nano-optics first?
A: Let me see, so I think I mean the the names nano and optics separately. I’ve probably heard a long time ago, but in connection I probably heard this when I came back from industry so this would be the early 90s or mid 90s early 90s .

Q: How would you describe nano-optics today?
A: So if you say that nano optics is everything in optics which happens in on a on a scale smaller than a wavelength, one might think on such a small scale things are more or less homogeneous enough, not much, not much is happening. But that is not true. So when you have interaction with atoms, small plasmonic particles you have all this interaction all the near field aspects , you can have, you can have zeros of the field, which are surrounded by vortices. And these are structures which can be much much smaller than the wavelengths, and can be highly complex, and interesting, and have uh interesting um consequences. And the interesting thing I find is that, in this old field of optics, one discovers new things again and again. So if I think about my own research, so we discovered this focusing is possible in a different better way, and we did this in the we started to think about this in the 90s. More than 100 years after after unstubber, and so many people having thought about light and focusing and vector properties. All books have been written on this, but this was not there, and then another thing that happened more recently is that um an old optics topic is totally eternal reflection, and for a long time. I thought I know everything about it, and then it was between the group of Anatoly zayats Zayats and Ann Rauschenboitel, and and my group, that we understood in different contexts. And in quite different experiments, we understood that in the polarization has a very special property, in, in near the interface, under the condition of total internal reflection. Um, and you can have these spinning fields that we call transverse angular momentum. Um in the evanescent wave, in total internal reflection, which is uh was it was a strong, um it was a strong correlation between the sense of the spinning and the emission direction, which these groups that I mentioned, have now shown that you can make use of this. Um, so this I think is really nice that sometimes. So this shows you that you can, you can have topics where much research has been done, but if you look at it with fresh eyes, uh you can still discover something that people haven’t recognized before which can have a big impact. So then of course, when, when this happens, you go back in literature, you think maybe, maybe we missed something, and so there is the famous paper by Richardson and Wolff from 59 1959, where they describe the field in the vicinity of a focus in great detail, which you could call a nano optics paper because it shows all these very strange, uh, the Poynting vector pointing in all directions, and all these things so describe all this in much detail. It’s a long paper, and in one half sentence, they say by the way there is a spinning field, which spins in a plane containing the propagation direction, which is exactly this transverse angle momentum. But it was just a remark, so they noticed this and the reason they I mean the, the fact that, they wrote this sentence down means that they thought this is something special, but they didn’t discuss it further. They there was, it was not so obvious what this might lead to, and then the next thing is in the seventies, there was a paper by, um, by two French people written in French, and they wanted to have circular polarized microwaves, but they only had a linearly polarized source, and in microwaves not so easy to have to change the polarization. So they took a big prism probably parafine, and send in the microwave from the side so that there was an evanescent wave, and with the proper linear polarization. It produces exactly the spinning field with this transverse angular momentum. I mean not calling it transverse angle momentum, they also didn’t talk about the coupling between the spinning direction, and, the and the, and the possible emission. If this would be a dipole but basically they understood these special polarization dynamics, there so, so i think this often happens that if you, if there’s a breakthrough, and suddenly people think, oh my god, this is really nice, and has consequences. Then if you look in the past people have been very close to this, but, but not really doing the final step. And um, so this I think can also be a lesson. If you want to do something new, when uh, should look at the things that people have done, and uh with a with all the curiosity and the openness that you have and then chances are that you discover something new.

Q: What do you think the future challenges in nano-optics?
A: So I think there’s, there’s certainly a wide range, and iI probably cannot think of all of these. But one thing that that I’m struggling with at the moment. And I have a small project on this is that, there’s so no luminescence that I started to talk about, was a focus of sound wave in a homogeneous medium, and still you had this effect which this small bubble that was created at a scale which is much smaller than the wavelength of these sound waves. So this we have not achieved in optics yet, and i wonder whether you can do this when you focus to a small plasmonic particle gold bead, an atom. You have this enhancement, and this near field which is probably a little bit like it, and you can have this structure is much smaller than the wavelengths. But then you have to, have the structure, there the medium is not homogeneous. And if there would be due to some nonlinear interaction, so in the sound in the case of a sound wave says surely some non-linear interaction. If there could be some non-linear interaction, then um in a homogeneous medium. I, my feeling is there could be something, but nobody has looked at that. So we are looking, but we haven’t found yet ultimately. This could even happen in the vacuum, because the vacuum we know is not a void space, we know that if you put enough, in enough energy. You can create electron positron pairs and uh um and you can calculate at which electric field this should happen. And so this is far away from what people can do so far. The new eli project the extreme light infrastructure project of the European union. This is one of their goals so sometime, in the future they hope to do this but maybe it can happen at lower intensities. Then what they think will happen if there is such an enhancement vector. So we, we have an experiment in, not in vacuum. Because then you need so high. So we don’t have the lasers that they have in the eli project. But you can do this in a in a homogeneous non-linear medium like a, like a gas. So, so this I think from myself this is a challenge we are tackling at the moment. We are trying to understand, and uh and, do the experiment. Um, so I think there are some, some loose end if you some things, if you think them through. Then there are some final things missing and uh, so to, to get this, to gain full control of these aspects, is a, is a big challenge, and what I mentioned before this, all these issues associated with this. What people call the quantum internet that has many many aspects, which is, this is full of challenges for for the quantum optic side for the uh, for the nano optic side. This transverse angular momentum, and directed emission is uh could be a central ingredient to the to this quantum internet. So this is, uh, full of challenges. So we kind of build it now, because it has to be has to allow for has to have all this flexibility and of the steering but you also need very low losses you cannot put up with losses you need high efficiency and so that i think is a big challenge

Q: How would you describe the difference between photonics and optics?
A: So first of all, I know that different people use it completely differently. So I could describe you what people different people think of photonics, and of optics, which is controversial, or which is not, which is not matching. So for me personally, optics is everything. So optics include classical optics and quantum optics and so. This is, this is everything. Photonics is probably a bit weird because it carries the name of the quantum of light, but in the past during much of my lifetime, it was mostly used by at least initially, by people in solid state physics, where the fact that light is quantized does not play a role. They still call it photonics, so that was kind of like a misnomer, but now things are changing. Also I think if you ask people in Germany and in anglo-saxon countries, you’ve got different answers. So in general things have been changing so much that for me, photonics and optics is two different sides of the same coin, both basically you can use to describe everything that has to do with light.

Interview with Mikhail Lukin

Q: What was the topic of your PhD work and where did you do that?
A: So my phd work was on quantum coherence and interference effects in atomic systems, and the applications in quantum optics. During that time we were exploring how we can manipulate the properties of atomic systems of optical by creating coherent superposition states, and using them for applications like laser silver inversion. So this was in a group of marlon scali texas a m university in college station texas very interesting place, I finished my phd in 1998

Q: How did you become interested in nano-photonics?
A: First of all, i’m still working with atoms and also others atom-like systems, and I view this kind of interplay between quantum optics, and also kind of modern photonics as a very fruitful playground, in which the two disciplines learn from each other, and influence each other. And certainly you know, I saw my own interest in nanophotonics came um from kind of ideas to try basically do atomic physics better in new ways to create functionality, to create new quantum optical devices, to explore new quantum optical effects, to improve control over interactions between light, and atoms, and for those kind of applications, for doing science in these areas. It turns out that nanophotonic devices offer pretty unique possibilities, and that what motivated me to think about nanophotonics, to think about plasmonics, to think about photonic crystals. And, uh I really see that you know already now, uh this interplay between quantum optics, and you know nanophotonics really opens completely new frontiers. And I think we are now only at the beginning of we just tipped you know the iceberg you know so see the top of the iceberg in terms of the things which are about to come.

Q: Do you remember when your first encountered scanning near-field microscopy?
A: Not exactly the day, but you know it is I mean definitely. So as when i was a, when i was a graduate student, when going to uh optics quantum, and photonics conferences, I realized at the time that you know there is a kind of a community being developed, which is you know really kind of looking, how to kind of explore the possibilities of really isolating individual particles, individual molecules. And it became clear with me over time that you know basically, know borrowing, you know some ideas from this community, and also supplementing it with kind of. Let’s say the ideas for coherent control, which originate from the field of quantum optics uh, is a very promising new direction, and uh I mean this is the kind of stuff which we’re exploring today right, and I think by doing that not only we basically just borrowing ideas from the field of nanophotonics, and use them for example, building quantum computers, but also we realized that some of the kind of techniques which we developed, and quantum optics, and science can now influence, and are now influencing the nanophotonics field with new applications, such as for example nanoscale sensing emerging from this work

Q: How would you describe you research field today?
A: So my research is now in the field which can be perhaps called quantum science or quantum science and technology. And basically in this field what we are trying to do is, we are trying to use the control over coherent control over individual quantum neutral particles, individual atoms, individual atom-like systems, to build more and uh complicated uh quantum systems, and you know one example of that is you know building quantum computers. But there is much more for example another direction, that I mentioned involves applications of the systems to build normal types of sensors. And you know these sensors you know, potentially allow one to measure things, and you know do things which know just about few years ago, or fought to be completely impossible. And that’s you know the kind of stuff that you are doing

Q: What comes to your mind when you think of the word nano-optics?
A: So I am uh fascinated by the idea of breaking a diffraction limit of confining the light down to sub-wavelength dimension. And what really comes to mind is the possibility of really molding of using this you know interact kind of confinement to really mold the interaction between light and atoms individual atoms in particular. And that’s where at least in my kind of world that’s where nanophotonics really offers unique new possibilities.

Q: What are the challenges in quantum nano-optics？
A: I think the key challenge from my point of view is really kind of extending the techniques for kind of confined light confinement and interactions, uh with atoms to kind of fully coherent, and fully reproducible domain. So basically uh, what you know in my research, we are trying to do now yes, we are trying to basically confront the usually kind of contradictory challenge, challenges of you know basically controllability and scalability. So on one hand, you know we would like to really be able to control uh atoms and photons that level down to the individual quanta, but at the same time would like to have a larger systems kind of practical systems associated of them. So it’s basically these are two contradictory requirements, where you know nanophotonics can really play the very important role, but you know basically you know what we would like to do is really you know be able to squeeze systems down to subwavelength dimensions, while without losing the control, without using kind of reproducibility, such that we can potentially you know have hundreds thousands of millions the systems coupled to each other, or used for applications. From my point of view, nano-photonics is still very young field you know, and I think the the best is still ahead. So i think as I said, we just tipped you know see the top of iceberg and there’s a lot to come.