Unblinded By the Light: Controlling the Shape of Light in Space – Dr. Hasan Yilmaz – Yale University, Department of Applied Physics

Richard Jacobs: Hello, this is Richard Jacobs with the future tech podcast. I have Dr. Hasan Yilmaz. He received his PhD from the University of Twente in 2015 for his work on advanced optical imaging with scattering lenses. He is currently a postdoc at Yale University Department of Applied Physics and Dr. Hassan or Hassan. Thanks for coming. How are you doing?

Hasan Yilmaz: Thanks, Richard. Thanks for the introduction.

Richard Jacobs: So tell me about your current work. What are you researching and working on now?

Hasan Yilmaz: Currently actually I work on a system that scattered light very strongly. One of the examples of this system, its actually very typical example is a piece of white paper. The reason why we can see through glass but we cannot see through the paper is because of light scattering, like the image information distorted due to this random scattering. So currently, I am developing methods to control light scattering so that we can see through a piece of paper as an example.

Richard Jacobs: So by controlling how light scatters, we can see through opaque objects that we can’t see through right now.

Hasan Yilmaz: Yes.

Richard Jacobs: How would that be possible? What would be the mechanism for something like that to work?

Hasan Yilmaz: It only works for a specific type of light, for example, the laser light. So if we sent a laser light to the scattering materials such as a piece of paper or white paint, what happens is like randomly follow the path inside this system. Even though this path looks like random, it’s not random, it’s deterministic, and it’s just too complicated. So what we can do is we can use a specific device that we call special light modulator, which we can control the shape of light in space. So we can measure the little amount of light that passes through the system. So we can just right a specific way front on this light pattern, like a specific image such that the light can be focused through this scattering media. So after focusing, you can scan this and construct an image or directly send image information.

Richard Jacobs: So this would allow you to see through objects that normally can’t be seen through?

Hasan Yilmaz: Yes, exactly. But of course, it has limitations. Currently, we can only do this practically through like a few million meters take samples. So it cannot be done to very thick walls for instance. But this is only a technological limitation in order to control it to very thick materials you need very large numbers of pixels maybe million to billion number of pixels which is not currently possible.

Richard Jacobs: Okay. So what would be some of the applications of doing this?

Hasan Yilmaz: One of the applications is actually that people are working on, I am not myself, and I’m not working on immediate applications currently, but there are other groups in Caltech or there are other groups in Paris. What they do is actually one application imaging through scattering mediums such as biological tissue. The reason is again, that’s scattering, and that’s why we cannot use optical light fields like visible light to see inside our body. We can currently only use x-ray because x-ray doesn’t scatter, but x-ray doesn’t give us too much information. So currently there are groups that are working on imaging, deep inside scattering to show using this kind of methods. It’s one example of an application.

Richard Jacobs: So where would you put the device to collimate the lights. You put it like right before the material, like right on top of it. Or you need also a receiving panel behind the objects so you can collect the light that’s coming out of it?

Hasan Yilmaz: Exactly. That’s a very good question. Let’s say if we want to image like inside my body, if there’s not enough light passing through my whole, let’s say full-body, what are the ways to get reflected light, for example, we can like label some targets molecules or target tissue with some sort of the fluorescent dye. So what we can do is we can optimize to enhance this amount of fluorescence that coming from this particular place by shaping way front of the light. So in this case, we will sound bite on the material and look at the reflected light at a different color.

Richard Jacobs: Hmm. Okay. Interesting. How close is this to being used as it been tested in the lab and know what kind of things can be seen?

Hasan Yilmaz: Currently there’s no method that can be applied in clinical studies. It’s still ongoing research. On one hand, there are groups like for example me, I am mostly working on physical properties of this light, understanding how light propagates through these scattering systems. But there are other groups who are trying to develop new techniques, microscopy techniques and for example, usually what they do is they can do imaging inside brain tissue because the brain is also a scattering medium. For example specifically, focus on certain positions in brain-like certain narrows and one another application which is also closely related to imaging is Optogenetics. In Optogenetics, what you do is you have a genetic modified like an animal. This animal’s brain cell can respond to light fields so you can basically control the behavior of animals by stimulating certain neurons with light and the problem is again light scattering here because you cannot address certain specific parts of the brain, but using these kinds of methods, some groups are already can focus at specific areas of the brain. So basically change the behavior of the animal in a different way. That’s one of the applications that people do. But this is not what I do. I am just trying to understand what happens, how light propagates when we do wait from shaping.

Richard Jacobs: Can you control the penetration depth of the light?

Hasan Yilmaz: Yes. I can do that. For example, there is already a theory that is there were Lopes way back in a couple of decades. The very different community conducts metaphysics community that what they study is actually how electrons transport through a copper wire. So electrons are also considered as waves, from quantum mechanics we know that. So what they find is there are some specific solutions of these electron waves, like specific shapes of which that the material behaves like a superconductor. So basically the resistance becomes zero. So which means all of the electrical currents just transports to the system.

So we apply the same theory to optical waves because the light is also optical waves. So it also scatters through this random system. So which means there are specific wave solutions like shapes of a wave that all of the light can propagate through the material independent of its thickness. So essentially this is what I’m working on. What we can, we can enhance the light transmitters by a couple of like two times, three times depending on the thickness. So, which means we can make reflecting material like completely transparent almost. So the advantage of this is, for example, if you want to do imaging very deep inside scattering tissue, the first thing you need is light. You want the light to be there, otherwise, you cannot image anything. So this is the first step of imaging, deep tissue imaging.

Richard Jacobs: So depending on the wavelength of light, you can have them pass through someone’s entire body or pass through a certain depth?

Hasan Yilmaz: Yes, theoretically it’s possible under the condition that there is no absorption. So if there’s only scattering, like if you find a particular wavelength, which is usually like near-infrared, usually our body does not absorb near-infrared very much. So it only scatters. In principle, if you have a lot of degrees of freedom, which means like you can give a very complicated pattern to the way front, it’s theoretically possible to find such a way for all of it can propagate. But this is right now experimentally not possible. But we can see that in our numerical simulations. We also know that already it works.

Richard Jacobs: Okay. So what are you specifically trying to learn about how light moves within an object and how it scatters?

Hasan Yilmaz: For example, we recently discovered it was discovered because it was like a physical effect. As I said, there are specific solutions which we call them open channels. So all of the light can propagate. What we find out is, for example, if you have a light, like a flashlight in a very foggy day. The light that emitted from this flashlight will spread due to fog. It will spread more than ever. So it will behave like diffusion in the system. So we know that typically in scattering systems light diffusers, it lateral spread. What we find is actually in a system where its width is very large and it’s like a thick layer of paint, you can imagine it’s really wide width it is thick but its width is much larger than its thickness. What we find out that if we send this open channel, which totally transmits to the system, this light doesn’t spread, it just goes most straight as if there is no scattering in the transverse direction. So you can imagine it as if like light is propagating through an optical fiber.

Richard Jacobs: How do you find an open channel though? The human body is a complex structure, is there such a thing as an open channel? How could you find that?

Hasan Yilmaz: How we define it. First, we measure the transmission metrics of the system. So what I mean by transmission metrics, choose a certain set of input vectors, which can be different angles. You’ll send light at different things. But a lot of angles, like thousands to 10,000 of angles and then you measure the transmitter light field behind. You can also measure the reflected, but we are measuring the transmitters then at this point you can find such a metrics that, this metrics has the all the information that if you a certain specific light at inputs, you can predict that light at the output basically. So these metrics give you the whole information about the input and output of light fields. So this is a kind of metrics. So which means for example, if we want a specific output on the other side, we know what to send from the input.

Richard Jacobs: Okay. But again, to find a channel through a material, does it have to be a homogenous material or a very structured material like a crystal or can it be something that’s a worthless?

Hasan Yilmaz:  It can be anything. It can be like an amorphous, like for example, opaque systems are amorphous. We know that’s what we study actually particularly to study complex systems. It can be any type of material that you can measure, this is very general, but as long as the light field behaves linear, well what I mean by linear is let’s say, I sent an input field one and I get an input field of two and I sent another input field of three and I get an output field of four then I add one and three I should get one and three, two and four, two plus four. So which means I can add this input fields. So as long as this applies, it will always work, which means this will work for biological materials, opaque materials, glass or opaque glass or even very dark materials as long as there’s some light propagating.

Richard Jacobs: Hmm. Interesting. So what are some of the stumbling blocks for this which could be used?

Hasan Yilmaz: One of the things is, this is a very large amount of information as you can imagine like it’s a big metrics you measure. So it takes time to do this measurement. Currently, for example, the device that I’m using is a liquid crystal special light modulator, which is actually not so different than the CD screen you have in our daily lives, like in many devices and this kind of measurement for 2000 inputs of the metrics it takes about half an hour. But there are other types of devices like digital micrometer devices. They are called DMD in short and they’re a lot faster. It composes of very tiny meters of millions of meters. So you can just control that angle. With these devices, you can do this measurement sub cycles and there are also a new type of devices being developed which is targeting for sub milliseconds, which is especially very necessary for doing this through brain tissue because we know that brain tissue changes very fast. So you need to do this measurement before the brain tissue changes. This is one of the challenges, as a measurement speed, but it will be overcome in time, I’m pretty sure. I would say this is the main challenge now and also the number of pixels. Right now we can address like 1 million to 5 million pixels. So in the future, it would be better if we can have like 10 million 100 million. But this is currently not possible.

Richard Jacobs: So what are some of the uses of this that you envisioned and when you can think that it would be commercially available?

Hasan Yilmaz: Specialized modulators already has been used for different purposes. Like for example, one name is adapt to optics. This kind of imaging to scattering medium, I think it will still take a while before it becomes commercial. Because the main reason is you cannot classify biological tissues one type of material. Like it’s not like the white paint. It’s much more complicated. So, which means we still need to understand more about the structures other than the techniques.

Richard Jacobs: Okay. Well, what’s your estimation, how long is it going to take? So we can go through a simple structures? Imaging through biological structures would take quite a while, like going through more simple structures. When do you think that would be possible or is it possible now?

Hasan Yilmaz: Actually in principle it’s already possible to image very simple structures, but there is no market for it. The problem is there is not enough number of customers for this purpose. So that’s why if you want to use this device for brain imaging, like a kind of some different version of tomography in Dr. Collegium. I think this will take still one or two decades at least until we see something really useful.

Richard Jacobs: Oh Wow. One or two decades.

Hasan Yilmaz: Because there are a lot of problems, not only technological problems, but still there are some scientific questions to be addressed.

Richard Jacobs: Okay. What is the best way for listeners to read papers that you put out or to get in contact with you or to find out more?

Hasan Yilmaz:  The best way to learn all these things you mean?

Richard Jacobs: Yes. For people to learn more. Maybe to contact you.

Hasan Yilmaz:  I would say, even though its seem like an advanced topic. All the basis are actually in studying electromagnetic theory. If for example, people understand how electromagnetic waves behave in the different environments, like just basic electromagnetics together with wave physics, it’s pretty straightforward to learn all this topic. In terms of experiments, like experimental techniques, I think it’s mostly the way to learn it is experience, to do experiments to work in the lab. I would say that’s the easiest way.

Richard Jacobs: Okay. Very good. Well Hasan thank you for coming on the podcast. I appreciate it

Hasan Yilmaz: Thank you.