For the Lowly Incandescent Bulb, a Ray of Hope

MANOUSH ZOMORODI: OK, once upon a time there was an ordinary incandescent light bulb, like Edison made. It shone brightly, but it wasted a lot of energy put into it as heat. Then came the compact fluorescent. And as costs have dropped in recent years, consumers have increasingly been switching to LED technology for their lighting needs. But in work published this week in the journal Nature Nanotechnology, researchers report that nanophotonic devices may be able to breathe new life into that old school incandescent technology. And joining me now are two of the authors of that study, Marin Soljacic, Professor of Physics at MIT, and Ognjen Ilic, a post-doctoral student in the Physics Department there. Welcome to you both. Marin, apologies for not saying your name right.

MARIN SOLJACIC: Hi. No, you said it quite well.

MANOUSH ZOMORODI: Oh, OK. Good. OK, good. So can you describe how this works? How do you deal with all that lost heat energy that typically escapes from a light bulb?

MARIN SOLJACIC: So as you described, when you heat up a filament in an incandescent light bulb, it starts glowing. And a portion– it’s actually a fairly small portion, only a few percent– goes into visible light, which is desired. But the majority, like 95%, goes into infrared light, which I cannot see, and it therefore presents losses. And in this current work, what we did is we implemented this nanophotonic filter, which is placed in front of the filament and lets visible light through. But it reflects infrared light back towards the filament, so thereby recycling light. And then there it gets absorbed into the filament, and hopefully next time emitted into visible light. And this way, thereby enhances the overall efficiency of the system.

MANOUSH ZOMORODI: Huh. And Ognjen, how efficient is it, the one that you created?

OGNJEN ILIC: So to be technical, in our experiment, when we send the same amount of power through the device we see about three times more light in the visible, so to put it.

MANOUSH ZOMORODI: And so why– explain why bother. LEDs are getting cheaper, they’re pretty efficient, they’re improving a little bit slowly.

MARIN SOLJACIC: Absolutely. LEDs are a great thing, and people should be buying LEDs. People should be working on LEDs. They’re actually improving quite rapidly. Our group does work on LEDs, so our work was by no means a motivation to try to displace LEDs, or anything like that. The main motivation of our work was to study this principle of recycling light, and how well it can be implemented. And we picked this particularly challenging system of incandescent light bulb as a showcase to demonstrate how well this can be done.

And the motivation for studying recycling of light is because that has many potential applications for various different types of energy conversion. So that is really the reason to be doing this, by no means to displace LEDs, or anything like that.

MANOUSH ZOMORODI: We’ll get to some of those deeper reasons. But I have to ask you a rather superficial question. We put LED lights on our Christmas tree, and the color was absolutely atrocious. So is light color something that you are looking at that you could bring back, have the efficiency and the beauty that comes with an incandescent?

MARIN SOLJACIC: Right. That was the reason why people liked incandescent light bulbs for such a long time, because they had this really nice color. But LEDs are getting rapidly better on that front, also.

MANOUSH ZOMORODI: OK. Go ahead, Ognjen, yeah.

OGNJEN ILIC: As Marin said, yes. So in our case, it’s something that in our approach we could also, based on the application optimize for color, as well, if we wanted to, efficiency or color, or whatever. Because the underlying variating element has all the richness of the color. So the approach can in principle preserve it.

MANOUSH ZOMORODI: So could you think of this as a filter for light, a filter that only lets through the colors of the light that you want?

OGNJEN ILIC: Yes, certainly. That is a very good way to think about it.

MANOUSH ZOMORODI: So how easy is it to tune these structures to different wavelengths? Could you make a material that works in other parts of the spectrum, Ognjen?

OGNJEN ILIC: That’s actually a great point. So we have, especially nowadays, given the nanophotonic techniques that we have, we can really tailor this concept to a lot of different applications. And in particular, lighting was just something we wanted to do as a little proof of concept. But for other applications, a lot of them energy conversion applications, you would kind of shift these filters to different regions of the spectrum that would give you the best performance.

MANOUSH ZOMORODI: So tell me about how you actually did this in the lab.

OGNJEN ILIC: So in the lab, basically the idea is that this filtering structure is there. They’re kind of like a nano layered cake. So you stack different materials of different optical properties, and their cumulative effect will give you the optical properties that you want. It’s a combination of photonic theory, optimisation, and in the end, actually fabricating that thing.

MANOUSH ZOMORODI: I’m Manoush Zomorodi. This is Science Friday from PRI, Public Radio International.

So Marin, can you just explain– so what other applications are there for this potentially new light bulb? Where can we see this being used, but also what can we learn from it, more broadly?

MARIN SOLJACIC: So applications are really for this concept of recycling light. And another one that we are working on has to do with this thing called thermal photovoltaics, which is a way of converting actually heat or thermal energy into electricity.

Now, there are many different ways of converting heat into electricity. And this particular one, the way that it works is you get something to a pretty high temperature, let’s say 1,200 Celsius or so, so it starts glowing. And then you use a photovoltaic, so it’s sort of like a solar cell, but not the identical one that you would use on the roof of your house. You place it in front of this hot object, and then it converts this emitted light into electricity.

And as you can see here, also, it’s desirable that you have certain wavelengths which you can convert well into electricity, and very desirable. But the ones that you cannot convert, you want to reflect them back so they will be absorbed, and hopefully next time emitted in the right wavelength range. So this is just one example of where this concept of recycling light could be beneficial for energy conversion.

MANOUSH ZOMORODI: And in terms of what you’ve done so far, you mentioned that you’d made 6% efficiency. How high could the efficiency potentially get, and what would need to happen for it to really be worthwhile?

OGNJEN ILIC: So it can go up quite high. So really, in terms of the photonics theory, we know all the pieces to the puzzle. Now, actually making something like this in the lab would mean that we have to solve some challenges of fabricating these complicated structures, but also ensuring their mechanical stability, their reliability, and so on. So there’s a lot of engineering challenges, as well, that would have to be solved before we can really fulfill the so to speak, the potential of this approach.

MANOUSH ZOMORODI: And Marin, just to sum up for our listeners, why should they be following along with this? Is this Just a matter of energy conservation, or because for a lot of consumers it was like, oh, I got to switch my light bulbs over. They’re more expensive, but I understand that the long term repercussions will be worthwhile. Can you just sort of make it usable for us?

MARIN SOLJACIC: Yeah, so I think the main point of this is that our society faces this huge, huge challenge of global warming, and all the energy conversion challenges that come with it. And in order to address this huge challenge, we kind of need to have as many tools in our tool box to handle it. And you can think of this recycling of light as one more tool, which will then allow us to try to tackle a wide variety of energy conversion problems.

MANOUSH ZOMORODI: I’d like to thank you both so much for being with me here today. Marin Soljacic, Professor of Physics at MIT, an Ognjen Ilic, a post doctoral student in the physics department there. Thank you both so, so much.

MARIN SOLJACIC: Thank you.

OGNJEN ILIC: Thank you very much.

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