Professor Nina Vaidya has developed a new kind of optical concentrator — Axially Graded Index Lens (AGILE) — that can passively focus the sun onto a photovoltaic cell from any angle to reduce the amount of photovoltaic material needed for a given amount of power generation.

Tech Briefs: The thing that first caught my attention was memories of when I was a little boy with my father in the park and a magnifying glass burning leaves — that was great fun.

Professor Nina Vaidya, who has developed a new kind of optical concentrator.

Professor Nina Vaidya: The writer of the Stanford news article on our work, Laura Castañón, suggested that way of communicating our project. As scientists, we aim to articulate ideas in a way that can reach the public. All day, in the lab or meetings with other engineers and scientists we talk in a certain way and use subject-specific jargon. The Stanford news article helped me explain our Axially Graded Index Lens (AGILE) technology and its impact, while reporting on our recent manuscript.

Tech Briefs: What led you to this project?

Vaidya: That’s a good question. When I first started out, I moved from a business consulting job in Europe to Stanford for my PhD as I was eager to get back to scientific research, especially in clean energy and sustainability. In my first quarter at Stanford, I distinctly remember Professor Olav Solgaard’s ‘Micro and Nano Optical Device Design’ class. He asked us to do an individual report on a new idea linked to optics, photonics, or MicroElectroMechanical Systems (MEMS).

Olav, at one point, asked whether it was possible to design an optical device that can take all the light from all angles and focus it at the same point without moving it towards the source — and he mentioned gradient index optics.

That got me thinking; so, after more discussions and simulations I came up with a design. That individual project report then became my PhD project with Olav as my PhD advisor. Then came two patents, prototypes, and two later papers.

Tech Briefs: Was this just a theoretical idea of his — to ask that question?

Vaidya: Yes, we started from that impossible theoretical dream to design an ideal optical concentrator that does not need to track the source. Then I worked on the feasibility study of our idea and comparison with literature searches, simulations, the theory, and the design optimization. There were people I talked to who told me that to make this idea as a real device with real materials cannot be done — but I thought it could. Olav gave me the research freedom and encouragement to research various optical materials, new fabrication techniques, and to build equipment to explore this unknown space.

It took trial and error and talking to a lot of theorists and experimentalists, companies selling optical materials, lenses, and various solar companies. I even tried to make some materials in the lab with polymers, nanoparticles, nanoporosity, and things like that, to match the theoretical gradient index. Finally, as I described in our paper , I used different types of glasses and polymers. Some of them are custom made by Ohara Glass Corporation, and some are optical grade UV curable polymers from Norland Products Inc. The main thing was that we wanted to grade the refractive index from low to high in a bulk material. But the range of the refractive index difference needed is very large.

Something like a glass fiber optic cable is graded from about 1.45 to 1.5, which works well for communications but is very little for our application. What I wanted was to go all the way from the refractive index of air, which is approximately 1.0, to photovoltaic material, around 3.5 — that’s huge. And not only that, I wanted to be able to use the whole broadband spectrum of sunlight — make AGILE with materials that have this large range of indices but is also highly transparent across the solar spectrum, with compatible thermal expansion coefficients, and robustness so all the different layers can stay together to create the graded index shape.

Sunlight goes from about 250 nanometers to 1300 nanometers — from ultraviolet (short wavelengths) to almost infrared. There are very few materials that have low and high refractive indices but are also transparent and have low losses over this solar spectrum. There is a whole field of applied physics and metamaterials, which is great for theory and research, but a lot of the ideas and prototypes are only functional in a specific narrow-band wavelength for specific applications.

So, I made, not just the material layers from low to high index, but also new fabrication techniques and equipment to make, bond, shape, and test the prototypes. We named our device AGILE (Axially Graded Index Lens).

Tech Briefs: What made it possible to cover these wavelengths?

Vaidya: I did an extensive material search. I bought glasses and polymers from different companies and tested them to measure their light transmission and refractive index across the solar spectrum — basically characterizing various materials and then matching them to be used as the layers in AGILE.

So, I knew, for example, that my first layer needed a very low refractive index but also had to be robust to the environment, I searched each layer with those criteria. Another problem is that some materials are fragile or have very different mechanical properties. For example, they might have very different coefficients of thermal expansion. So, that couldn't work because they all have to be completely bonded on top of each other with perfect interfaces, with no air in the middle or particles/dust. And they have to be machined together and be able to heat/cool together.

So, those were my constraints — it was a huge material search and characterization process.

Tech Briefs: How did you bond the layers?

Vaidya: By basically machining it in a way similar to glass polishing, but at a much higher level, with the help of the crystal shop at Stanford. We wanted to have the surfaces almost nanometer smooth so we could bond them to each other with anodic bonding. For the anodic bonding, I built equipment to be able to heat the joint and pass electricity through it at the flexible cleanroom  at Stanford.

AGILE (Axially Graded Index LEns) concept and concentrator array system. (a) Depiction of the optical concentration action, (b) repeating unit of AGILE, (c) concentrator arrays with built-in anti-reflection and encapsulation. Spatially separated PV cells have the advantage of reduced photovoltaic material use, hence there is lower cost and additional space for cooling and circuitry (Image: Nina Vaidya).

With the polymers, it was easier, because when you start with a liquid, there is some mixing in at the interfaces — it becomes more of the ideal gradient index than the graded index, and it's even better in terms of performance. But of course, glass is more robust in terms of standing up to the environment. I even made AGILE using 3D printing and optical polymers as published in an earlier paper . This opens the door to ultralightweight design-flexible 3D printed precision optics of various shapes, tuned to custom applications, which are as efficient as devices made traditionally using metal or glass.

So, there are pros and cons and application-specific materials and designs. I have also been doing a lot of materials research for aerospace and built space solar power prototypes at Caltech since the AGILE work at Stanford.

Tech Briefs: I’m puzzled as to now it actually can work. When I was burning dry leaves with my magnifying glass, I always had to change the angle of the glass as the day wore on. So, I can’t imagine how your device can hold the same focal point as the sun moves across the sky.

Vaidya: Great question. If you have a magnifying glass and then the sun, you know that the spot on the leaf or your hand will move as the sun does. Now imagine instead of that lens, you have a material with a graded refractive index, so now we are talking about metamaterial — a material engineered in a way that is not found naturally. It's not a homogeneous material like a traditional glass lens. Now we have media that we are not really used to thinking of — it goes from a low index to a high index gradually across the height of the AGILE from a larger aperture to a smaller aperture, with reflective sidewalls. And because of that gradient index, light slows down and curves due to refraction instead of travelling in straight lines.

The light enters our funnel, then starts turning toward the normal. The light doesn't go in straight lines anymore because it is refracted due to the gradient index. Because the index gradient changes gradually — in the theoretical case the light curves perfectly — it reflects from the side walls so that it's always kicked back inside and does not escape out of the AGILE concentrator. This way we can collect light from all angles without solar tracking and can even collect light diffused from cloud cover and atmosphere.

Tech Briefs: So, as the position of the sun changes, how is it that the light reflects to the same point because the sun is entering your funnel from different angles?

Vaidya: The first image in our paper can help clarify how it works.

Since the opening is just air, the gradient index goes from an index of one at the input to a high index at the PV cell. The two important things are that the index changes gradually and that the walls are reflective. So, even if the light comes in at quite an acute angle, say at sunrise or sunset, it will come in, bend toward the normal and either go directly in, or if it's near the edge of the opening it will bend, reflect, and then bend again — focusing all the light at the smaller output aperture of the solar cell. So even at very acute angles, the focus will be at the same position.

Tech Briefs: I think I understand. So, do I have it right that the outer surface of this structure is a mirror, and then the glass is inside of that?

Vaidya: Yes, however, if we design it in a specific way, we can have total internal reflection at the sidewalls, so the metallic reflection would not be needed — but it doesn't work in all the AGILE designs.

Tech Briefs: Has this kind of design been used elsewhere?

Vaidya: No, in fact we have a patent on AGILE. We rigorously looked at all prior art.

I have also worked out AGILE’s generalized design equation to inform how different AGILE concentrators can be made. So, for example, the height or the concentration factor — the geometry of it — and the refractive index range can all be changed and tuned, depending on the application. And I have made several theoretical analyses and simulations to be able to plug in the numbers — sort of a generalized mathematical design guide for applications using AGILE.

A different application, from solar, is solid-state lighting, for example LEDs. You could place an AGILE on top of a LED, to accomplish the reverse of the solar concentrator. Instead of collecting light, we want to emit light efficiently out of the LED into the room. That would work because the optical system is reversible. It’s a passive optical device — it’s not using energy or has moving parts, which makes it a simple and robust solution.

Tech Briefs: What are some of the applications and potential impact of your AGILE technology?

Vaidya: The technology has several applications: Laser coupling, solar aerial vehicles, energy- saving solid-state lighting, for example, LEDs and displays, could all utilize AGILE’s ability to focus light passively.

In today’s energy and urgent climate crisis, AGILE’s potential to make solar arrays more effective and cheaper is going to be its most impactful use. To be able to use this new AGILE concept to create better solar concentrator prototypes has been a meaningful engineering adventure. We need to catalyze engineering solutions to make clean energy and sustainable future a reality.