Leaving Convention Behind: Bending Multicolored Light with a Flat Lens

A good pair of lenses can transform your life, assuming you are one of the 4.2 billion people in the world with less-than-perfect eyesight. A good lens can also transform our understanding of life and this world we inhabit. From the discovery of microorganisms to the moons of Jupiter, lenses shine a light on things too small or too faint to see with the naked eye. They also help us capture and preserve the milestones and everyday moments that make up a life. That’s a lot to ask of ground glass.

SEAS researchers have developed the first flat lens that works within a continual bandwidth of colors, from blue to green. This bandwidth, close to that of an LED, paves the way for new applications in imaging, spectroscopy and sensing.
Image Credit: Courtesy of Vyshakh Sanjeev/ Harvard SEAS.

In work just published in the American Chemical Society’s journal Nano Letters, a team of scientists in the lab of Federico Capasso at Harvard University demonstrates a new type of flat lens that could replace the curved, polished pieces of glass that live in our phones, cameras, and microscopes. With the potential to resolve finer details and be more cost-effective, the new lenses are also much tinier and easier to manufacture. This work builds on a previous version by the same lab, but overcomes one of its key limitations.

Lenses gather and focus light. When appropriately designed for the task at hand, they create a sharp, clear image of the thing you want to see. This is accomplished through geometry and physics. Air and glass have different optical properties, which means that light travels differently through them. Light traveling from air into glass, or vice versa, is bent in a direction and amount that depends on the composition of the glass and the curvature of the lens.

We’ve gotten pretty good at making modern lenses that are barely noticeable, like those in your cell phone camera, but compared to the ever-shrinking electronic components in that same cell phone, lenses are huge. Is it possible to shrink them down to a much tinier scale, to a design that could be manufactured more easily, cost loss, weigh less, and produce better quality images? In a nutshell, it appears so.

In the summer of 2016, the same lab at Harvard demonstrated a new, tiny, flat lens that can efficiently focus visible light, using an entirely different approach than traditional lenses. Instead of curved glass, they made the lens from a metamaterial, a material with a structure designed to give it properties that aren’t found in nature. This “metalens” consisted of a thin layer of transparent quartz covered in strategically placed nanoscale pillars. Although it was through a different physical process and required much less material, the metalens did the same thing as a glass lens—bend light toward a focal point. This technique could lead to lenses that are lighter, easy to manufacture, and have the potential to resolve even finer details. However, there was one major drawback. The lens could only focus light of one color at a time.

Here’s the problem. When you send white light into a prism, you get a rainbow on the other side. This is because the prism bends each wavelength (color) of light by a slightly different amount. This effect isn’t unique to a prism or its geometry. It’s called chromatic aberration and, in the cases of a lens, it means that each wavelength will focus at a slightly different point. That’s not a good thing if you want a sharp image of a multicolored object.

Chromatic aberration is an issue for traditional lenses as well as metalenses. In the case of traditional lenses, you can counter chromatic aberration by adding additional lenses to the system. This works well, but adds bulk, complexity, and expense. In this new research, the scientists demonstrate the first flat lens that effectively focuses visible light over a continuous range of wavelengths. This means that metalenses could be a viable option for many applications—some that have never been possible before.

The key, the researchers found, was in optimizing the design so that a kind of corrective factor emerged. With the aid of a mathematical model, the team carefully considered how the height, width, and shape of the nanoscale pillars and the spacing between them influenced the light. In doing so, they landed on a pattern of nanoscale pillars that introduces bending in a way that counteracts the effects of chromatic aberration. With this technique, they were able to focus light with wavelengths between 490 and 550 nanometers (blue to green) all in exactly the same place.

A scanning electron microscope image shows a side-view of the metalens, with nanopillars optimized to focus colors without chromatic aberration. Scale bar: 200 nm.
Image Credit: Courtesy of the Capasso Lab/Harvard SEAS.

This is an exciting step forward. The team is working on broadening the bandwidth even further, but this new work shows that metalenses have the potential to detect fluorescent signals and image objects illuminated by LEDs, among other possibilities. It is also a vibrant demonstration that advances in science aren’t bound by convention and tradition. This isn’t just an upgraded prescription, it’s a whole new approach to seeing our world more clearly.

Kendra Redmond

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