Lenses are almost as old as civilization itself. The ancient Egyptians, Greeks, and Babylonians all developed lenses made from polished quartz and used them for simple magnification. Later, 17th-century scientists combined lenses to make telescopes and microscopes, instruments that changed our view of the universe and our position within it.
Now lenses are being reinvented by the process of photolithography, which carves subwavelength features onto flat sheets of glass. Today, Alan She and pals at Harvard University in Massachusetts show how to arrange these features in ways that scatter light with greater control than has ever been possible. They say the resulting “metalenses” are set to revolutionize imaging and usher in a new era of optical processing.
Lens making has always been a tricky business. It is generally done by pouring molten glass, or silicon dioxide, into a mold and allowing it to set before grinding and polishing it into the required shape. This is a time-consuming business that is significantly different from the manufacturing processes for light-sensing components on microchips.
So a way of making lenses on chips in the same way would be hugely useful. It would allow lenses to be fabricated in the same plants as other microelectronic components, even at the same time.
She and co show how this process is now possible. The key idea is that tiny features, smaller than the wavelength of light, can manipulate it. For example, white light can be broken into its component colors by reflecting it off a surface into which are carved a set of parallel trenches that have the same scale as the wavelength of light.
Physicists have played with so-called diffraction gratings for centuries. But photolithography makes it possible to take the idea much further by creating a wider range of features and varying their shape and orientation.
Since the 1960s, photolithography has produced ever smaller features on silicon chips. In 1970, this technique could carve shapes in silicon with a scale of around 10 micrometers. By 1985, feature size had dropped to one micrometer, and by 1998, to 250 nanometers. Today, the chip industry makes features around 10 nanometers in size.
Visible light has a wavelength of 400 to 700 nanometers, so the chip industry has been able to make features of this size for some time. But only recently have researchers begun to investigate how these features can be arranged on flat sheets of silicon dioxide to create metalenses that bend light.
The process begins with a silicon dioxide wafer onto which is deposited a thin layer of silicon covered in a photoresist pattern. The silicon below is then carved away using ultraviolet light. Washing away the remaining photoresist leaves the unexposed silicon in the desired shape.
She and co use this process to create a periodic array of silicon pillars on glass that scatter visible light as it passes through. And by carefully controlling the spacing between the pillars, the team can bring the light to a focus.
Specific pillar spacings determine the precise optical properties of this lens. For example, the researchers can control chromatic aberration to determine where light of different colors comes to a focus.
In imaging lenses, chromatic aberration must be minimized—it otherwise produces the colored fringes around objects viewed through cheap toy telescopes. But in spectrographs, different colors must be brought to focus in different places. She and co can do either.
Neither do these lenses suffer from spherical aberration, a common problem with ordinary lenses caused by their three-dimensional spherical shape. Metalenses do not have this problem because they are flat. Indeed, they are similar to the theoretical “ideal lenses” that undergraduate physicists study in optics courses.
Of course, physicists have been able to make flat lenses, such as Fresnel lenses, for decades. But they have always been hard to make.
The key advance here is that metalenses, because they can be fabricated in the same way as microchips, can be mass-produced with subwavelength surface features. She and co make dozens of them on a single silica wafer. Each of these lenses is less than a micrometer thick, with a diameter of 20 millimeters and a focal length of 50 millimeters.
“We envision a manufacturing transition from using machined or moulded optics to lithographically patterned optics, where they can be mass produced with the similar scale and precision as IC chips.” say She and co.
And they can do this with chip fabrication technology that is more than a decade old. That will give old fab plants a new lease on life. “State-of-the-art equipment is useful, but not necessarily required.” say She and co.
Metalenses have a wide range of applications. The most obvious is imaging. Flat lenses will make imaging systems thinner and simpler. But crucially, since metalenses can be fabricated in the same process as the electronic components for sensing light, they will be cheaper.
So cameras for smartphones, laptops, and augmented-reality imaging systems will suddenly become smaller and less expensive to make. They could even be printed onto the end of optical fibers to acts as endoscopes.
Astronomers could have some fun too. These lenses are significantly lighter and thinner than the behemoths they have launched into orbit in observatories such as the Hubble Space Telescope. A new generation of space-based astronomy and Earth observing beckons.
But it is within chips themselves that this technology could have the biggest impact. The technique makes it possible to build complex optical bench-type systems into chips for optical processing.
And there are further advances in the pipeline. One possibility is to change the properties of metalenses in real time using electric fields. That raises the prospect of lenses that change focal length with voltage—or, more significant, that switch light.