Harvard Metamaterial Flat lens
July 4, 2016
After a first, unfortunate, experience in elementary school with a pinhole camera advertised on the back pages of a comic book, I saved enough money to buy a real camera, a 35-mm film format rangefinder focusing camera, a used Argus C3.
In those days, long before the Internet, the best way to learn about technology was by subscribing to a magazine on the subject. A side-benefit of having such a magazine was the interesting array of products advertised therein, with the affordable products located towards the back. These ads were an education in themselves. The principal part of a camera is its lens, and there was a plethora of articles and ads on lenses alone.
After a while, I purchased items to outfit a home darkroom. Photography in those days was a chemical process, and it was interesting to me from that aspect, also. Not having enough money to buy a commercial enlarger for my darkroom, I decided to build my own. I was happy to realize that the lens of my camera would work as an enlarger lens, but I needed to buy another type of lens, a condenser lens.
Condenser lenses might be familiar to my readers who are microscope users. Their purpose in an enlarger is to transform the point source of light from a tungsten lamp to a wide, parallel shaft of light. The parallel light passes through the photographic negative, which is imaged onto photographic paper by the exit lens. More light passes through the exit lens when another condenser lens is placed below the first.
As I found, a condenser lens for this purpose was huge and heavy, since a lot of glass is needed to form parallel rays of light over a large area (I designed the enlarger to accommodate negatives up to 120-size, which were 60 mm x 60 mm). Over the years, high refractive index were developed and the processing of optical materials has progressed to the point at which extremely small lenses of exceptional quality are now integrated into every cellphone and tablet computer.
The technological battle cry of the last few decades has been, "smaller is better," with computer chips now containing more than a billion transistors. While cellphone lenses are quite small, there are still applications for lenses that are even smaller. As one example, small, lightweight lenses are needed for drones, some of which are the size of flying insects. I wrote about one such drone in an earlier article (Pop-Up Robots, February 23, 2012).
While a single large lens will focus light, it's possible to combine many, smaller optical elements to do the same thing. The problem, however, is arranging these so that they act together and do not distort the image. Harvard University's Federico Capasso and his research group have successfully made an extremely thin optical lens using an array of smaller optical elements.[1-3] The lens is made from a computed array of high refractive index titanium dioxide (TiO2) pillars on a silica glass (SiO2) substrate (see figures).
The metalens, which is just 2 mm in dimension, has a numerical aperture of 0.8, and it focuses light with efficiencies as high as 86%.[1,3] The focusing was superb, being diffraction-limited, but such a lens operates at just a particular wavelength of light. It is out-of-focus at other wavelengths. However, such single wavelength lenses are useful for such things as laser microscopy, photolithography, and spectroscopy.
Once the technology of being able to build the phase-shifting titanium dioxide pillars is developed, the device design is quite straightforward. Geometrical optics provides a solution for the phase-shift at each region of the plate (x,y) that's needed to focus a spot of light at a focal length f for a wavelength λ.
Since the length L is much greater than the width W, the phase shift of light through the length at each point can be adjusted through rotation of the pillars.
Modern computer technology allows an easy way to prepare a lithography mask of pillar elements that satisfies this equation.
Says Capasso, the senior author of the paper describing this lens,
"This technology is potentially revolutionary because it works in the visible spectrum, which means it has the capacity to replace lenses in all kinds of devices, from microscopes to cameras, to displays and cell phones... In the near future, metalenses will be manufactured/a> on a large scale at a small fraction of the cost of conventional lenses, using the foundries that mass produce microprocessors and memory chips."
patent applications have been filed, and commercial opportunities are being explored. This work was supported in part by the Air Force Office of Scientific Research, the Charles Stark Draper Laboratory, and Thorlabs Inc. Fabrication was carried out in the Harvard Center for Nanoscale Systems, which is supported by the National Science Foundation.
- Mohammadreza Khorasaninejad, Wei Ting Chen, Robert C. Devlin, Jaewon Oh, Alexander Y. Zhu, and Federico Capasso, "Metalenses at visible wavelengths: Diffraction-limited focusing and subwavelength resolution imaging," Science, vol. 352, no. 6290 (June 3, 2016), pp. 1190-1194, DOI: 10.1126/science.aaf6644.
- Leah Burrows, "Metalens works in the visible spectrum, sees smaller than a wavelength of light," Harvard University Press Release, June 2, 2016.
- Roland Pease, "Flat lens promises possible revolution in optics," BBC, June 3, 2016.
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