Power/Performance Bits: Dec. 1

Self-erasing chip
Researchers from the University of Michigan developed self-erasing chips that could be used to prevent counterfeiting or detect tampering. The technology is based on a new material that temporarily stores energy, changing the color of the light it emits. It self-erases in a matter of days, or it can be erased on demand.

“It’s very hard to detect whether a device has been tampered with. It may operate normally, but it may be doing more than it should, sending information to a third party,” said Parag Deotare, assistant professor of electrical engineering and computer science at University of Michigan.

The self-erasing chips are built from a three-atom-thick layer of a tungsten diselenide (WSe₂) semiconductor laid atop a thin film of molecules based on azobenzenes, which shrink in reaction to UV light. Those molecules tug on the semiconductor in turn, causing it to emit slightly longer wavelengths of light.

The stretched azobenzene naturally gives up its stored energy over the course of about seven days in the dark. The time is shorter when exposed to heat and light, or longer if kept in dark, cool conditions.

A bar code could be written or the chip and placed on integrated circuit chips or circuit boards, to detect whether they had been opened or replaced in transit. If the lifespan of the bar codes was extended, the team sees potential for writing them into devices as hardware analogues of software authorization keys.

Whatever was written on the chip, be it an authentication bar code or a secret message, would disappear when the azobenzene stopped stretching the semiconductor. Alternatively, it can be erased all at once with a flash of blue light. Once erased, the chip can record a new message or bar code.

Next, the researchers will work on extending the amount of time that the material can keep the message intact for use as an anti-counterfeit measure.

Lensless imaging
Researchers at Pennsylvania State University built a camera that uses reconfigurable particle-based masks, rather than lenses, to image objects.

Created from microscopic gold wires, the mask scatters the light reflected off the nearby object it is imaging. An image sensor collects the light. An electric current rearranges the particles in the mask, producing a new mask with every iteration, and the system records each new image. The multiple light captures are then computationally reconstructed into the original object image, resulting in highly improved resolution and quality.

“We are not the only group to do lens-free imaging,” explained Jennifer Miller, a doctoral candidate in chemistry at Penn State. “What is different about our work is that typically you would need to make multiple masks and physically move them around to get multiple images. This becomes bulky and expensive and negates some of the simplicity that is the advantage of lens-free imaging.”

Another advantage lies in microscopy, where field of view and power of resolution trade off. With the lens-free imaging, it is possible to combine a wide field of view with high magnification.

“Traditional masks are passive,” said Cheng-Yu Wang, doctoral candidate in electrical engineering at Penn State. “We can add functionalization to our microwire, like polarization, selectivity and plasmonic effects, that make our imaging system more powerful.”

The researchers see potential applications in lower-cost and faster disease diagnosis and improved optical microscopy, particularly in developing countries where high-end microscopes are not available.

Flat fisheye lens
Engineers at MIT and the University of Massachusetts at Lowell have designed a wide-angle lens that is completely flat.

Typically, ultra-wide-angle images are captured by fisheye lenses that use highly curved pieces of glass to distort incoming light. Instead, the new metalens consists of a single flat, millimeter-thin piece of calcium fluoride with a thin film of lead telluride deposited on one side. Lithographic techniques are used to carve a pattern of optical structures into the film.

These structures scatter incoming light to produce panoramic images, just as a conventional curved, multielement fisheye lens assembly would. The lens works in the infrared part of the spectrum, but modifying it to capture images using visible light is possible.

“This design comes as somewhat of a surprise, because some have thought it would be impossible to make a metalens with an ultra-wide-field view,” says Juejun Hu, associate professor in MIT’s Department of Materials Science and Engineering. “The fact that this can actually realize fisheye images is completely outside expectation.”

On the front of the lens is an optical aperture. “When light comes in through this aperture, it will refract at the first surface of the glass, and then will get angularly dispersed,” explained Mikhail Shalaginov of MIT. “The light will then hit different parts of the backside, from different and yet continuous angles. As long as you design the back side properly, you can be sure to achieve high-quality imaging across the entire panoramic view.”

In one demonstration, the new lens was tuned to operate in the mid-infrared region of the spectrum. The team used the imaging setup equipped with the metalens to snap pictures of a striped target. They then compared the quality of pictures taken at various angles across the scene, and found the new lens produced images of the stripes that were crisp and clear, even at the edges of the camera’s view, spanning nearly 180 degrees.

Another test, at near-infrared, utilized a simulation used to test imaging instruments. “The key question was, does the lens cover the entire field of view? And we see that it captures everything across the panorama,” said Tian Gu of MIT. “You can see buildings and people, and the resolution is very good, regardless of whether you’re looking at the center or the edges.”

The team says the new lens can be adapted to other wavelengths of light. To make a similar flat fisheye lens for visible light, for instance, Hu said the optical features may have to be made smaller than they are now, to better refract that particular range of wavelengths. The lens material would also have to change. But the general architecture remains the same.

Applications range include wide-angle but compact lenses for phones, panoramic projectors, depth sensors, and medical imaging devices like endoscopes.