Power/Performance Bits: July 3

Transient electronics; lensless camera; reducing screen reflection.


Transient electronics
Researchers at Vanderbilt University took a new approach to transient electronics, creating circuits that, rather than requiring active behavior to destruct, will dissolve if not kept above a certain temperature.

Using silver nanowires embedded in a polymer that dissolves in water below 32 degrees Celsius – between body and room temperature – the team made a simple circuit board that, so far, just turns on an LED light. However, they believe its potential applications are far more promising.

“Let’s say you use this technology to make an RFID wireless tag,” said Leon Bellan, assistant professor of mechanical and biomedical engineering at Vanderbilt University. “You could implant important information in a person, and body temperature would keep it intact. If the tag were removed or the bearer died, it would dissolve. You could use it for implanted medical devices as well – to cause them to disintegrate, it would only require applying ice to the skin.”

In the lab, the tiny circuits stay operational in water warmed by a hot plate. Turn off the hot plate, and they start dissolving in minutes.

(Source: Vanderbilt University)

The technique used was developed last year, when a polymer pushed through a cotton candy machine was used to build capillary networks, potentially of use for artificial organs.

In this system, silver nanowires are held together in the polymer so that they touch, and as long as the polymer doesn’t dissolve, the nanowires will form a path to conduct electricity. Trigger the polymer to dissolve by lowering the temperature, and the nanowire network disintegrates, destroying the conductive path.

The team’s next steps are integrating semiconductors to make transistors and ensuring users can interact wirelessly with the device.

Lensless camera
Engineers at Caltech developed a new camera design that replaces the lenses with an ultra-thin optical phased array (OPA). The OPA does computationally what lenses do using large pieces of glass: it manipulates incoming light to capture an image.

Lenses have a curve that bends the path of incoming light and focuses it onto a piece of film or an image sensor. The OPA has a large array of light receivers, each of which can individually add a tightly controlled time delay (or phase shift) to the light it receives, enabling the camera to selectively look in different directions and focus on different things.

“Here, like most other things in life, timing is everything. With our new system, you can selectively look in a desired direction and at a very small part of the picture in front of you at any given time, by controlling the timing with femto-second–quadrillionth of a second–precision,” said Ali Hajimiri, Professor of Electrical Engineering and Medical Engineering at Caltech.

“We’ve created a single thin layer of integrated silicon photonics that emulates the lens and sensor of a digital camera, reducing the thickness and cost of digital cameras. It can mimic a regular lens, but can switch from a fish-eye to a telephoto lens instantaneously — with just a simple adjustment in the way the array receives light,” Hajimiri said.

The OPA chip placed on a penny for scale. (Source: Caltech/Hajimiri Lab)

Phased arrays are collections of individual transmitters, all sending out the same signal as waves. These waves interfere with each other constructively and destructively, amplifying the signal in one direction while canceling it out elsewhere. Thus, an array can create a tightly focused beam of signal, which can be steered in different directions by staggering the timing of transmissions made at various points across the array.

A similar principle is used in reverse in an optical phased array receiver, which is the basis for the new camera. Light waves that are received by each element across the array cancel each other from all directions, except for one. In that direction, the waves amplify each other to create a focused “gaze” that can be electronically controlled.

Last year, the team rolled out a one-dimensional version of the camera that was capable of detecting images in a line, such that it acted like a lensless barcode reader but with no mechanically moving parts. This year’s advance was to build the first two-dimensional array capable of creating a full image. This 2D lensless camera has an array composed of just 64 light receivers in an 8 by 8 grid. The resulting image has low resolution. But this system represents a proof of concept for a fundamental rethinking of camera technology, Hajimiri and his colleagues say.

Next, the team will work on scaling up the camera by designing chips that enable much larger receivers with higher resolution and sensitivity.

Reducing screen reflection
Researchers at the University of Central Florida created an antireflective film inspired by the nanostructures found on moth eyes.

The antireflection film exhibits a surface reflection of just 0.23%, much lower than the iPhone’s surface reflection of 4.4%, the researchers say. Reflection is the major reason it’s difficult to read a phone screen in bright sunlight, as the strong light reflecting off the screen’s surface washes out the display.

A major advantage would be improving device battery power, if increasing a screen’s brightness in response to ambient light is no longer necessary.

“Using our flexible anti-reflection film on smartphones and tablets will make the screen bright and sharp, even when viewed outside,” said  Shin-Tson Wu, a professor at the College of Optics and Photonics, University of Central Florida. “In addition to exhibiting low reflection, our nature-inspired film is also scratch resistant and self-cleaning, which would protect touch screens from dust and fingerprints.”

The eyes of moths are covered with a pattern of antireflective nanostructures that allow moths to see in the dark and prevent eye reflections that might be seen by predators. The nanostructure pattern has found use in reducing the sunlight reflected off the surface of solar cells.

The new film contains tiny uniform dimples, each about 100 nanometers in diameter. The coating can also be used with flexible display applications.

Researchers created a film of moth-eye-like nanostructures that can improve the sunlight visibility of screens on mobile phones and tablets. The images show the nanostructures from above (left) and from the side (right). (Source: Shin-Tson Wu, College of Optics and Photonics, University of Central Florida)

The researchers developed a fabrication technique that uses self-assembled nanospheres to form a precise template that can be used to create the moth-eye-like structure on a coating. The process allowed fabrication of the intricate structure in a film large enough to apply to a mobile screen.

Tests of the film after optimization showed that when viewed in sunlight, glass covered with the new film exhibited a more than four-fold improvement in contrast ratio — the difference between the brightest white and darkest black. When viewed in the shade, glass with the new film showed about a ten-fold improvement in contrast ratio.

The researchers are now working to further improve the anti-reflection film’s mechanical properties, including finding the best balance of surface hardness and flexibility, to make the film surface rugged enough for long-term use on touch screens. They are also using the simulation model to further optimize the moth-eye structure’s shape and size to obtain even better optical performance.