Power/Performance Bits: April 21

Focus-free lens
Researchers from the University of Utah developed a new lens that doesn’t require focusing. They present it as an alternative to the multiple lenses common in smartphone cameras.

“Our flat lenses can drastically reduce the weight, complexity and cost of cameras and other imaging systems, while increasing their functionality,” said research team leader Rajesh Menon from the University of Utah. “Such optics could enable thinner smartphone cameras, improved and smaller cameras for biomedical imaging such as endoscopy, and more compact cameras for automobiles.”

The new flat lens can maintain focus for objects that are about 6 meters apart from each other. It uses nanostructures patterned on a flat surface rather than bulky glass or plastic to achieve the optical properties that control the way light travels.

“The new lens eliminates the need for focusing and allows any camera to keep all the objects in focus simultaneously,” said Menon. “Conventional cameras also use multiple lenses to keep different colors of light in focus simultaneously. Since our design is very general, we can also use it to create a single flat lens that focuses all colors of light, drastically simplifying cameras even further.”

Traditional lenses transform parallel light waves into spherical waves that converge into a focal spot. The researchers found that waves with other shapes could provide a similar effect, increasing the number of possible lens designs.

“In stark contrast to what is taught in optics textbooks, our research has shown that there is more than one way that light transmission is affected by an ideal lens – a concept known as pupil function,” said Menon. “This opened essentially infinite possibilities for the lens pupil function, and we searched through these possibilities for one that achieved an extreme depth of focus.”

After choosing the best lens design for depth of focus, the researchers used nanofabrication techniques to make a prototype lens. Experiments confirmed that the new lens performed as expected and achieved a depth of focus several orders of magnitude larger than that of an equivalent conventional lens.

The researchers demonstrated the new lens using infrared light and relatively low numerical aperture, which characterizes the range of angles over which the lens can accept or emit light. They plan to extend the lens to larger numerical apertures and to use it with the full visible light spectrum. Work to ensure that the lenses could be mass manufactured is also needed before they could be commercialized.

“This new lens could have many interesting applications outside photography such as creating highly efficient illumination for LIDAR that is critical for many autonomous systems, including self-driving cars,” said Menon.

The researchers say the design approach they used could be expanded to create optical components with any number of properties such as extreme bandwidth, easier manufacturability or lower cost.

Battery separator
Nanoengineers at the University of California San Diego developed a safety feature that prevents lithium metal batteries from rapidly heating up and catching fire in case of an internal short circuit.

The team’s work focused on the separator that serves as a barrier between the anode and cathode, so that it slows down the flow of energy (and thus heat) that builds up inside the battery when it short circuits.

“We’re not trying to stop battery failure from happening. We’re making it much safer so that when it does fail, the battery doesn’t catastrophically catch on fire or explode,” said Matthew Gonzalez, a Ph.D. student at UC San Diego.

In lithium metal batteries, needle-like dendrites can form and pierce the separator, causing a short circuit that can lead to the battery no longer working and, in extreme cases, fires.

The separator that the team developed essentially softens this blow. One side is covered by a thin, partially conductive web of carbon nanotubes that intercepts any dendrites that form. When a dendrite punctures the separator and hits this web, electrons now have a pathway through which they can slowly drain out rather than rush straight towards the cathode all at once.

To make the new separator, a slurry of polymer gel is casted onto a glass slide, followed by a layer of carbon nanotubes. (Credit: David Baillot/UC San Diego Jacobs School of Engineering)

Gonzales used the metaphor of a dam’s spillway to explain. “When a dam starts to fail, a spillway is opened up to let some of the water trickle out in a controlled fashion so that when the dam does break and spill out, there’s not a lot of water left to cause a flood,” he said. “That’s the idea with our separator. We are draining out the charge much, much slower and prevent a ‘flood’ of electrons to the cathode. When a dendrite gets intercepted by the separator’s conductive layer, the battery can begin to self-discharge so that when the battery does short, there’s not enough energy left to be dangerous.”

In tests, lithium metal batteries equipped with the new separator showed signs of gradual failure over 20 to 30 cycles. Batteries with a normal, slightly thicker separator experienced abrupt failure in a single cycle.

“In a real use case scenario, you wouldn’t have any advance warning that the battery is going to fail. It could be fine one second, then catch on fire or short out completely the next. It’s unpredictable,” Gonzalez said. “But with our separator, you would get advance warning that the battery is getting a little bit worse, a little bit worse, a little bit worse, each time you charge it.”

While they focused on lithium metal batteries, the researchers say the separator can also work in lithium ion and other battery chemistries. The team will be working on optimizing the separator for commercial use. A provisional patent has been filed by UC San Diego.