System Bits: Nov. 12

3D Printers
When thinking about 3D printers, most people probably think about creating small plastic parts or prototypes. 3D printing now can be used to print lithium-ion microbatteries the size of a grain of sand.

The printed microbatteries could supply electricity to tiny devices in fields from medicine to communications, including many that have lingered on lab benches for lack of a battery small enough to fit the device, yet providing enough stored energy to power it.

This image shows the interlaced stack of electrodes that were printed layer by layer to create the working anode and cathode of a microbattery. Photo by Jennifer Lewis.

To make the microbatteries, a team based at Harvard University and the University of Illinois at Urbana-Champaign printed precisely interlaced stacks of tiny battery electrodes, each less than the diameter of a human hair.

Manufacturers traditionally have deposited thin films of solid materials to build the electrodes. However, because of their ultra-thin design, these solid-state micro-batteries do not pack sufficient energy to power tomorrow’s miniaturized devices.

The scientists realized they could pack more energy if they could create stacks of tightly interlaced, ultrathin electrodes that were built out of plane. For this they turned to 3-D printing. Jennifer Lewis, Hansjörg Wyss Professor of Biologically Inspired Engineering at the Harvard School of Engineering and Applied Sciences (SEAS), and her group have expanded the capabilities of 3-D printing. They have designed a broad range of functional inks that have useful chemical and electrical properties. And they have used those inks with their custom-built 3-D printers to create precise structures with the electronic, optical, mechanical or biologically relevant properties they want.

Inks developed for extrusion-based 3-D printing must fulfill two difficult requirements. They must exit fine nozzles like toothpaste from a tube, and they must immediately harden into their final form.

In this case, the inks also had to function as electrochemically active materials to create working anodes and cathodes, and they had to harden into layers that are as narrow as those produced by thin-film manufacturing methods. To accomplish these goals, the researchers created an ink for the anode with nanoparticles of one lithium metal oxide compound, and an ink for the cathode from nanoparticles of another. The printer deposited the inks onto the teeth of two gold combs, creating a tightly interlaced stack of anodes and cathodes. Then the researchers packaged the electrodes into a tiny container and filled it with an electrolyte solution to complete the battery.

The electrochemical performance is comparable to commercial batteries in terms of charge and discharge rate, cycle life and energy densities.

Optical Tuning Fork
Stabilizing the voltage in a chip can be hard. Power spikes cause voltage droop and can upset sensitive circuitry such as analog components. Now we have another reason to worry about power rail stability as we start to integrate photonics devices onto chips. Lasers require a stable frequency for best operation and energy surges can create instability. Interestingly, while we attempt to minimize track length in electronics because of the delays it adds, this is not the case with light. Researchers at Caltech have created the optical equivalent of a tuning fork—a device that can help steady the electrical currents needed to power high-end electronics and stabilize the signals of high-quality lasers. The work marks the first time that such a device has been miniaturized to fit on a chip and may pave the way to improvements in high-speed communications, navigation, and remote sensing.

A photograph of the spiral chip-based optical resonator developed at Caltech, shown next to a quarter to provide scale. Credit: Hansuek Lee/Caltech

A good tuning fork controls the release of its acoustical energy, ringing just one pitch at a particular sound frequency for a long time; this sustaining property is called the quality factor. Kerry Vahala, Ted and Ginger Jenkins Professor of Information Science and Technology and Applied Physics, and his colleagues transferred this concept to their optical resonator, focusing on the optical quality factor and other elements that affect frequency stability.

The researchers were able to stabilize the light’s frequency by developing a silica glass chip resonator with a specially designed path for the photons in the shape of what is called an Archimedean spiral. “Using this shape allows the longest path in the smallest area on a chip. We knew that if we made the photons travel a longer path, the whole device would become more stable,” says Hansuek Lee, a senior researcher in Vahala’s lab.

Because the new resonator has a longer path, the energy changes are diluted, so the power surges are dampened—greatly improving the consistency and quality of the resonator’s reference signal, which, in turn, improves the quality of the electronic or optical device.

In the new design, photons are applied to an outer ring of the spiraled resonator with a tiny light-dispensing optic fiber; the photons subsequently travel around four interwoven Archimedean spirals, ultimately closing the path after traveling more than a meter in an area about the size of a quarter—a journey 100 times longer than achieved in previous designs. In combination with the resonator, a special guide for the light was used, losing 100 times less energy than the average chip-based device.

In addition to its use as a frequency reference for lasers, a reference cavity could one day play a role equivalent to that of the ubiquitous quartz crystal in electronics. Most electronics systems use an oscillator to provide power at very precise frequencies. In the past several years, optical-based oscillators—which require optical reference cavities—have become better than electronic oscillators at delivering stable microwave and radio frequencies. While these optical oscillators are currently too large for use in small electronics, there is an effort under way to miniaturize their key subcomponents—like Vahala’s chip-based reference cavity.