Power/Performance Bits: Aug. 14

All-optical logic
Researchers from Aalto University developed multifunction all-optical logic gates using a network of nanowires.

To build the nanostructure, the team assembled two different semiconductor nanowires, indium phosphide and aluminum gallium arsenide. The nanowires have a unique one-dimensional structure, which allows them to function like nanosized antennas for light.

Using a combing technique, the nanowires were able to be aligned in any specific direction, which differs from the as-fabricated randomly aligned nanowires typically used.

“Repeating the combing method allows us to build integrated devices of nanostructures in which two different types of nanowires are perpendicular to each other,” said Professor Zhipei Sun, who leads the Photonics group at Aalto University.

“The one-dimensional and crossbar structures are the core of our calculations because they enable the input light to choose which nanowire it interacts with–either the indium phosphide or the aluminum gallium arsenide,” added He Yang of Aalto.

The graphs, color-coded, correspond with the input above. (Source: Aalto University)

Depending on the input, in this case the linearly-polarized light direction and its wavelength, the nanowires either interact with the input light or not. Since the response of the different nanomaterials is different, the light output of the fabricated nanowire structure can be switched with different wavelengths and light direction for the realization of logic operations.

The researchers found the networks were perform various logic operations, such as AND, OR, NAND, and NOR binary logic functions. Additionally, the networks successfully enabled all-optical arithmetic binary calculations, such as n-bit addition, to be conducted.

3D printed battery electrodes
Researchers at Carnegie Mellon University and Missouri University of Science and Technology developed a new way to fabricate battery electrodes using Aerosol Jet 3D printing.

While 3D printing has been used to create porous electrodes for lithium-ion batteries, they have been limited to only a few architectures. The best performing of those, interdigitated geometry, has interlocking metal prongs with the lithium shuttling between the two sides. However, such an electrode lacks pores and channels, which improve battery capacity.

“In the case of lithium-ion batteries, the electrodes with porous architectures can lead to higher charge capacities,” said Rahul Panat, an associate professor of mechanical engineering at Carnegie Mellon. “This is because such architectures allow the lithium to penetrate through the electrode volume leading to very high electrode utilization, and thereby higher energy storage capacity. In normal batteries, 30-50% of the total electrode volume is unutilized. Our method overcomes this issue by using 3D printing where we create a microlattice electrode architecture that allows the efficient transport of lithium through the entire electrode, which also increases the battery charging rates.”

The microlattice structure (Ag) used as lithium-ion batteries’ electrodes was shown to improve battery performance in several ways, such as a fourfold increase in specific capacity and a twofold increase in areal capacity when compared to a solid block (Ag) electrode. Furthermore, the electrodes retained their complex 3D lattice structures after forty electrochemical cycles. The batteries can thus have high capacity for the same weight or alternately, for the same capacity, a vastly reduced weight, an important attribute for transportation applications.

SEM images of 3D printed electrodes for Li-ion batteries used for electrochemical cycling in the researchers’ study. Image taken from top of microlattice electrodes with height of about 250mm. (Source: Rahul Panat, Carnegie Mellon University College of Engineering / Additive Manufacturing 23 (2018) 70-78)

Previously, 3D printed battery electrodes used extrusion-based printing, which creates continuous structures. The new method is capable of rapidly assembling individual droplets one-by-one into three-dimensional structures. The resulting structures have complex geometries impossible to fabricate using typical extrusion methods.

The researchers estimate that this technology will be ready to translate to industrial applications in about 2-3 years.

The team is also working on creating more complex three-dimensional structures, which can simultaneously be used as structural materials and as functional materials. For example, a part of a drone can act as a wing, a structural material, while simultaneously acting as a functional material such as a battery.

On-chip optical filter
Researchers at MIT designed an optical filter on a chip that can process optical signals from across an extremely wide spectrum of light at once.

Existing optical filters, which are used to separate one light source into wanted and unwanted wavelengths, have tradeoffs and disadvantages. Discrete (off-chip) “broadband” filters, called dichroic filters, process wide portions of the light spectrum but are large, can be expensive, and require many layers of optical coatings that reflect certain wavelengths. Integrated filters can be produced in large quantities inexpensively, but they typically cover a very narrow band of the spectrum, so many must be combined to efficiently and selectively filter larger portions of the spectrum.

The team’s new on-chip filter mimics dichroic filters in many ways, but can be manufactured using traditional silicon-chip fabrication methods. They created two sections of precisely sized and aligned silicon waveguides that coax different wavelengths into different outputs.

“This new filter takes an extremely broad range of wavelengths within its bandwidth as input and efficiently separates it into two output signals, regardless of exactly how wide or at what wavelength the input is. That capability didn’t exist before in integrated optics,” said Emir Salih Magden, an assistant professor of Electrical Engineering at Koç University.

The researchers used waveguides to precisely guide the light input to the corresponding signal outputs. One section of the researchers’ filter contains an array of three waveguides, while the other section contains one waveguide that’s slightly wider than any of the three individual ones.

In the paper, the researchers created a single waveguide measuring 318nm, and three separate waveguides measuring 250nm each with gaps of 100nm in between. This corresponded to a cutoff of around 1,540nm, which is in the infrared region. When a light beam entered the filter, wavelengths measuring less than 1,540nm could detect one wide waveguide on one side and three narrower waveguides on the other. Those wavelengths move along the wider waveguide. Wavelengths longer than 1,540nm, however, can’t detect spaces between three separate waveguides. Instead, they detect a massive waveguide wider than the single waveguide, so move toward the three waveguides.

A colorized scanning electron micrograph showing the fabricated waveguide cross-section. Scale bar, 400 nm. (Source: Emir Salih Magden et al. / Nature Communications volume 9, Article number: 3009 (2018))

“That these long wavelengths are unable to distinguish these gaps, and see them as a single waveguide, is half of the puzzle. The other half is designing efficient transitions for routing light through these waveguides toward the outputs,” said Magden.

The researchers also provided guidelines for exact widths and gaps of the waveguides needed to achieve different cutoffs for different wavelengths. In that way, the filters are highly customizable to work at any wavelength range. Added Magden, “Once you choose what materials to use, you can determine the necessary waveguide dimensions and design a similar filter for your own platform.”

The team sees the technology as offering greater precision and flexibility for designing optical communication and sensor systems as well as studying photons and other particles through ultrafast techniques.