Power/Performance Bits: Jan. 14

Optical memory; millimeter wave light modulator; origami RF filter.


Optical memory
Researchers at the University of Oxford, University of Exeter, and University of Münster propose an all-optical memory cell that can store more optical data, 5 bits, in a smaller space than was previously possible on-chip.

The optical memory cell uses light to encode information in the phase change material Ge2Sb2Te5. A laser causes the material to change between ordered and disordered states, which have different optical indices of refraction. Mixing different ratios of ordered and disordered states in an area of the material allows information to be stored in a continuum of levels instead of just a zero and a one as in traditional electronic memory.

“Although our team has previously used this approach to optical memory, we’ve now been able to push the resolution limits of this memory cell by storing a larger number of intermediate states between zero and one,” said Nathan Youngblood, a postdoctoral fellow in photonic computing at Oxford. “This allowed us to store information in 34 levels, while only 10 could be achieved previously.”

The optical memory chip. (Source: Oxford University)

Key to the increased resolution was using a laser light with a single, double-stepped pulse.

“Instead of heating the material with a single laser pulse, we shape the pulse in a way that allows us to control the material’s temperature over time,” said Xuan Li, a graduate student at Oxford. “This provides the ability to adjust how that material interacts with light and the state it will reach after heating. It also greatly speeds up the writing process because we can change the material’s state with just one laser pulse instead of the hundreds or thousands of pulses required previously.”

The team argues that this memory could help overcome the processor-memory bottleneck. “A lot of work has gone into improving the communication between these two units using fiber optics,” said Harish Bhaskaran, professor of applied nanomaterials at Oxford. “However, linking these two units optically still requires expensive electro-optical conversions at both ends. Our memory cell could be used in a hybrid optical-electrical setup to eliminate the need for that conversion on the memory side by allowing data to be stored and retrieved optically.”

Next, the researchers plan to integrate multiple memory cells and individually program them. While the team says that they can already replicate the devices extremely well, they will need to develop light signal processing techniques to integrate multiple optical memory cells.

Millimeter wave light modulator
Researchers at ETH Zurich and the University of Washington developed a light modulator that could be part of an efficient, low-cost way to cover the “last mile” of connectivity between fiber optic cables and the home using high-frequency microwaves.

The light modulator receives and translates data contained in these millimeter waves into light pulses entirely without batteries and electronics.

“That makes our modulator completely independent of external power supplies and, on top of that, extremely small so that it can, in principle, be mounted on any lamppost. From there, it can then receive data via microwave signals from individual houses and feed them directly into the central optical fiber”, said Yannick Salamin, a PhD student at ETH Zurich.

The modulator consists of a chip measuring less than a millimeter that also contains the microwave antenna. That antenna receives the millimeter waves and converts them into an electric voltage, which acts on a thin slot at the center of the chip. There, a narrow slit, just a few micrometers long and less than a hundred nanometers wide, is filled with an electrically sensitive nonlinear material.

The light beam from the fiber is fed into that slit, where the light propagates as plasmons. When the plasmons propagate, the electric field created by the antenna influences their oscillatory phase, which is conserved when the plasmons are converted back into light waves at the end of the slit. This transfers the data bits contained in the millimeter waves directly onto the light waves without electronics.

The “last mile” to the internet connection at home is also the most demanding. The new modulator is a viable alternative. In it, data transmitted by millimeter waves (red arrows) can be directly converted into pulses for the optical fiber (yellow). (Source: Salamin Y et al. Nature Photonics 2018 / ETH Zurich)

In a laboratory experiment with microwave signals at 60 Gigahertz, the researchers were able to demonstrate data transmission rates of up to 10 Gigabits per second over a distance of five meters, and 20 Gigabits per second over one meter.

The researchers say the modulator is already compatible both with new 5G technology and with future industry standards based on millimeter-wave and terahertz frequencies of 300 Gigahertz and data transmission rates of up to 100 Gigabits per second. Additionally, it can be produced at a comparatively low cost using conventional silicon technology.

Origami RF filter
Researchers at the Georgia Institute of Technology built an adjustable radio frequency filter that can change what signals it blocks on the fly. Based on the Miura-Ori origami pattern, the filter expands and contracts like an accordion: flat when fully extended and compact when fully compressed. This structure could be useful for antenna systems that need to stay in compact spaces until deployed, such as in space applications.

“The Miura-Ori pattern has an infinite number of possible positions along its range of extension from fully compressed to fully expanded,” said Glaucio Paulino, Chair of Engineering and a professor at Georgia Tech. “A spatial filter made in this fashion can achieve similar versatility, changing which frequency it blocks as the filter is compressed or expanded.”

To create the filter, a special printer was used to score paper so it could be folded into the pattern. An inkjet printer deposited silver ink across the perforations to form dipole elements. “The dipoles were placed along the fold lines so that when the origami was compressed, the dipoles bend and become closer together, which causes their resonant frequency to shift higher along the spectrum,” said Manos Tentzeris, professor in flexible electronics at Georgia Tech.

Silver dipoles are arranged across the folds of a Miuri-Ori pattern to enable frequency blocking. (Source: Rob Felt / Georgia Tech)

In testing the filter, the team found that a single-layer Miura-Ori-shaped filter blocked a narrow band of frequencies while multiple layers of the filters stacked could achieve a wider band of blocked frequencies.

The origami pattern also lent the filter strength, according to Larissa Novelino, a Georgia Tech graduate student. “The Miura-Ori pattern exhibits remarkable mechanical properties, despite being assembled from sheets barely thicker than a tenth of a millimeter. Those properties could make light-weight yet strong structures that could be easily transported.”

“A device based on Miura-Ori could both deploy and be re-tuned to a broad range of frequencies as compared to traditional frequency selective surfaces, which typically use electronic components to adjust the frequency rather than a physical change,” said Abdullah Nauroze, a Georgia Tech graduate student. “Such devices could be good candidates to be used as reflectarrays for the next generation of cubesats or other space communications devices.”

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