Power/Performance Bits: March 23

Optical media metasurface; 300 GHz CMOS transceiver; energy-harvesting backpack.


Metasurface for optical media
Researchers at Purdue University proposed a new way to store information in optical media, such as CDs and DVDs, that could improve both storage capacity and read times. The development focuses on encoding information in the angular position of tiny antennas, allowing them to store more data per unit area.

“The storage capacity greatly increases because it is only defined by the resolution of the sensor by which you can determine the angular positions of antennas,” said Alexander Kildishev, an associate professor of electrical and computer engineering in Purdue’s College of Engineering. “We map the antenna angles into colors, and the colors are decoded.”

Their method follows the typical process of mass production for Blu-Ray discs, where a silicon stamper replicates the standard dot-and-dashes format the final disc is getting, with a thin nickel coating added to get a negative stamp.

“Our metasurface-based ‘optical storage’ is just like that,” said Di Wang, a former Ph.D. student who fabricated the prototype structure. “Whereas in our demo prototype, the information is ‘burnt in’ by electron-beam lithography, it could be replicated by a more scalable manufacturing process in the final product.”

The proposed anisotropic metasurface from Purdue University innovators has significant potential for high-density optical data storage, dynamic color image display, and encryption. (Source: Purdue University)

As well as allowing more information to be stored, it also increases the readout rate. It uses a surface‐relief aluminum metasurface that reflects polarization‐tunable plasmonic colors, providing high spatial resolution and mechanical/chemical stability.

A single storage unit, called a nanopixel, stores multiple‐bit information in the orientation of its constituent nanoantennae, which is retrieved by inspecting the reflected color sequence with two linear polarizers.

“You can put four sensors nearby, and each sensor would read its own polarization of light,” Kildishev said. “This helps increase the speed of readout of information compared to the use of a single sensor with dots and dashes.”

The team is looking for industry partners to further develop the technology, and say applications include security tagging and cryptography.

300 GHz CMOS transceiver
Scientists at Tokyo Institute of Technology and NTT Corporation developed a CMOS-based transceiver for wireless communications at the 300 GHz band, targeting beyond-5G applications. The researchers say it represents the first wideband CMOS phased-array system to operate at such frequencies.

Amplification becomes difficult near 300 GHz. Although a few CMOS-based transceivers for 300 GHz have been proposed, they either lack enough output power, can only operate in direct line-of-sight conditions, or require a large circuit area to be implemented.

The 300 GHz CMOS-based transceiver design is bidirectional; a great portion of the circuit, including the mixer, antennas, and local oscillator, is shared between the receiver and the transmitter. This means the overall circuit complexity and the total circuit area required are much lower than in unidirectional implementations.

It also uses four antennas in a phased array configuration. (Typically, 300 GHz CMOS transmitters use a single radiating element, which limits the antenna gain and the system’s output power.) Beamforming capability of phased arrays allows the device to adjust the relative phases of the antenna signals to create a combined radiation pattern with custom directionality. The antennas used are stacked “Vivaldi antennas,” which can be etched directly onto PCBs, making them easy to fabricate.

Chip micrograph of 300 GHz-band phased-array transceiver implemented by 65nm CMOS. (Source: Tokyo Institute of Technology)

The proposed transceiver uses a subharmonic mixer, which is compatible with a bidirectional operation and requires a local oscillator with a comparatively lower frequency. However, this type of mixing results in low output power, which the team turned to the outphasing technique to boost it.

Professor Kenichi Okada from Tokyo Tech explained, “Outphasing is a method generally used to improve the efficiency of power amplifiers by enabling their operation at output powers close to the point where they no longer behave linearly–that is, without distortion. In our work, we used this approach to increase the transmitted output power by operating the mixers at their saturated output power.”

Another notable feature of the new transceiver is its cancellation of local oscillator feedthrough (a “leakage” from the local oscillator through the mixer and onto the output) and image frequency (a common type of interference for the method of reception used).

The transceiver was implemented in an area of 4.17 mm2. It achieved maximum rates of 26 Gbaud for transmission and 18 Gbaud for reception.

Energy-harvesting backpack
Researchers from Tsinghua University, University of Geosciences Beijing, and Chinese Academy of Sciences propose a backpack design that makes loads feel lighter and harvests energy.

The backpack utilizes two elastomers that stretch and shrink, keeping the bag steady as the wearer walks. This resulted in about a 20% reduced force on the wearer.

In addition, movement between the frame of the backpack and its load during walking drove a triboelectric nanogenerator, or TENG, to convert mechanical energy into electricity with 14% efficiency. The researchers showed that the bag could power LEDs, an electric watch, and fluorescent tubes.

The team plans to work on improving the energy conversion efficiency, but said that if that can be done the backpack has promising potential as a power source for small-scale wearable and portable electronics, GPS, and health care sensors.

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