Power/Performance Bits: Feb. 24

Simulating ultrafast phenomena

Interesting phenomena can happen when electronic states in materials are excited during dynamic processes. As an example, electrical charge transfer can take place on quadrillionth-of-a-second, or femtosecond, timescales. Numerical simulations in real-time provide the best way to study these processes.

Such simulations, however, can be extremely expensive. Real-time simulations of ultrafast phenomena require “small time steps” to describe the movement of an electron, which takes place on the attosecond timescale – a thousand times faster than the femtosecond timescale. Simulating a 10 femtosecond process can take a supercomputer several weeks.

What if that time could be taken down to ten hours? Lin-Wang Wang, senior staff scientist at Berkeley Lab, and visiting scholar Zhi Wang from the Chinese Academy of Sciences, have developed a new algorithm which increases the small time step from about one attosecond to about half a femtosecond. They have published their findings in a new paper for Physical Review Letters.

Model of ion (Cl) collision with atomically thin semiconductor (MoSe2). Collision region is shown in blue and zoomed in; red points show initial positions of Cl. The simulation calculates the energy loss of the ion based on the incident and emergent velocities of the Cl. (Source: Berkeley Lab)

The traditional algorithms work by directly manipulating time-dependent quantum mechanical equations that describe the movement of electrons. Wang’s new approach is to expand the equations into individual terms, based on which states are excited at a given time. The trick is to figure out the time evolution of the individual terms. The advantage is that some terms in the expanded equations can be eliminated.

This new algorithm opens the door for efficient real-time simulations of ultrafast processes and electron dynamics, such as excitation in photovoltaic materials and ultrafast demagnetization following an optical excitation.

Wrap-around solar

You can’t wrap a solar panel around a car now, but improvements in polymer technology may make flexible solar-coatings feasible for such applications.

Researchers led by Yang Yang, professor of engineering at UCLA, have identified key principles for improving the architecture and performance of polymer solar cells. The group successfully blended different pairs of polymers — or synthetic plastics — to enable devices to absorb light from a larger part of the solar spectrum (for another way of increasing the spectrum available to solar cells, see Power/Performance Bits: Feb. 10). They also identified criteria that could lead to even greater solar cell efficiency and absorption of light as researchers develop new polymers.

Over the years, polymers have been created with different molecular structures in an effort to create materials that can absorb light from different parts of the solar spectrum. They also have blended two or more polymers together on one device to further improve absorption. However, blending has not yielded great improvement.

In the new study, published in Nature Photonics, the researchers demonstrated the problem could be solved by carefully selecting polymers with molecular structures that are compatible with each other.  Using different combinations of polymers and device architectures, they determined which blends improved the solar cells’ efficiency and which were incompatible with each other.

Efficient radios for IoT

Power is the always-on issue of IoT devices. It’s on the minds of a team of researchers from MIT, too.

The group, led by Anantha Chandrakasan, professor of electrical engineering at MIT, will present a new low-power radio chip at the IEEE International Solid-State Circuits Conference this week. The transmitter design reduces off-state leakage 100-fold, while still providing adequate power for Bluetooth transmission or for the 802.15.4 protocol.

They reduced the leakage by applying a negative charge to the gate when the transmitter is idle, a strategy that only works if generating the negative charge consumes less energy than would otherwise be lost. In tests conducted on a prototype chip fabricated through TSMC’s research program, the MIT researchers found that their circuit spent only 20 picowatts of power to save 10,000 picowatts in leakage.

Additionally, to make the transmitter more efficient when it’s active, the team divided the problem of generating an electromagnetic signal into discrete steps, only some of which require higher voltages. For those steps, the circuit uses capacitors and inductors to increase voltage locally.

What those efficiencies mean for battery life depends on how frequently the transmitter is operational. As a datapoint, if it broadcasts once per hour, the researchers believe that they can reduce power consumption 100-fold.