Power/Performance Bits: Sept. 22

Photonic memories

A team of researchers from Oxford University, the University of Münster, the Karlsruhe Institute of Technology, and the University of Exeter produced the first all-photonic nonvolatile memory chip. The new device uses the phase-change material Ge2Sb2Te5 (GST), used in rewritable CDs and DVDs, to store data. This material can be made to assume an amorphous state, like glass, or a crystalline state, like a metal, by using either electrical or optical pulses. The device uses a small section of GST on top of a silicon nitride ridge as a waveguide.

The team has shown that intense pulses of light sent through the waveguide can carefully change the state of the GST. An intense pulse causes it to momentarily melt and quickly cool, causing it to assume an amorphous structure; a slightly less-intense pulse can put it into a crystalline state.

When light with much lower intensity is sent through the waveguide, the difference in the state of the GST affects how much light is transmitted. The team was able to measure that difference to identify its state — and in turn read off the presence of information in the device as a 1 or 0. “This is the first ever truly non-volatile integrated optical memory device to be created,” according to Carlos Ríos, one of two lead authors of the paper. “And we’ve achieved it using established materials that are known for their long-term data retention — GST remains in the state that it’s placed in for decades.”

A scanning electron microscope image of the device. The GST phase-change material, highlighted in yellow, sits on top of the silicon nitride waveguide, highlighted in red. (Source: Oxford University)

By sending different wavelengths of light through the waveguide at the same time, the team also showed that they could use a single pulse to write and read to the memory at the same time. “In theory, that means we could read and write to thousands of bits at once, providing virtually unlimited bandwidth,” explained Wolfram Pernice, professor at the University of Munster.

The researchers found that different intensities of strong pulses can accurately and repeatedly create different mixtures of amorphous and crystalline structure within the GST. When lower intensity pulses were sent through the waveguide to read the contents of the device, they were also able to detect the subtle differences in transmitted light, allowing them to reliably write and read off eight different levels of state composition — from entirely crystalline to completely amorphous. This multi-state capability could provide memory units with more than the usual binary information of 0 and 1, allowing a single bits of memory to store several states or even perform calculations themselves instead of at the processor.

Storing solar energy

Researchers at Missouri University of Science and Technology have developed a relatively inexpensive and simple way to split water into hydrogen and oxygen through a new electrodeposition method. The method produces highly efficient solar cells that can gather solar energy for use as fuel.

“The work helps to solve the problem that solar energy is intermittent,” said Jay A. Switzer, professor at Missouri S&T. “Obviously, we cannot have the sun produce energy on one spot the entire day, but our process converts the energy into a form that is more easily stored.”

The team used silicon wafers to absorb solar energy. The silicon is submerged in water, with the front surface exposed to a solar energy simulator and the back surface covered in electrodes to conduct the energy. Unlike current research standards that cover the surface of the silicon entirely in a catalyst layer, this has small clusters of cobalt catalysts dotted on the silicon’s surface. This process was found to be just as protective and produced much higher voltages.

A working cell from Switzer’s research, with gas evolution. (Source: Sam O’Keefe / Missouri S&T)

The generated voltage from the solar rays splits liquid water into hydrogen and oxygen. Oxygen evolves at the cobalt-coated silicon electrode, and hydrogen evolves at the platinum counter electrode.

“Initially, we had set out to produce a uniform layer of metal that both catalyzed and protected the silicon, but found that this method actually produces a higher voltage and did not need an additional step that typically has to be applied to the silicon,” said Switzer. “These nano-islands of cobalt produce a much more efficient cell.”