Power/Performance Bits: June 7

Tiny lasers on silicon; rewriteable magnetic charge ice.

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Tiny lasers on silicon

A group of scientists from Hong Kong University of Science and Technology, the University of California, Santa Barbara, Sandia National Laboratories, and Harvard University were able to fabricate tiny lasers directly on silicon.

To do this, they first had to resolve silicon crystal lattice defects to a point where the cavities were essentially equivalent to those grown on lattice-matched gallium arsenide (GaAs) substrates. Nano-patterns created on silicon to confine the defects made the GaAs-on-silicon template nearly defect free and quantum confinement of electrons within quantum dots grown on this template made lasing possible.

The group was then able to use optical pumping, a process in which light, rather than electrical current, “pumps” electrons from a lower energy level in an atom or molecule to a higher level, to show that the devices work as lasers.

“Putting lasers on microprocessors boosts their capabilities and allows them to run at much lower powers, which is a big step toward photonics and electronics integration on the silicon platform,” said professor Kei May Lau, Department of Electronic and Computer Engineering, Hong Kong University of Science and Technology.

“Our lasers have very low threshold and match the sizes needed to integrate them onto a microprocessor,” Lau pointed out. “And these tiny high-performance lasers can be grown directly on silicon wafers.”

In terms of applications, the group’s tiny lasers on silicon are ideally suited for high-speed data communications.

The researchers expect to see this technology emerge in the market within 10 years.

Rewriteable magnetic charge ice

A team of scientists working at Argonne National Laboratory, Northern Illinois University, and University of Notre Dame created a new material, called rewriteable artificial magnetic charge ice, which shows strong potential for technological applications from information encoding, reprogrammable magnonics, and also to spintronics.

The new magnetic metamaterial forms eight types of ‘magnetic charge’ ordering and follows the “two-positive two-negative” charge ice rule. The study demonstrates techniques to switch the charge ordering both globally and locally. The ‘read-write-erase’ multiple recording functionalities are conveniently realized at room temperature.

Artificial spin ice is a class of lithographically created arrays of interacting magnetic nano-islands. Due to its geometrical anisotropy, the elongated nano-scale island forms a single magnetic domain which behaves like ‘macro spin’ with a binary degree of freedom. The ‘spins’ in artificial spin ice follows the ‘two-in two-out’ ice rule that determines the proton positional ordering in water ice.

A depiction of the global order of magnetic charge ice. Orange-red areas represent the positive charges; blue areas represent negative charges. (Source: Yong-Lei Wang and Zhili Xiao)

A depiction of the global order of magnetic charge ice. Orange-red areas represent the positive charges; blue areas represent negative charges. (Source: Yong-Lei Wang and Zhili Xiao)

Due to the plethora of spin configurations, artificial spin ices have great potential for applications in data storage, memory, and logic devices. However, because of the large magnetic energy scales of these nanoscale islands at room temperature, achieving the magnetic ground and higher ordered states in traditional artificial spin ices have been a big challenge for nearly a decade since the first artificial spin ice was created. This essentially limits the practical application of artificial ices.

“Instead of focusing on spins, we tackled the associated magnetic charges that allow us to design and create artificial magnetic charge ices with more control,” said Yong-Lei Wang of Notre Dame, who designed the new magnetic nano-structures and built a custom magnetic force microscope for the research.

Magnetic charge ice is two-dimensional, meaning it consists of a very thin layer of atoms, and could be applied to other thin materials, such as graphene. The team says the material also is environmentally friendly and relatively inexpensive to produce.