System Bits: July 26

Mixing topology, spin
MIT researchers are studying new compounds, such as topological insulators (TIs), which support protected electron states on the surfaces of crystals that silicon-based technologies cannot as part of the pursuit of material platforms for the next generation of electronics.

They report new physical phenomena being realized by combining this field of TIs with the subfield of spin-based electronics known as spintronics, and the success within spintronics of realizing important magnetic technologies — such as the spin valve — have increased the expectations that new results in TIs might have near-term applications.

But, the researchers point out, combining these two research threads has relied on “shoehorning” magnetism by forcing magnetic atoms to partially occupy elemental positions in TIs or by applying a conventional magnetic field, and that realizing an integrated material that is both intrinsically magnetic and has a topological character has proven more challenging.

Now however, a team of researchers based in the group of Joseph G. Checkelsky, assistant professor of physics at MIT, and collaborators at the NIST Center for Neutron Research (NCNR), Carnegie Mellon University, and the Beijing Institute of Technology have experimentally demonstrated a hybrid material solution to this problem.

A: Crystal structure of a gadolinium (Gd), platinum (Pt), and bismuth (Bi) compound that is composed of [B] a magnetic gadolinium-face-centered cubic sublattice and [C] a platinum-bismouth zinc-blende sublattice. An MIT-led study found that these crystals at the same time were exotic magnets and exhibited signatures of electronic topology.
(Source: MIT)

They studied a ternary compound of gadolinium, platinum and bismuth, whereby gadolinium supplies the magnetic order while the platinum-bismuth components support a topological electronic structure. These two components acting in concert make a correlated material that is more than the sum of its parts, showing quantum mechanical corrections to electrical properties at an unprecedented scale.

Laser focus
In the quest to miniaturize photonic technologies for dense integration onto tiny semiconductor chips, Lehigh University researchers are seeking to develop even smaller nanolasers, of which plasmonic lasers are the tiniest.

Sushil Kumar, associate professor of electrical and computer engineering, explained that the plasmonic laser uses metal films or nanoparticles to confine light energy inside the cavity from which laser light is generated. By storing light energy inside the cavity through a combination of electron oscillations in the integrated metal films or nanoparticles, plasmonic lasers utilize surface-plasmon-polaritons (SPPs) to store energy in dimensions that can be made smaller than the wavelength of light that they generate.

This ability of plasmonic lasers makes them attractive for potential applications in integrated (on-chip) optics, for transporting large swathes of data on-chip and between neighboring chips, and for ultrafast digital information processing.

Before plasmonic lasers can be widely used, several problems must be solved, including the difficulty of extracting light from the cavity of a plasmonic laser. Also, the lasers are extremely poor emitters of light, and whatever light does come out is highly divergent rather than focused, which severely limits their usefulness.


A semiconductor laser chip (lower left, center) measuring approximately 3mm x 1.5mm contains 10 lasers. A scanning electron microscopy magnification (right) shows one of the laser cavities. Periodic slits in the thin-film top metal layer provide the distributed feedback in the cavity. (Source: Lehigh University)

While most plasmonic lasers emit visible or near-infrared radiation, Kumar’s group develops plasmonic lasers that emit long-wavelength terahertz radiation, which are also known as terahertz quantum-cascade lasers, or QCLs. As the brightest solid-state sources of terahertz radiation, QCLs are well suited for applications in biology and medicine for sensing and spectroscopy of molecular species, in security screening for remote detection of packaged explosives and other illicit materials, and in astrophysics and atmospheric science. But terahertz QCLs emit highly divergent beams, which poses an obstacle to commercialization.

Now, Kumar and his group have demonstrated that it is possible to induce plasmonic lasers to emit a narrow beam of light by adapting a technique called distributed feedback.

The researchers have filed a patent application on their invention, which could help plasmonic lasers, especially terahertz QCLs with narrow beams, find commercial applications. They said there is a very strong interest in security spectrometry given that approximately 80 to 95 percent of explosives, and all commonly used ones, have unique and identifiable terahertz signatures.

Virtual reality for driving instruction
Joining the ranks of astronauts, pilots, truck drivers, and Formula One race car drivers, there is now a virtual reality simulator specifically designed to help teenagers with autism spectrum disorder (ASD) learn how to drive, thanks to Vanderbilt University researchers.

Amy Weitlauf, a psychologist who specializes in autism, and who is an assistant professor of pediatrics at Vanderbilt University Medical Center explained that in the past 15 years, there has been an appropriate emphasis on early identification and early treatment of children with ASD, and she has started to work on providing them with the support they need to become independent adults. One of those key life skills for independence is, for many people, the ability to drive.

She is collaborating with a team of Vanderbilt engineers to develop a special adaptive virtual-reality driving environment for individuals with ASD. Although there is no single accepted treatment for ASD, there is growing agreement that individualized behavioral and educational interventions can have a positive impact on the lives of these individuals and their families.

When the driver fails to look at critical elements in the virtual surroundings, then the simulator will stop and highlight the critical objects. (Source: Robotics and Autonomous Systems Lab/Vanderbilt University)

While there are a number of off-the-shelf driving simulators available, none have the capabilities built into the Vanderbilt VR Adaptive Driving Intervention Architecture (VADIA). Not only is it specifically designed to teach adolescents with ASD the basic rules of the road, but VADIA also gathers information about the unique ways that they react to driving situations. This will allow the system to alter driving scenarios with varying degrees of difficulty to provide users with the training they need while keeping them engaged in the process. Ultimately, it may also help screen individuals whose deficits are too severe to drive safely.