Manufacturing Bits: April 1

Portable laser weapons
For years, the U.S. military has been developing high-energy laser (HEL) weapons. But the massive size, weight and power requirements of laser systems limit their use on many military platforms.

The Defense Advanced Research Projects Agency (DARPA) has made a breakthrough in its so-called Excalibur program. The program will develop laser weapons that are 10 times lighter and more compact than existing high-power chemical laser systems.

This is an image of the optical phased array used in the Excalibur demo. (Source: DARPA)

To accomplish this feat, DARPA has developed a 21-element optical phased array (OPA). These phased arrays will combine lower-power electrically driven lasers, such as diode lasers and fiber laser amplifiers. All told, this low power array was used to precisely hit a target 7 kilometers—more than 4 miles away. The OPA used in these experiments consisted of three identical clusters of seven fiber lasers, with each cluster only 10 centimeters across.

The demonstration helps advance Excalibur’s goal of a 100-kilowatt-class laser system in a scalable configuration. Continued development and testing of Excalibur fiber optic laser arrays may one day lead to multi-100 kilowatt-class HELs in a package 10 times lighter and more compact than legacy high-power laser systems.

“The success of this real-world test provides evidence of how far OPA lasers could surpass legacy lasers with conventional optics,” said Joseph Mangano, DARPA program manager, on the agency’s Web site. “It also bolsters arguments for this technology’s scalability and its suitability for high-power testing. DARPA is planning tests over the next three years to demonstrate capabilities at increasing power levels, ultimately up to 100 kilowatts—power levels otherwise difficult to achieve in such a small package.”

Topological insulators
Topological insulators are creating a buzz. These 3D materials conduct electricity on the surface, while the interior is an insulator. Topological insulators are a promising material for the emerging field of spintronics.

In theory, the electrons will move in these materials without scattering. The trick is to control the electrons in operation. Until now, the only way to change the electronic state was to apply a magnetic field.

The study found that the atoms are either stretched apart or pushed together at the grain boundaries and that strain can be used to “tune” the material’s unique electronic properties. Here, the boundaries appear dotted or puckered, while the Bi2Se3 grain forms in a triangular shape. (Source: UWM)

The University of Wisconsin-Milwaukee (UWM) has devised a new method. Researchers demonstrated that the states can be enhanced or destroyed by strain. This is done in the vicinity of grain boundaries on the surface of epitaxial bismuth selenide (Bi2Se3) thin films. Bi2Se3 is crystalline solid used in semiconductor and related applications.

In the lab, researchers used scanning tunneling and transmission electron microscopy (TEM) to prove the technology. They demonstrated that the low-angle tilt grain boundaries in Bi2Se3 films consist of arrays of alternating edge dislocation pairs.

These dislocations introduce periodic compressive and tensile strains. Researchers found that while the energy of the Dirac state shifts in regions under tensile strain, a gap opens in regions under compressive strain at the atomic scale.

“(Topological insulators) would work well in spintronics, because the spin and velocity of their surface electrons is locked in at right angles,” said UWM Physics Professor Lian Li, on the university’s Web site.

Still, researcher must find ways to create a simple on-off switch. “So, when we apply compression at the boundaries, then you have no spin movement. All of the sudden, it becomes a switch,” said Michael Weinert, UWM Distinguished professor of physics. “The advantage here is control. You don’t have to apply an electrical field, you can apply stress.”

Spinning around
Spintronics is a promising field, but controlling the electron spin states is a challenge. In the lab, quantum theory predicts that certain energy states could be used to control electron spin.

Riken Center for Emergent Matter Science, the University of Tokyo and the SLAC National Accelerator Laboratory have detected a quantum property known as Berry’s phase in a semiconductor.

Berry’s phase gives rise to a quantum version of the classical Hall effect. This is the transverse electrical current induced by an external magnetic field. It also possibly enables new materials like topological insulators.

Electron spin can be either up or down. The two states are difficult to distinguish. To overcome the challenges, researchers focused on a material called bismuth tellurium iodide (BiTeI). Researchers found the interfaces between the layers in BiTeI destroy the symmetry of the atomic lattice. This, in turn, gives rise to a strong coupling between electron spin and motion. All told, the spin states can be easily observed.

“When the spin Berry’s phase exists, novel phenomena such as spin-polarized charge flow without energy dissipation can be realized. Despite its ubiquity and importance, however, experimental observation of Berry’s phase stemming from electron spin is challenging,” said Hiroshi Murakawa of Riken.

The crystal structure of the semiconductor bismuth tellurium iodide (BiTeI) (left) is made up of stacked layers of bismuth (Bi), tellurium (Te) and iodine (I) atoms. (Source: Riken)