Invisibility cloaks in the offing

Scientists have developed a new technique that sprays metallic nanoparticles into semiconductors, an advance that could boost the efficiency of LED lighting and even pave the way for invisibility cloaking devices.
The technique can inexpensively grow metal nanoparticles both on and below the surface of semiconductors, said researchers from University of Michigan in the US.
The process adds virtually no cost during manufacturing and its improved efficiency could allow manufacturers to use fewer semiconductors in finished products, making them less expensive.
The metal nanoparticles can increase the efficiency of LEDs in several ways. They can act as tiny antennas that alter and redirect the electricity running through the semiconductor, turning more of it into light, researchers said.They can also help reflect light out of the device, preventing it from being trapped inside and wasted.

The process can be used with the gallium nitride that's used in LED lighting and can also boost efficiency in other semiconductor products, including solar cells, they said.
"This is a seamless addition to the manufacturing process, and that's what makes it so exciting," said Rachel Goldman, professor at University of Michigan.

"The ability to make 3-D structures with these nanoparticles throughout is going to open a lot of possibilities," Goldman said.
Researcher discovered a simple way that integrates easily with the molecular beam epitaxy process used to make semiconductors.
Molecular beam epitaxy sprays multiple layers of metallic elements onto a wafer. This creates exactly the right conductive properties for a given purpose, researchers said.

The team applied an ion beam between these layers-a step that pushes metal out of the semiconductor wafer and onto the surface.
The metal forms nanoscale particles that serve the same purpose as the pricey gold and platinum flecks in earlier research.
Their size and placement can be precisely controlled by varying the angle and intensity of the ion beam. And applying the ion beam over and over between each layer creates a semiconductor with the nanoparticles interspersed throughout, researchers said.
"If you carefully tailor the size and spacing of nanoparticles and how deeply they're embedded, you can find a sweet spot that enhances light emissions. This process gives us a much simpler and less expensive way to do that," said Myungkoo Kang from, University of Michgian.

Because the technique allows precise control over the nanoparticle distribution,it may one day be useful for cloaks that render objects partially invisible by inducing a phenomenon known as "reverse refraction, researchers said.
"For invisibility cloaking, we need to both transmit and manipulate light in very precise ways, and that's very difficult today," Goldman said.
"We believe that this process could give us the level of control we need to make it work," she added.
The findings were published in the Journal of Applied Physics.