Power-Performance Bits: Dec 3

In this week’s edition of Power/Performance Bits we look at two very different types of antennas, in one case a combined antenna and solar panel and in the second nanoantennas that can create holograms.

Combining Antennas with Solar Panels
When it comes to satellites weight is everything, and historically telecommunication antennas and solar cells have never really worked well together, as they have to function independently of each other in order to avoid interference. In a satellite, the surface area has to be large enough for both antenna systems, which emit and receive data, and solar panels, which supply the electricity. But researchers at Ecole Polytechnique Federale de Lausanne, Switzerland (EPFL) have managed to combine antennas and solar cells to work together with unprecedented efficiency.

Philippe Dreyer, who is part of Julien Perruisseau-Carrier Group, in collaboration with the Transparent Conductive Oxides group (TCOs), which is part of the Photovoltaics and Thin Film Electronics Laboratory (PV-Lab), has developed a mixed function surface. In addition to usage in satellites, it could also be used for mobile and autonomous communications systems that are often necessary to keep in contact with people in the wake of a natural disaster. This new technology would make these systems lighter and therefore easier to carry around. “Our device could also support flexible implementation. It could be folded up so that it isn’t deployed until the relief area has been reached,” states Julien Perruisseau-Carrier, who supervised the project.

A basic structure

Figure courtesy of EPFL

For their study, the researchers used so-called reflectarray (RA) antennas, which have the advantage of being flat, relatively cheap and highly efficient. These antennas were combined with thin-film amorphous-silicon solar cells developed by PV-Lab.
In the device, a set of conductors (resonators) is placed on top of the solar cells. This layering makes it possible for the cells to maintain up to 90% of their photovoltaic efficiency.

It is not the first time that scientists try to merge solar cells and antennas, but the advantage of this method is that it preserves good performance for both the antenna and the photovoltaic system, while leaving intact the solar cell’s original structure. You just have to take existing solar cells, and to add a conductive layer.

Devices have been developed with two different types of antenna: one with a copper conductor, which lets no light rays through where the metal is deposited, but guarantees very high antenna efficiency, and one with a so-called transparent conductor (TCOs), which gives priority to solar-cell efficiency.

Thin film amorphous Si Solar cells, which have been under study for more than 20 years in the PV-Lab, have significant benefits: they are light and resistant to radiation. “Their efficiency is lower than the traditional multi-junction solar cells, but they allow better power/weight performances”, state Christophe Ballif and Monica Morales Masis , from PV-Lab.

New Hologram Technology Created With Tiny Nanoantennas
Migrating from the very large to the very small, researchers at Purdue University have created tiny holograms using a “metasurface” capable of ultra-efficient control of light, representing a potential new technology for advanced sensors, high-resolution displays and information processing.

The metasurface is composed of thousands of V-shaped nanoantennas formed into an ultrathin gold foil and could enable planar photonic devices and optical switches small enough to be integrated into chips for information processing, sensing and telecommunications. Laser light shines through the nanoantennas, creating the hologram 10 microns above the metasurface.

To demonstrate the technology, researchers created a hologram of the word PURDUE smaller than 100 microns wide, or roughly the width of a human hair. (Xingjie Ni, Birck Nanotechnology Center)

“If we can shape characters, we can shape different types of light beams for sensing or recording, or, for example, pixels for 3-D displays. Another potential application is the transmission and processing of data inside chips for information technology,” Alexander Kildishev, associate research professor of electrical and computer engineering at Purdue University said. “The smallest features – the strokes of the letters – displayed in our experiment are only 1 micron wide. This is a quite remarkable spatial resolution.”

Metasurfaces could make it possible to use single photons for switching and routing in future computers. While using photons would dramatically speed up computers and telecommunications, conventional photonic devices cannot be miniaturized because the wavelength of light is too large to fit in tiny components needed for integrated circuits.
Nanostructured metamaterials, however, are making it possible to reduce the wavelength of light, allowing the creation of new types of nanophotonic devices, said Vladimir M. Shalaev, scientific director of nanophotonics at Purdue’s Birck Nanotechnology Center and a distinguished professor of electrical and computer engineering.

“The most important thing is that we can do this with a very thin layer, only 30 nanometers, and this is unprecedented,” Shalaev said. “This means you can start to embed it in electronics, to marry it with electronics.”

The researchers have shown how to control the intensity and phase, or timing, of laser light as it passes through the nanoantennas. Each antenna has its own “phase delay” – how much light is slowed as it passes through the structure. Controlling the intensity and phase is essential for creating working devices and can be achieved by altering the V-shaped antennas.