electromagnetic waves

Stanford engineer aims to connect the world with ant-sized radios

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Costing just pennies to make, tiny radios-on-a-chip are designed to serve as controllers or sensors for the ‘Internet of Things.’

 

The tiny radio-on-a-chip gathers all the power it needs from the same electromagnetic waves that carry signals to its receiving antenna. (Photo courtesy of Amin Arabian)

A Stanford engineering team has built a radio the size of an ant, a device so energy efficient that it gathers all the power it needs from the same electromagnetic waves that carry signals to its receiving antenna – no batteries required.

Designed to compute, execute and relay commands, this tiny wireless chip costs pennies to fabricate – making it cheap enough to become the missing link between the Internet as we know it and the linked-together smart gadgets envisioned in the “Internet of Things.”

“The next exponential growth in connectivity will be connecting objects together and giving us remote control through the web,” said Amin Arbabian, an assistant professor of electrical engineering who recently demonstrated this ant-sized radio chip at the VLSI Technology and Circuits Symposium in Hawaii.

Much of the infrastructure needed to enable us to control sensors and devices remotely already exists: We have the Internet to carry commands around the globe, and computers and smartphones to issue the commands. What’s missing is a wireless controller cheap enough to so that it can be installed on any gadget anywhere.

“How do you put a bi-directional wireless control system on every lightbulb?” Arbabian said. “By putting all the essential elements of a radio on a single chip that costs pennies to make.”

Cost is critical because, as Arbabian observed, “We’re ultimately talking about connecting trillions of devices.”

A three-year effort

Arbabian began the project in 2011 while he was completing a PhD program and working with Professor Ali Niknejad, director of the Wireless Research Center at the University of California, Berkeley. Arbabian’s principal collaborator was his wife, Maryam Tabesh, then also a student in Niknejad’s lab and now a Google engineer.

Arbabian joined the Stanford faculty in 2012 and brought a fourth person onto the team, Mustafa Rangwala, who was then a postgraduate student but is now with a startup company.

The work took time because Arbabian wanted to rethink radio technology from scratch.

“In the past when people thought about miniaturizing radios, they thought about it in terms of shrinking the size of the components,” he said. But Arbabian’s approach to dramatically reducing size and cost was different. Everything hinged on squeezing all the electronics found in, say, the typical Bluetooth device down into asingle, ant-sized silicon chip.

This approach to miniaturization would have another benefit – dramatically reducing power consumption, because a single chip draws so much less power than conventional radios. In fact, if Arbabian’s radio chip needed a battery – which it does not – a single AAA contains enough power to run it for more than a century.

But to build this tiny device every function in the radio had to be reengineered.

The antenna

The antenna had to be small, one-tenth the size of a Wi-Fi antenna, and operate at the incredibly fast rate of 24 billion cycles per second. Standard transistors could not easily process signals that oscillate that fast. So his team had to improve basic circuit and electronic design.

Many other such tweaks were needed but in the end Arbabian managed to put all the necessary components on one chip: a receiving antenna that also scavenges energy from incoming electromagnetic waves; a transmitting antenna to broadcast replies and relay signals over short distances; and a central processor to interpret and execute instructions. No external components or power are needed.

And this ant-sized radio can be made for pennies.

Based on his designs, the French semiconductor manufacturer STMicroelectronics fabricated 100 of these radios-on-a-chip. Arbabian has used these prototypes to prove that the devices work; they can receive signals, harvest energy from incoming radio signals and carry out commands and relay instructions.

Now Arbabian envisions networks of these radio chips deployed every meter or so throughout a house (they would have to be set close to one another because high-frequency signals don’t travel far).

He thinks this technology can provide the web of connectivity and control between the global Internet and smart household devices. “Cheap, tiny, self-powered radio controllers are an essential requirement for the Internet of Things,” said Arbabian, who has created a web page to share some ideas on what he calls battery-less radios.

 

Silicon nanorods bend light in new directions.

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Ultrathin coatings that arbitrarily manipulate the phase and polarization of electromagnetic waves have been created by researchers in the US. The coatings are made from silicon nanorods using a technique that is compatible with industrial processes such as photolithography. The researchers say that the coatings could be used in new types of optical components that are much less bulky than traditional lenses. The technique could even be used to bend light in ways not possible with conventional lenses.

Fermat’s principle – the rule that light travels along the path of least time – says that electromagnetic waves travel along the path on which they accumulate the least phase. In a medium of higher refractive index, the wavelength shortens and so a wave accumulates more phase across the same distance. A wave therefore bends towards the normal to reduce the distance travelled in the medium and the phase accumulated.

Manipulative metasurface

In a conventional optical component such as a lens, phase accumulates continuously as the wave propagates and this determines the nature of the wave that emerges from the lens. However, if the phase of a wave could be changed discontinuously at a surface (called a metasurface), then the wave could, in principle, be manipulated in ways not possible with conventional optics.

While this is straightforward in theory, the challenge facing physicists is how to create such a phase discontinuity using real materials. In 2011 researchers at Harvard University led by Federico Capasso andZeno Gaburro covered a surface with V-shaped gold antennas so that the surface could be used to introduce any desired phase shift to optical waves passing through it. While this allows the arbitrary redirection of visible light, there are two major problems with this approach. First, the metallic nature of the surface means that most of the visible light is lost as it travels through the surface. Second, thin layers of metal are very difficult to work with and incompatible with the complementary metal-oxide semiconductor (CMOS) process used to make modern electronic devices.

In the new research, Mark Brongersma and colleagues at Stanford University in California use lossless silicon optical antennas. When illuminated by a particular frequency of light (which can be selected by varying its diameter), the antenna will resonate strongly. This causes the light wave to pick up a phase shift that depends on the relative orientations of its polarization axes to the antenna. By appropriately tailoring the orientations and distances between the antennas, the surface can impart any desired phase shift to the light. This allowed the researchers to reproduce the functions of a bulk lens with a single layer of nanorods just 100 nm thick.

Axicons and Bessel beams

The team was able to create various types of “lenses” using this technique. These include traditional focusing lenses and an axicon. The latter is a specialized type of conical lens that transforms an ordinary laser beam into a Bessel beam – a ring-shaped beam used in optical tweezers and eye surgery.

Optics expert John Pendry of Imperial College London is impressed. “If anyone in the electronics or photonics game wanted to use a material, it would have to be silicon,” he explains. “You can lay down silicon extremely flat and shape it very precisely. Metals are nowhere near silicon in terms of the precision and the control you can exert over them; so, if you can translate a technology like metasurfaces into a silicon environment, you’re on to a real winner because you can hook on to this bandwagon that’s been rolling for half a century now.”

Scanning electron micrograph of an axicon lens made of silicon nanorods

I think that Intel or other companies based on CMOS technology can implement such a metasurface now
Erez Hasman, Technion-Israel

In the experiment, the metasurfaces were fabricated by electron-beam lithography, but team member Erez Hasman, now at the Technion-Israel Institute of Technology in Haifa, says that commercial companies could produce large quantities using industrial processes such as photolithography. “I think that Intel or other companies based on CMOS technology can implement such a metasurface now,” he says.

“The theoretical concept is not surprising at this point, but the fact that they built it and it works is interesting,” agrees Andrea Alù, an expert on metasurfaces at the University of Texas at Austin. He looks forward to the development of optical components that are not possible with normal lenses. Hasman suggests that one of the first such uses might be to interface waveguides with free space. “In general, the modes of a laser resonator or a waveguide are very complex and different from the modes of free space,” he says. Coupling the two together to allow signals to pass between them, he explains, is very difficult using a lens or a prism but should be no problem using the 2D metasurface.