Graphene-based nano-antennas could enable intelligent swarms of dust to work together

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This schematic representation of a plasmonic nano-antenna made of graphene shows how short-wave EM waves are converted into long-wave surface plasmon polaritons in a graphene layer (Image: Georgia Tech)

Smart dust. Utility fog. Programmable matter. Gray and blue goo. Cooperating swarms of micrometer-sized devices (motes) offer completely new solutions and possibilities that can hardly be imagined.

However, collaboration requires communication, and traditional radio or optical networks are simply not practical at this size. Now researchers at Georgia Tech have invented a plasmonic graphene nano-antenna that can be used efficiently in the millimeter radio range and takes another step towards smart dust.

Graphene is a two-dimensional layer of carbon atoms, which has become one of the miracle materials of the 21st. One of the unique properties of graphene is that its transport electrons behave as if they had no mass and independently of their energy with about 0.3 Move percent of the speed of light.

This speed limit means that the wavelength of the surface plasmon polaritons at a certain frequency is several hundred times smaller than the wavelength of freely propagating electromagnetic waves of the same frequency.

The team at Georgia Tech showed that this speed difference enables graphene-based antennas to be much smaller than antennas made of conventional materials with roughly the same efficiency.

Why do we need tiny electromagnetic antennas?

One of the barriers to creating smart dust, that is, cooperating swarms of micrometer-sized devices, is enabling the communication necessary for self-directed group activities. With such small sizes, a communication system encounters a number of limitations, such as: B. the available energy and power, the size of the resonance structures, the signal diffraction and the quantization limits. Let’s look at an example to illustrate these factors.

Imagine a smart speck of dust in a one-micron cube. The mass of this cube is on the order of one picogram. If a tenth of the cube were a next generation supercapacitor, the amount of energy stored would be around 10 picojoules and the power density around 1 picowatt.

A radiation source or a radiation detector with a size of one micrometer means that the electromagnetic waves used for communication should have a smaller wavelength than this, ie over 300 THz (near infrared light). This would currently limit our mote to the use of optoelectronic semiconductors for communication. Lasers can be made small enough, but they require too much energy to reach their lasing threshold, so smart dust is limited to LEDs and photodetectors.

With a photon efficiency of around ten percent, an electrical picowatt would produce around 600,000 IR photons per second. The IR radiation would be emitted in an almost hemispherical pattern due to the diffraction of the emitted light at the edges of the LED chip. In addition, only ten percent of the photons that hit another mote are converted into electrons.

To achieve a data rate of one bit per second with a signal-to-noise ratio of 10: 1, the communicating intelligent dust particles only have to be about 35 micrometers apart.

While long-range messages can be routed anywhere within the swarm through intermediaries, a swarm large enough for most purposes would need to contain a huge number of motes to provide connectivity. The number is so large that it makes sense to look for a different communication concept.

Graphene-based plasmonic antennas

The latest approach is plasmonic antennas based on graphene. In contrast to plasmonic antennas, which are based on precious metals, these intelligent dusts could work at frequencies that are at least 100 times (and in principle maybe 1000 times) smaller than with a conventional metal antenna.

The principle of operation is that an electromagnetic (EM) wave, which is directed onto a graphene surface perpendicular to this surface, excites the electrons in the graphene to vibrate. These electrons interact with those of the dielectric material on which the graphene is attached to form surface plasmon polaritons (SPP).

When the antenna goes into resonance (meaning that a whole number of SPP wavelengths fit into the physical dimensions of the graph), the coupling of the SPP to the external EM waves increases greatly, resulting in efficient energy transfer between the two.

When receiving, the energy from the SPP is diverted to a transceiver. When transmitting, the electron density of the graph is modulated to drive the formation of the SPP, which then convert to EM waves and propagate, taking with them the energy pumped into the SPP. Researchers at Georgia Tech are also working on compatible graphene-based transceivers, an effort to watch out for.

Note that the lower frequency means you can generate 100-1000 times more photons for the same amount of energy, which brings the communication range between two micrometer-sized subjects to about 0.35-1.0 mm (0.014-0.04 in ) and thus the number of motifs for a swarm with a given physical dimension is reduced by at least a million times.

There are other uses for such small antennas. For example, a phased array antenna as small as 100 microns in diameter could be used to produce 300 GHz beams just a few degrees in diameter, rather than the hemispherical radiation you’d expect from a conventional metal antenna as large as 100 microns.

This would enable wireless terabit-per-second networks to be set up for smartphones and computers without the loss of performance that occurs when sub-THz radio waves propagate in the atmosphere. Whatever the outcome of the story, it is another example of the novel behaviors graphs are capable of.

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