Researchers design first battery-powered invisibility cloak.

Researchers at The University of Texas at Austin have proposed the first design of a cloaking device that uses an external source of energy to significantly broaden its bandwidth of operation.

Andrea Alù, associate professor at the Cockrell School of Engineering, and his team have proposed a design for an active cloak that draws energy from a battery, allowing objects to become undetectable to radio sensors over a greater range of frequencies.

The team’s paper, “Broadening the Cloaking Bandwidth with Non-Foster Metasurfaces,” was published Dec. 3 in Physical Review Letters. Alù, researcher Pai-Yen Chen and postdoctoral research fellow Christos Argyropoulos co-authored the paper. Both Chen and Argyropoulos were at UT Austin at the time this research was conducted. The proposed active cloak will have a number of applications beyond camouflaging, such as improving cellular and radio communications, and biomedical sensing.

Cloaks have so far been realized with so-called passive technology, which means that they are not designed to draw energy from an external source. They are typically based on metamaterials (advanced artificial materials) or metasurfaces (a flexible, ultrathin metamaterial) that can suppress the scattering of light that bounces off an object, making an object less visible. When the scattered fields from the cloak and the object interfere, they cancel each other out, and the overall effect is transparency to radio-wave detectors. They can suppress 100 times or more the detectability at specific design frequencies. Although the proposed design works for radio waves, active cloaks could one day be designed to make detection by the human eye more difficult.

“Many cloaking designs are good at suppressing the visibility under certain conditions, but they are inherently limited to work for specific colors of light or specific frequencies of operation,” said Alù, David & Doris Lybarger Endowed Faculty Fellow in the Department of Electrical and Computer Engineering. In this paper, on the contrary, “we prove that cloaks can become broadband, pushing this technology far beyond current limits of passive cloaks. I believe that our design helps us understand the fundamental challenges of suppressing the scattering of various objects at multiple wavelengths and shows a realistic path to overcome them.”

The proposed active cloak uses a battery, circuits and amplifiers to boost signals, which makes possible the reduction of scattering over a greater range of frequencies. This design, which covers a very broad frequency range, will provide the most broadband and robust performance of a cloak to date. Additionally, the proposed active technology can be thinner and less conspicuous than conventional cloaks.

In a related paper, published in Physical Review X in October, Alù and his graduate student Francesco Monticone proved that existing passive cloaking solutions are fundamentally limited in the bandwidth of operation and cannot provide broadband cloaking. When viewed at certain frequencies, passively cloaked objects may indeed become transparent, but if illuminated with white light, which is composed of many colors, they are bound to become more visible with the cloak than without. The October paper proves that all available cloaking techniques based on passive cloaks are constrained by Foster’s theorem, which limits their overall ability to cancel the scattering across a broad frequency spectrum.

In contrast, an active cloak based on active metasurfaces, such as the one designed by Alù’s team, can break Foster’s theorem limitations. The team started with a passive metasurface made from an array of metal square patches and loaded it with properly positioned operational amplifiers that use the energy drawn from a battery to broaden the bandwidth.

“In our case, by introducing these suitable amplifiers along the cloaking surface, we can break the fundamental limits of passive cloaks and realize a ‘non-Foster’ surface reactance that decreases, rather than increases, with frequency, significantly broadening the of operation,” Alù said.

The researchers are continuing to work both on the theory and design behind their non-Foster active cloak, and they plan to build a prototype.

Alù and his team are working to use active cloaks to improve wireless communications by suppressing the disturbance that neighboring antennas produce on transmitting and receiving antennas. They have also proposed to use these cloaks to improve biomedical sensing, near-field imaging and energy harvesting devices.

Nanomechanical FM transmitter is smallest yet.

Researchers at Columbia University in the US have built the smallest frequency-modulated (FM) radio transmitter ever. Based on a graphene nanomechanical system (NEMS), the device oscillates at a frequency of 100 MHz. It could find use in a variety of applications, including sensing tiny masses and on-chip signal processing. It also represents an important first step towards the development of advanced wireless technology and the design of ultrathin mobile phones, says team co-leader James Hone.

“Our device is much smaller than any other radio-signal source ever made and, importantly, can be put on the same chip that is used for data processing,” he explains.

Graphene is a sheet of carbon atoms arranged in a honeycomb-like lattice that is just one atom thick. Since its discovery in 2004, this “wonder material” has continued to amaze scientists with its growing list of unique electronic and mechanical properties, which include high electrical conductivity and exceptional strength. Indeed, some researchers believe that graphene might even replace silicon as the electronic industry’s material of choice in the future.

Ideal for making NEMS

Graphene is ideal for making NEMS – which are scaled-down versions of the microelectromechanical systems (MEMS) that are routinely employed in vibration-sensing applications. The new device made by Hone and colleagues is a NEMS version of a common electronic component known as a voltage-controlled oscillator (VCO) and generates a frequency-modulated (FM) signal of about 100 MHz. This frequency lies exactly in the middle of the FM radio band (87.7–108 MHz) and the researchers say that they have already succeeded in using low-frequency music signals to modulate the 100 MHz carrier signal from their graphene NEMS and recover the signals again using an ordinary FM receiver.

While graphene NEMS might not replace conventional radio transmitters yet, they will certainly be used in many other wireless signal-processing applications. Although electrical circuits have been continuously shrinking over the last few decades (as described by Moore’s law), there are still some types of devices – especially those involved in creating and processing radio-frequency (RF) signals – that are notoriously difficult to miniaturize, explains team co-leader Kenneth Shepard. Called off-chip components because they cannot be integrated with miniaturized devices, they require a lot of space and electrical power, and their frequency cannot be easily tuned.

Graphene NEMS offer a solution to this problem because they are very small – the active area is only a few microns across – and they can potentially be integrated directly onto conventional CMOS chips. Most importantly, it is easy to tune their frequency thanks to graphene’s exceptional strength.

Adjusting the tension

The Columbia researchers made their devices by contacting graphene sheets to source and drain electrodes and freely suspending the sheets over metal gates. In this configuration, the graphene functions like the skin of a drum. A DC gate voltage pulls the graphene down towards the gate and this adjusts the tension and, therefore, the mechanical resonance frequency, explains Hone. A radio-frequency signal on the gate drives sheet vibrations. “Finally, we apply a DC bias across the graphene and when the graphene vibrates it acts as a transistor whose gate capacitance is constantly changing – and it is this that creates an RF source–drain current,” he says.

The team studied the vibrational properties of the device at room temperature in a vacuum chamber. “To make an oscillator, we first adjust the signal gain to just above unity (using a variable amplifier) and the phase to zero (using a phase shifter) at the resonance frequency,” says Hone. “We then connect the output to the gate. This creates a closed loop that amplifies random thermal vibrations and makes the device oscillate.”

The researchers say they are now busy looking at how to put their devices directly onto integrated circuits that already contain all the necessary drive and readout circuitry. They also hope to improve the performance of their oscillators and reduce device noise.

New Irish technology to reduce water and pesticide usage, increase crop yields and make GMOs obsolete.

New technology invented by Irish scientists could lead to more productive crop yields, lower water and pesticide usage and make GMOs obsolete.

The technology, radio wave energized water, massively increases the output of vegetables and fruits by up to 30 percent. Plants that are watered using this technology are said to grow bigger and have increased resistance to disease.

Professor Austin Darragh and Dr. JJ Leahy of Limerick Chemistry and Environmental Science developed this technology. The pair of scientists invented a device, called Vi-Aqua, which converts 24 volts of electricity into a radio signal, which charges the water via an antenna. Attached to a hose, this device can charge thousands of gallons of water in less than 10 minutes at the cost of mere pennies.

According to, the technology has been successfully tested in many countries in Europe and in India.

Explaining the technology, Darragh said, “Vi-Aqua makes water wetter and introduces atmospheric nitrogen into the water in the form of nitrates – so it is free fertiliser. It also produces the miracle of rejuvenating the soil by invigorating soil-based micro-organisms.”

He then went on to say, “We can also make water savings of at least 30 per cent. When the water is treated it becomes a better solvent, which means it can carry more nutrients to the leaves and stem and percolate better down into the soil to nourish the roots, which in turn produces a better root system. Hence the reason you need less water and why you end up with larger and hardier crops.”

The Royal Botanical Gardens at Kew, London, were so impressed by the technology that they granted Darragh and his team the right to use their official coat of arms on the device, the first time anyone has received such an honor.