Super surfaces use terahertz waves to help bounce wireless communication into the next generation

 

Suresh Venkatesh, a postdoctoral research associate, Hooman Saeidi, a graduate student, and Kaushik Sengupta, associate professor of electrical engineering, have developed a device that focuses and directs terahertz waves for use in high-speed communications. Photo by Aaron Nathans

https://research.princeton.edu/news/super-surfaces-use-terahertz-waves-help-bounce-wireless-communication-next-generation

by Adam Hadhazy

Assembling tiny chips into unique programmable surfaces, Princeton researchers have created a key component toward unlocking a communications band that promises to dramatically increase the amount of data wireless systems can transmit.

The programmable surface, called a metasurface, allows engineers to control and focus transmissions in the terahertz band of the electromagnetic spectrum. Terahertz, a frequency range located between microwaves and infrared light, can transit much more data than current, radio-based wireless systems. With fifth generation (5G) communications systems offering speeds 10 to 100 times faster than the previous generation, demand for bandwidth is ever increasing. Facing the emergence of technologies such as self-driving cars and augmented reality applications, the terahertz band presents an opportunity for engineers seeking ways to increase data transmission rates.

To harness the expanded space in the terahertz band, engineers will need to overcome some challenges, and that is where the metasurface comes in. Unlike radio waves, which easily pass through obstructions such as walls, terahertz works best with a relatively clear line of sight for transmission. The metasurface device, with the ability to control and focus incoming terahertz waves, can beam the transmissions in any desired direction.  This can not only enable dynamically reconfigurable wireless networks, but also open up new high-resolution sensing and imaging technologies for the next generation of robotics, cyberphysical systems and industrial automation. Because the metasurface is built using standard silicon chip elements, it is low-cost and can be mass produced for placement on buildings, street signs and other surfaces.

"A terahertz beam would be like a laser pointer, whereas today's radio wave transmitters are like light bulbs that send light everywhere. A programmable metasurface is one that produces any possible fields; it’s the ultimate projector," said Kaushik Sengupta(link is external), an associate professor of electrical engineering at Princeton and a lead author of a new study in reporting the results. Sengupta, whose research focuses on integrated chip-scale systems across radio waves, terahertz to optical frequencies, said the metasurface’s low production cost and its programmability means it could be “a major enhancer for communications and network capabilities.”

In a study(link is external) published Dec.14 in Nature Electronics, researchers from Sengupta’s Integrated Micro-systems Research Lab described the design of the metasurface, which features hundreds of programmable terahertz elements, each less than 100 micrometers (millionths of a meter) in diameter and a mere 3.4 micrometers tall, made of layers of copper and coupled with active electronics that collectively resonate with the structure. This allows adjustments to their geometry at a speed of several billions of times per second. These changes — which are programmable, based on desired application — split a single incoming terahertz beam up into several dynamic, directable terahertz beams that can maintain line of sight with receivers.

The Princeton researchers commissioned a silicon chip foundry to fabricate the metasurface as tiles onto standard silicon chips. In this way, the researchers showed that the programmable terahertz metasurface can be configured into low-cost, scalable arrays of tiles. "The tiles are like Lego blocks and are all programmable," said Sengupta. As a proof of concept, the Princeton researchers tested tile arrays measuring two-by-two with 576 such programmable elements and demonstrated beam control by projecting (invisible) terahertz holograms.These elements are scalable across larger arrays.

One possible way to incorporate these sorts of flat tiles into the built environment as next-generation communications network components would be to plaster them as a sort of "smart surface" wallpaper, Sengupta said,

Daniel Mittleman, a professor of engineering at Brown University who was not involved in the study, commented that the research represents a significant step toward terahertz communications.

"This new work demonstrates a fascinating approach which, unlike most previous efforts, is scalable into the terahertz range," said Mittleman. "The key takeaway is that we are now getting a handle on practical methods for actively controlling the wave front, beam size, beam direction, and other features of terahertz beams."

Numerous other applications for the technology include gesture recognition and imaging, as well as industrial automation and security. Another potential application is autonomous or self-driving cars. These vehicles require precise sensing and imaging to make sense of the external world in real time, and ideally even faster than a human driver. Semi-autonomous cars increasingly sold today use 77 GHz radars to detect pedestrians and other vehicles for the purposes of adaptive cruise control and engaging automatic emergency braking. For full, driverless autonomy, though, cars would benefit by "seeing" the road and obstacles better with terahertz-band sensors and cameras, along with being able to communicate with other vehicles more rapidly.  

Looking ahead, the programmable metasurfaces will need further development, Sengupta said, to be integrated as components in innovative, next-generation networks.

"There are so many things that people would like to do that are not possible with current wireless technology," said Sengupta. "With these new metasurfaces for terahertz frequencies, we're getting a lot closer to making those things happen."

Besides Sengupta, the researchers included Suresh Venkatesh, a postdoctoral research associate, Xuyang Lu, and graduate student Hooman Saeidi. The work was supported in part by the Office of Naval Research, the Air Force Office of Scientific Research, and the Army Research Office.

Abstract -On Chip Antenna Measurement: A Survey of Challenges and Recent Trends

M. Rashid Karim,  Xiaodong Yang, Muhammad Farhan Shafique

https://ieeexplore.ieee.org/document/8328817/

Exponential increase in the requirements of cost effective and highly compact wireless modules has put system-on-chip (SoC) technology in high demand. On-chip-antenna (OCA) is an integral component of SoC-based wireless communication systems and has emerged as a perfect candidate for plethora of promising applications, especially at millimeter wave and terahertz frequencies. OCAs also support compact and low power applications of wireless sensor networks and Internet-of-Things. Since OCAs are manufactured on a single substrate along with other components, therefore their successful realization is subject to several challenges; the most significant of which is their accurate measurement. OCA's precise characterization is considered to be one of the toughest challenges to overcome since traditional off-chip antenna measurement setups are not suitable for this job. This calls for innovative measurement techniques, setups and solutions to enable their true characterization. Inspired by the significance of OCA characterization, this paper presents a comprehensive survey of the key recent developments in the field of OCA measurements. The techniques used to measure conventional off-chip antennas are briefly outlined followed by a succinct description of OCA's characterization challenges. The most recent trends and techniques of OCA measurement are expansively compiled. Some avenues for future trends in this regards are also delineated. It is anticipated that the presentation of this review will inspire the research community to come up with the novel methods and proposals to facilitate the OCA characterization process.https://ieeexplore.ieee.org/document/8328817/

OT-New class of zero-moment half metallic magnets may enhance data storage, wireless transmission speeds

http://phys.org/news/2015-03-class-zero-moment-metallic-magnets-storage.html

Attempting to develop a novel type of permanent magnet, a team of researchers at Trinity College in Dublin, Ireland has discovered a new class of magnetic materials based on Mn-Ga alloys

Described as a zero-moment half metal this week in the journal Applied Physics Letters, from AIP Publishing, the new Mn2RuxGa magnetic alloy has some unique properties that give it the potential to revolutionize data storage and significantly increase wireless data transmission speeds.

The discovery realizes a goal researchers have sought for several decades: to make a material with no net magnetic moment, but full spin polarization. Having no magnetic moment—essentially a measure of the net strength of a magnet—frees the material from its own demagnetizing forces and means that it creates no stray magnetic fields. Zero moment also means being immune to the influence of any external magnetic fields, unlike conventional ferromagnets. As a result, there would be no radiation losses during magnetic switching of the material, which occurs as data is read or written, for instance. This property, coupled with full spin polarization means that the material should be extremely efficient in spintronics - the electronics of magnetized electrons.

Furthermore, it promises to shift the ferromagnetic resonance frequency, the maximum speed at which data is written or retrieved, into the low terahertz range. This range is currently of great interest for fast data transmission, but it is unexploited since it is difficult to make effective, yet reasonably-priced emitters and detectors that operate at such extremely high frequencies.

Though scientists have long recognized the merits of such a 'zero-moment half metal', nobody has been able to synthesize one. Several have been proposed through the years, but none of them delivered this combination of properties.

Now the Trinity College team, led by Michael Coey, studying spin-dependent transport properties of Mn2RuxGa (MRG) thin-films as a function of the Ru concentration, developed a zero-moment half metal free from demagnetizing forces that created no stray fields, essentially removing two of the obstacles to integrating magnetic elements in densely packed, nanometer-scale memory elements, and millimeter-wave generators.

The secret was in combining the Manganese with the Ruthenium, said Karsten Rode, a co-author on the new paper.

"Mn is in the Goldilocks zone - the magnetic coupling of the electrons is neither too strong nor too weak - just right," he said. "Ruthenium plays a critical role since without any Ru, even if one were able to crystallize the alloy in the right structure, the electronic bands contributing to the conduction would be only slightly spin polarized."

Building a better magnet

The solution the Trinity College team came up with was to design a material such that the moments of two inequivalent, oppositely aligned magnetic Mn sublattices perfectly compensated for one another—essentially cancelling each other out and giving no net moment. But, in a simplified picture, only one of these sublattices actually carries current—so that the result was a 100 percent spin polarized current with no net magnetic moment.

The development of this new material required a delicate balance. Spin-polarized current is due to the coupling of electrons in localized magnetic states (d-states) with mobile electrons in current-carrying states (s-states). If this coupling is too strong in a two-sublattice system, the spin polarization of the mobile carriers in the material tends to average to zero, but on the other hand, if the coupling is too weak, only a small fraction of the s-like electrons are spin polarized, and this would result in a very low spontaneous Hall effect. It is the spontaneous Hall effect that provides one piece of evidence of the spin polarization at room temperature.

Rode explained that the Manganese in the material was key to achieving this breakthrough because it allowed them to create a highly spin-polarized band of s-like electrons, yet keeping the magnetic coupling weak enough to allow for one of the spin bands to be pushed away from the Fermi level where all the conduction takes place. The addition by Ruthenium of both electrons and extra electronic states was also key because that made it possible to achieve zero net moment.

"The most difficult part was to understand that our new material was truly special," said Rode. "Our first experimental results could have been dismissed as a weakly-anisotropic ferrimagnet of no particular interest. Once we realized that there was a possibility that we could achieve full compensation of the magnetic moments, coupled with a large spin polarization, we started checking to see if the 'zero-moment half metal' hypothesis would stand intense scrutiny - and it did."

Now that the first example of this new type of magnet has been developed, the team will work to realize its benefits. "We need to demonstrate the spintronic functionality in a practical device" Rode said. "This is challenging for a Mn-based alloy. The manganese is easily oxidized and this has to be avoided in a fully-functional thin-film device stack. But now that we think we understand the conditions necessary to create a zero-moment half metal, it is likely that MRG will not long remain an only child."

 Explore further: Scientists resolve spin puzzle

More information: "Giant spontaneous Hall effect in zero-moment Mn2RuxGa," by Naganivetha Thiyagarajah, Yong-Chang Lau, Davide Betto, Kiril Borisov, J. M. D. Coey, Plamen Stamenov and Karsten Rode, Applied Physics Letters, Tuesday, March 24, 2015. DOI: 10.1063/1.4913687

Journal reference: Applied Physics Letters  

Abstract-TeraHertz Photonics for Wireless Communications

TeraHertz Photonics for Wireless Communications

Alwyn J. Seeds, Haymen Shams, Martyn J. Fice, and Cyril C. Renaud 

Journal of Lightwave Technology, Vol. 33, Issue 3, pp. 579-587 (2015)

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Acrobat PDF (621 KB) 

http://www.opticsinfobase.org/jlt/abstract.cfm?uri=jlt-33-3-579

Optical fibre transmission has enabled greatly increased transmission rates with 10 Gb/s common in local area networks. End users find wireless access highly convenient for mobile communication. However, limited spectrum availability at microwave frequencies results in per-user transmission rates limited to much lower values, e.g., 500 Mb/s for 5-GHz band IEEE 802.11ac. Extending the high data-rate capacity of optical fiber transmission to wireless devices requires greatly increased carrier frequencies. This paper will describe how photonic techniques can enable ultrahigh capacity wireless data distribution and transmission using signals at millimeter-wave and TeraHertz (THz) frequencies.

© 2014 OAPA

Abstract-Indoor Terahertz Communications: How Many Antenna Arrays Are Needed?

LIN, C. 
Cen Lin is with the School of Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, GA 30332, USA.(email:linc@gatech.edu) 
Li, G.
http://ieeexplore.ieee.org/xpl/abstractAuthors.jsp?reload=true&arnumber=7036065&sortType%3Dasc_p_Sequence%26filter%3DAND%28p_IS_Number%3A4656680%29%26rowsPerPage%3D50


Terahertz (THz) communications promise to be the next frontier for wireless networks. Novel solutions should be explored to overcome the hardware constraints and the severe path loss. In this paper, we study a low-complexity indoor THz communication system with antenna subarrays. The Saleh- Valenzuela (S-V) channel model is modified to characterize the THz channel. By exploiting the hybrid beamforming with multiple subarrays, we analyze the ergodic capacity of the system and obtain an upper bound. Furthermore, with the analysis of performance degradation for the uncertainty in THz phase shifters, we provide a guidance on the design of antenna subarray size and number for certain long-term data rate requirements with different distances. Simulation results validate the effectiveness of the ergodic capacity upper bound, and show that the proposed THz system and antenna array structure can efficiently achieve capacity gains and support THz communications.

Terahertz tech gets a major push at Rice

Mike Williams
http://news.rice.edu/2014/06/24/terahertz-tech-gets-a-major-push-at-rice-2/

Keck Foundation grant to Rice University bolsters cutting-edge research for communications, imaging

Rice University scientists have received a grant to develop terahertz-based technology that could enable a dramatic advance in wireless communications and other disciplines.

The $1 million grant by the W.M. Keck Foundation will let them tackle some of the knotty problems barring them from using the largely untapped terahertz region of the electromagnetic spectrum. Rice will supplement the grant with a $1.5 million commitment.

Chips the size of the one displayed above, which put out the shortest pulse ever generated by such a device, will be packaged into arrays that can steer terahertz beams. Click on the photo for a larger version. Photo by Jeff Fitlow

Potential benefits include much faster cellphone networks as well as sensors and detectors that may revolutionize medical imaging, security screening and manufacturing quality control.

Terahertz waves, which occupy the band from about 1 millimeter to 100 micrometers, are unique in the spectrum because few have figured out how to bend them to their purposes, said Daniel Mittleman, a professor of electrical and computer engineering and principal investigator on the project.

Longer waves power microwave ovens and radar; shorter waves include infrared and ultraviolet light, the visible spectrum and X-rays. While they don’t pass easily through water, or even travel long distances in the atmosphere, many non-metallic materials are transparent to terahertz. This unique feature opens the door for many interesting applications, such as detecting chemicals and hidden explosives.

“Terahertz technologies have been on the horizon for a long time, but people have only been thinking about specific applications seriously for a few years,” Mittleman said. “But there is such potential. For example, there are famous diagrams that show that the demand for wireless bandwidth is growing exponentially, and that six or eight years from now we’ll need tens or maybe 100 gigahertz of bandwidth. We’re nowhere near that now with existing wireless networks (which use a part of the spectrum well below terahertz frequencies).” Current phones only achieve hundreds of megabits per second even under the most ideal conditions, he said.

“If you want a higher data rate, you have to go to a higher carrier frequency. But almost all of the frequencies above what we use now are already claimed by someone and regulated,” he said.

Rice scientist Aydin Babakhani, left, one of four university researchers who have won a new grant to develop new terahertz technologies, shows a microchip that put out the shortest pulse ever measured from such a small device. The chip designed by Babakhani and graduate student Mahdi Assefzadeh, right, is seen as a step toward the ability to use terahertz for wireless communications and other applications. Photo by Jeff Fitlow

Until you get up into the terahertz range. The band between microwave and infrared offers a wide-open frontier, and a unique collection of talent at Rice is eager to explore it, Mittleman said. “We have some innovative approaches, and the combination of expertise we have is fairly unusual,” he said.

Co-investigators on the project are Junichiro Kono, a professor of electrical and computer engineering and of physics and astronomy; Edward Knightly, a professor of electrical and computer engineering and of computer science; and Aydin Babakhani, an assistant professor of electrical and computer engineering.

Mittleman’s research centers on devices like a terahertz version of Rice’s single-pixel camera and metamaterials that offer the possibility of high-speed modulation of terahertz beams. Kono specializes in the creation of advanced terahertz detectors that take advantage of Rice’s unique position as a world leader in carbon nanotechnology. Knightly, as director of Rice’s Wireless Networks Group, is a pioneer in the development of new wireless technologies. And Babakhani, director of the Rice Integrated Systems and Circuits laboratory and a winner of a DARPA Young Faculty Award in 2012, designs millimeter-wave and terahertz integrated circuits and antennas for communications, radar, medical imaging and biosensing.

Terahertz signals can help identify substances from the way they interact with the beams, but the beams themselves don’t travel as far in air as microwave signals. And there are other problems, primary among them the lack of a powerful, portable and practical source of terahertz.

The Rice team expects to chip away at those problems. Babakhani and his team have already developed a silicon-based microchip that puts out the shortest pulse ever generated by such a device, an 8-picosecond impulse radiator that won the best paper award at the recent IEEE International Microwave Symposium. While it’s not in the terahertz range, it could get there with the assistance of graphene, said Mittleman, who also serves as director of Rice’s Richard E. Smalley Institute for Nanoscale Science and Technology.

“Aydin thinks the power can scale,” he said. “We want to combine what Jun has done with the optical nonlinearities of graphene to frequency-double or triple it up to the range we need.”

Small, fast, inexpensive chips that can not only send and receive terahertz beams but also steer them are necessary for future wireless applications, Mittleman said. “We’re going to need lots of them, on the order of 100,000 of them to cover a city the size of New York, as opposed to the several thousand cell towers they use now.”

The world's smallest terahertz-enabled chip, similar to this one developed in the Rice lab of Aydin Babakhani, may be critical to the success of next-generation communications networks. Click on the photo for a larger version. Courtesy of the Rice Integrated Systems and Circuits lab

Babakhani’s speck-sized chips hold the key to steerable terahertz beams. “We’ve already demonstrated beam steering with two chips about 10 centimeters apart,” he said. “The timing synchronization between them is almost perfect. Now we’re looking at combining a signal from many chips with timing accuracy of a couple of hundred femtoseconds.” A femtosecond is one millionth of one billionth of a second.

Wireless terahertz communications would require new and smarter network architecture. Current cellular systems have to imperceptibly switch between users to keep the signals from interfering with each other, Knightly said, but that will no longer be an issue in a network that sends data through a narrow beam that tracks the user.

“With terahertz, we’ll have to coordinate distributed antennas, users and highly directional links, all wirelessly. This is a true paradigm shift requiring us to rethink the basic principles of wireless networking.” Knightly said.

Leaping the hurdles associated with wireless will help the team on other projects that require strong, dependable terahertz sources and detectors. Mittleman’s experience with the terahertz single-pixel camera and Kono’s carbon nanotube-based detectors hint at the possibility of multispectral terahertz imaging. A super spectrometer would be particularly useful for security screening of people and containers.

Kono’s successful efforts to detect and manipulate terahertz using graphene and carpets of nanotubes has inspired visions of many potential applications, he said. “Several pieces of the terahertz puzzle have reached a moderate level of maturity, but these pieces have emerged from diverse disciplines ranging from material science to optoelectronics to signal processing. The ultimate goal of our research is to fully eliminate the terahertz ‘technology gap.’”

Kono said the researchers intend to develop materials and build devices, but also aim to deepen their understanding of the physics and chemistry at the boundaries of electronics and optics.

“The great thing about the experimental capabilities we have right now is that, in terms of spectrum, we’re attacking the problems from above and below,” Knightly said. “Below terahertz are millimeter wave and 60 gigahertz technologies, and we have experimental state-of-the-art devices at those frequencies in our labs.

“From above, we have prototypes in the visible light range that, from a networking point of view, have characteristics like line-of-sight and short range in common with terahertz. With these and other fundamental advances, we’re very well positioned to realize our goals.”