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.

Closing the terahertz gap: Tiny laser is an important step toward new sensors

A new imaging technology rapidly measures the chemical compositions of solids. A conventional image of a sample pill is shown at left; at right, looking at the same surface with terahertz frequencies reveals various ingredients as different colors. Such images would aid quality control and development in pharmaceutical manufacturing, as well as medical diagnosis and treatment.

https://www.eurekalert.org/multimedia/pub/207021.php

In a major step toward developing portable scanners that can rapidly measure molecules in pharmaceuticals or classify tissue in patients' skin, researchers have created an imaging system that uses lasers small and efficient enough to fit on a microchip.

The system emits and detects electromagnetic radiation at terahertz frequencies -- higher than radio waves but lower than the long-wave infrared light used for thermal imaging. Imaging using terahertz radiation has long been a goal for engineers, but the difficulty of creating practical systems that work in this frequency range has stymied most applications and resulted in what engineers call the "terahertz gap."

"Here, we have a revolutionary technology that doesn't have any moving parts and uses direct emission of terahertz radiation from semiconductor chips," said Gerard Wysocki, an associate professor of electrical engineering at Princeton University and one of the leaders of the research team.

Terahertz radiation can penetrate substances such as fabrics and plastics, is non-ionizing and therefore safe for medical use, and can be used to view materials difficult to image at other frequencies. The new system, described in a paper published in the June issue of the journal Optica, can quickly probe the identity and arrangement of molecules or expose structural damage to materials.

The device uses stable beams of radiation at precise frequencies. The setup is called a frequency comb because it contains multiple "teeth" that each emit a different, well-defined frequency of radiation. The radiation interacts with molecules in the sample material. A dual-comb structure allows the instrument to efficiently measure the reflected radiation. Unique patterns, or spectral signatures, in the reflected radiation allow researchers to identify the molecular makeup of the sample.

While current terahertz imaging technologies are expensive to produce and cumbersome to operate, the new system is based on a semiconductor design that costs less and can generate many images per second. This speed could make it useful for real-time quality control of pharmaceutical tablets on a production line and other fast-paced uses.

"Imagine that every 100 microseconds a tablet is passing by, and you can check if it has a consistent structure and there's enough of every ingredient you expect," said Wysocki.

As a proof of concept, the researchers created a tablet with three zones containing common inert ingredients in pharmaceuticals -- forms of glucose, lactose and histidine. The terahertz imaging system identified each ingredient and revealed the boundaries between them, as well as a few spots where one chemical had spilled over into a different zone. This type of "hot spot" represents a frequent problem in pharmaceutical production that occurs when the active ingredient is not properly mixed into a tablet.

The team also demonstrated the system's resolution by using it to image a U.S. quarter. Fine details like the eagle's wing feathers, as small as one-fifth of a millimeter wide, were clearly visible.

While the technology makes the industrial and medical use of terahertz imaging more feasible than before, it still requires cooling to a low temperature, a major hurdle for practical applications. Many researchers are now working on lasers that will potentially operate at room temperature. The Princeton team said its dual-comb hyperspectral imaging technique will work well with these new room-temperature laser sources, which could then open many more uses.

Because it is non-ionizing, terahertz radiation is safe for patients and could potentially be used as a diagnostic tool for skin cancer. In addition, the technology's ability to image metal could be applied to test airplane wings for damage after being struck by an object in flight.

In addition to Wysocki, the paper's Princeton authors are former visiting graduate student Lukasz Sterczewski (currently a postdoctoral scholar at NASA's Jet Propulsion Laboratory) and associate research scholar Jonas Westberg. Other co-authors are Yang Yang, David Burghoff and Qing Hu of the Massachusetts Institute of Technology; and John Reno of Sandia National Laboratories. Support for the research was provided in part by the Defense Advanced Research Projects Agency and the U.S. Department of Energy.

Wave of the future: Terahertz chips a new way of seeing through matter

by Tien Nguyen for the Office of Engineering Communications

http://www.princeton.edu/main/news/archive/S48/64/99Q68/index.xml?section=topstories

Electromagnetic pulses lasting one millionth of a millionth of a second may hold the key to advances in medical imaging, communications and drug development. But the pulses, called terahertz waves, have long required elaborate and expensive equipment to use.

Now, researchers at Princeton University have drastically shrunk much of that equipment: moving from a tabletop setup with lasers and mirrors to a pair of microchips small enough to fit on a fingertip.

Princeton University researchers have drastically shrunk the equipment for producing terahertz — important electromagnetic pulses lasting one millionth of a millionth of a second — from a tabletop setup with lasers and mirrors to a pair of microchips small enough to fit on a fingertip (above). The simpler, cheaper generation of terahertz has potential for advances in medical imaging, communications and drug development. (Photos by Frank Wojciechowski for the Office of Engineering Communications)

In two articles recently published in the IEEE Journal of Solid State Circuits, the researchers describe one microchip that can generate terahertz waves, and a second chip that can capture and read intricate details of these waves.

"The system is realized in the same silicon chip technology that powers all modern electronic devices from smartphones to tablets, and therefore costs only a few dollars to make on a large scale" said lead researcher Kaushik Sengupta, a Princeton assistant professor of electrical engineering.

Terahertz waves are part of the electromagnetic spectrum — the broad class of waves that includes radio, X-rays and visible light — and sit between the microwave and infrared light wavebands. The waves have some unique characteristics that make them interesting to science. For one, they pass through most non-conducting material, so they could be used to peer through clothing or boxes for security purposes, and because they have less energy than X-rays, they don't damage human tissue or DNA.

Terahertz waves also interact in distinct ways with different chemicals, so they can be used to characterize specific substances. Known as spectroscopy, the ability to use light waves to analyze material is one of the most promising — and the most challenging — applications of terahertz technology, Sengupta said.

To do it, scientists shine a broad range of terahertz waves on a target then observe how the waves change after interacting with it. The human eye performs a similar type of spectroscopy with visible light — we see a leaf as green because light in the green light frequency bounces off the chlorophyll-laden leaf.

The challenge has been that generating a broad range of terahertz waves and interpreting their interaction with a target requires a complex array of equipment such as bulky terahertz generators or ultrafast lasers. The equipment's size and expense make the technology impractical for most applications.

Researchers have been working for years to simplify these systems. In September, Sengupta's team reported a way to reduce the size of the terahertz generator and the apparatus that interprets the returning waves to a millimeter-sized chip. The solution lies in re-imaging how an antenna functions. When terahertz waves interact with a metal structure inside the chip, they create a complex distribution of electromagnetic fields that are unique to the incident signal. Typically, these subtle fields are ignored, but the researchers realized that they could read the patterns as a sort of signature to identify the waves. The entire process can be accomplished with tiny devices inside the microchip that read terahertz waves.

"Instead of directly reading the waves, we are interpreting the patterns created by the waves," Sengupta said. "It is somewhat like looking for a pattern of raindrops by the ripples they make in a pond."

In two recently published articles, researchers Kaushik Sengupta (left), an assistant professor of electrical engineering, and Xue Wu (right), a Princeton graduate student in computer science, describe one microchip that can generate terahertz waves, and a second chip that can capture and read intricate details of these waves. Terahertz waves sit between the microwave and infrared light wavebands on the electromagnetic spectrum and have unique characteristics, such as the ability to pass through most non-conducting material such as clothing or boxes without damaging human tissue or DNA.

Daniel Mittleman, a professor of engineering at Brown University, said the development was "a very innovative piece of work, and it potentially has a lot of impact." Mittleman, who is the vice chair of the International Society for Infrared Millimeter and Terahertz Waves, said scientists still have work to do before the terahertz band can begin to be used in everyday devices, but the developments are promising.

"It is a very big puzzle with many pieces, and this is just one, but it is a very important one," said Mittleman, who is familiar with the work but had no role in it.

On the terahertz-generation end, much of the challenge is creating a wide range of wavelengths within the terahertz band, particularly in a microchip. The researchers realized they could overcome the problem by generating multiple wavelengths on the chip. They then used precise timing to combine these wavelengths and create very sharp terahertz pulses.

In an article published Dec. 14 in the IEEE Journal of Solid State Circuits, the researchers explained how they created a chip to generate the terahertz waves. The next step, the researchers said, is to extend the work farther along the terahertz band. "Right now we are working with the lower part of the terahertz band," said Xue Wu, a Princeton doctoral student in electrical engineering and an author on both papers.

"What can you do with a billion transistors operating at terahertz frequencies?" Sengupta asked. "Only by re-imagining these complex electromagnetic interactions from fundamental principles can we invent game-changing new technology."

The paper "On-chip THz spectroscope exploiting electromagnetic scattering with multi-port antenna" was published Sept. 2, and the paper "Dynamic waveform shaping with picosecond time widths" was published Dec. 14, both by IEEE Journal of Solid State Circuits. The research was supported in part by the National Science Foundation's Division of Electrical, Communications and Cyber Systems (grant nos. ECCS-1408490 and ECCS-1509560).

OT-Rice-sized laser, powered one electron at a time, bodes well for quantum computing

Princeton University researchers have built a rice grain-sized microwave laser. Credit: Jason Petta, Princeton University

http://phys.org/news/2015-01-rice-sized-laser-powered-electron-bodes.html#jCp

Princeton University researchers have built a rice grain-sized laser powered by single electrons tunneling through artificial atoms known as quantum dots. The tiny microwave laser, or "maser," is a demonstration of the fundamental interactions between light and moving electrons.

The researchers built the device—which uses about one-billionth the electric current needed to power a hair dryer—while exploring how to use quantum dots, which are bits of semiconductor material that act like single atoms, as components for quantum computers.

"It is basically as small as you can go with these single-electron devices," said Jason Petta, an associate professor of physics at Princeton who led the study, which was published in the journal Science.

The device demonstrates a major step forward for efforts to build quantum-computing systems out of semiconductor materials, according to co-author and collaborator Jacob Taylor, an adjunct assistant professor at the Joint Quantum Institute, University of Maryland-National Institute of Standards and Technology. "I consider this to be a really important result for our long-term goal, which is entanglement between quantum bits in semiconductor-based devices," Taylor said.

The original aim of the project was not to build a maser, but to explore how to use double quantum dots—which are two quantum dots joined together—as quantum bits, or qubits, the basic units of information in quantum computers.

"The goal was to get the double quantum dots to communicate with each other," said Yinyu Liu, a physics graduate student in Petta's lab. The team also included graduate student Jiri Stehlik and associate research scholar Christopher Eichler in Princeton's Department of Physics, as well as postdoctoral researcher Michael Gullans of the Joint Quantum Institute.

Double quantum dot as imaged by a scanning electron microscope. Current flows one electron at a time through two quantum dots (red circles) that are formed in an indium arsenide nanowire. Credit: Science

Because quantum dots can communicate through the entanglement of light particles, or photons, the researchers designed dots that emit photons when single electrons leap from a higher energy level to a lower energy level to cross the double dot.

Each double quantum dot can only transfer one electron at a time, Petta explained. "It is like a line of people crossing a wide stream by leaping onto a rock so small that it can only hold one person," he said. "They are forced to cross the stream one at a time. These double quantum dots are zero-dimensional as far as the electrons are concerned—they are trapped in all three spatial dimensions."

The researchers fabricated the double quantum dots from extremely thin nanowires (about 50 nanometers, or a billionth of a meter, in diameter) made of a semiconductor material called indium arsenide. They patterned the indium arsenide wires over other even smaller metal wires that act as gate electrodes, which control the energy levels in the dots.

To construct the maser, they placed the two double dots about 6 millimeters apart in a cavity made of a superconducting material, niobium, which requires a temperature near absolute zero, around minus 459 degrees Fahrenheit. "This is the first time that the team at Princeton has demonstrated that there is a connection between two double quantum dots separated by nearly a centimeter, a substantial distance," Taylor said.

When the device was switched on, electrons flowed single-file through each double quantum dot, causing them to emit photons in the microwave region of the spectrum. These photons then bounced off mirrors at each end of the cavity to build into a coherent beam of microwave light.

One advantage of the new maser is that the energy levels inside the dots can be fine-tuned to produce light at other frequencies, which cannot be done with other semiconductor lasers in which the frequency is fixed during manufacturing, Petta said. The larger the energy difference between the two levels, the higher the frequency of light emitted.

Claire Gmachl, who was not involved in the research and is Princeton's Eugene Higgins Professor of Electrical Engineering and a pioneer in the field of semiconductor lasers, said that because lasers, masers and other forms of coherent light sources are used in communications, sensing, medicine and many other aspects of modern life, the study is an important one.

"In this paper the researchers dig down deep into the fundamental interaction between light and the moving electron," Gmachl said. "The double quantum dot allows them full control over the motion of even a single electron, and in return they show how the coherent microwave field is created and amplified. Learning to control these fundamental light-matter interaction processes will help in the future development of light sources."

 Explore further: Arrays of electrons trapped in nanoscale circuitry could form the basis for future scalable quantum computers

More information: The paper, "Semiconductor double quantum dot micromaser," was published in the journal Science on Jan. 16, 2015. www.sciencemag.org/lookup/doi/… 1126/science.aaa2501

Journal reference: Science  

Provided by Princeton University