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Showing posts with label Rice University. Show all posts
Showing posts with label Rice University. Show all posts
Ultrathin, broadband polarization rotators are made possible
by ultrathin carbon nanotube films developed at RiceUniversity
in 2016. The films of highly aligned single-walled nanotubes were first made in
2016. Credit: Kono Laboratory/Rice University
It's always good when your hard work reflects well on you.
With the discovery of the giant polarization rotation of light, that is literally so.
The ultrathin, highly aligned carbon nanotube films first made by Rice University physicist Junichiro Kono and his students a few years ago turned out to have a surprising phenomenon waiting within: An ability to make highly capable terahertz polarization rotation possible.
This rotation doesn't mean the films are spinning. It does mean that polarized light from a laser or other source can now be manipulated in ways that were previously out of reach, making it completely visible or completely opaque with a device that's extremely thin.
The unique optical rotation happens when linearly polarized pulses of light pass through the 45-nanometer film and hit the silicon surface on which it sits. The light bounces between the substrate and film before finally reflecting back, but with its polarization turned by 90 degrees.
This only occurs, Kono said, when the input light's polarization is at a specific angle with respect to the nanotube alignment direction: the "magic angle."
The discovery by lead author Andrey Baydin, a postdoctoral researcher in Kono's lab, is detailed in Optica. The phenomenon, which can be tuned by changing the refractive index of the substrate and the film thickness, could lead to robust, flexible devices that manipulate terahertz waves.
RiceUniversity physicists
have made unique broadband polarization rotators with ultrathin carbon nanotube
films. The films optically rotate polarized light output by 90 degrees, but
only when the input light's polarization is at a specific angle with respect to
the nanotube alignment direction: the "magic angle." Credit: Kono
Laboratory/Rice University
Kono said easy-to-fabricate, ultrathin broadband polarization rotators that stand up to high temperatures will address a fundamental challenge in the development of terahertz optical devices. The bulky devices available until now only enable limited polarization angles, so compact devices with more capability are highly desirable.
Because terahertz radiation easily passes through materials like plastics and cardboard, they could be particularly useful in manufacturing, quality control and process monitoring. They could also be handy in telecommunications systems and for security screening, because many materials have unique spectral signatures in the terahertz range, he said.
"The discovery opens up new possibilities for waveplates," Baydin said. A waveplate alters the polarization of light that travels through it. In devices like terahertz spectrometers used to analyze the molecular composition of materials, being able to adjust polarization up to a full 90 degrees would allow for data gathering at a much finer resolution.
"We found that specifically at far-infrared wavelengths—in other words, in the terahertz frequency range—this anisotropy is nearly perfect," Baydin said. "Basically, there's no attenuation in the perpendicular polarization, and then significant attenuation in the parallel direction.
"We did not look for this," he said. "It was completely a surprise."
He said theoretical analysis showed the effect is entirely due to the nature of the highly aligned nanotube films, which were vanishingly thin but about 2 inches in diameter. The researchers both observed and confirmed this giant polarization rotation with experiments and computer models.
"Usually, people have to use millimeter-thick quartz waveplates in order to rotate terahertz polarization," said Baydin, who joined the Kono lab in late 2019 and found the phenomenon soon after that. "But in our case, the film is just nanometers thick."
"Big and bulky waveplates are fine if you're just using them in a laboratory setting, but for applications, you want a compact device," Kono said. "What Andrey has found makes it possible."
Representation of a transmitter (left) broadcasting a signal with strong angular dispersion. Each frequency is represented by a different color and comes out in a different direction, which produces a rainbowlike structure. Two of the frequencies make it to the receiver (right), one represented by yellow (LOS path) and another by blue (NLOS path incorporating a reflection off a surface).Mittleman Lab, Brown University
In a step toward enabling ultrafast wireless data transmission, researchers explore non-line-of-sight path issue for link discovery method using terahertz radiation.
WASHINGTON, April 27, 2021 -- If a base station in a local area network tries to use a directional beam to transmit a signal to a user trying to connect to the network -- instead of using a wide area network broadcast, as base stations commonly do -- how does it know which direction to send the beam?
Researchers from Rice University and Brown University developed a link discovery method in 2020 using terahertz radiation, with high-frequency waves above 100 gigahertz. For this work, they deferred the question of what would happen if a wall or other reflector nearby creates a non-line-of-sight (NLOS) path from the base station to the receiver and focused on the simpler situation where the only existing path was along the line-of-sight (LOS).
In APL Photonics, from AIP Publishing, those same researchers address this question by considering two different generic types of transmitters and exploring how their characteristics can be used to determine whether an NLOS path contributes to the signal received by the receiver.
"One type of transmitter sends all frequencies more or less in the same direction," said Daniel Mittleman, co-author and an engineering professor at Brown, "while the other type sends different frequencies in different directions, exhibiting strong angular dispersion. The situation is quite different in these two different cases."
The researchers' work shows that the transmitter sending different frequencies in different directions has distinct advantages in its ability to detect the NLOS path and distinguish them from the LOS path.
"A well-designed receiver would be able to detect both frequencies and use their properties to recognize the two paths and tell them apart," Mittleman said.
Many recent reports within academic literature have focused on various challenges involved in using terahertz signals for wireless communications. Indeed, the term 6G has become a buzzword to encompass future generations of wireless systems that use these ultrahigh-frequency signals.
"For terahertz signals to be used for wireless communications, many challenges must be overcome, and one of the biggest is how to detect and exploit NLOS paths," said Mittleman.
This work is among the first to provide a quantitative consideration of how to detect and exploit NLOS paths, as well as a comparison of the behavior of different transmitters within this context.
"For most realistic indoor scenarios we can envision for an above-100 gigahertz wireless network, the issue of NLOS path is definitely going to require careful consideration," said Mittleman. "We need to know how to exploit these link opportunities to maintain connectivity."
If, for example, the LOS path is blocked by something, an NLOS path can be used to maintain the link between the base station and receiver.
"Interestingly, with a transmitter creating strong angular dispersion, sometimes an NLOS link can provide even faster connectivity than the LOS link," said Yasaman Ghasempour, co-author formerly at Rice University and currently an assistant professor at Princeton University. "But you can't take advantage of such opportunities if you don't know the NLOS path exists or how to find it."
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The article "Line-of-sight and non-line-of-sight links for dispersive terahertz networks" is authored by Yasaman Ghasempour, Yasith Amarasinghe, Chia-Yi Yeh, Edward Knightly, and Daniel M. Mittleman. It appears in APL Photonics on April 27, 2021 (DOI: 10.1063/5.0039262) and can be accessed at https://aip.scitation.org/doi/10.1063/5.0039262.
Alessandro Alabastri (Photo by Jeff Fitlow/Rice University)
U.S. and Italian engineers have demonstrated the first nanophotonic platform capable of manipulating polarized light 1 trillion times per second.
“Polarized light can be used to encode bits of information, and we’ve shown it’s possible to modulate such light at terahertz frequencies,” said Rice University’s Alessandro Alabastri, co-corresponding author of a study published this week in Nature Photonics.
“This could potentially be used in wireless communications,” said Alabastri, an assistant professor of electrical and computer engineering in Rice’s Brown School of Engineering. “The higher the operating frequency of a signal, the faster it can transmit data. One terahertz equals 1,000 gigahertz, which is about 25 times higher than the operating frequencies of commercially available optical polarization switches.”
A pictorial schematic depicts the structure and action of a nanopatterned plasmonic metasurface that modulates polarized light at terahertz frequencies. An ultrashort laser pulse (green) excites cross-shaped plasmonic structures, which rotate the polarity of a second light pulse (white) that arrives less one picosecond after the first. (Image courtesy of A. Assié)
Each of the researchers work in nanophotonics, a fast-growing field that uses ultrasmall, engineered structures to manipulate light. Their idea for ultrafast polarization control was to capitalize on tiny, fleeting variations in the generation of high-energy electrons in a plasmonic metasurface.
A scanning electron microscope image of the nanopatterned plasmonic metasurface that engineers from Rice University, the Polytechnic University of Milan and the Italian Institute of Technology created to modulate polarized light at terahertz frequencies. (Image courtesy of Andrea Toma/IIT)
Metasurfacesare ultrathin films or sheets that contain embedded nanoparticles that interact with light as it passes through the film. By varying the size, shape and makeup of the embedded nanoparticles and by arranging them in precise two-dimensional geometric patterns, engineers can craft metasurfaces that split or redirect specific wavelengths of light with precision.
“One thing that differentiates this from other approaches is our reliance on an intrinsically ultrafast broadband mechanism that’s taking place in the plasmonic nanoparticles,” Alabastri said.
The Rice-Politecnico-IIT team designed a metasurface that contained rows of cross-shaped gold nanoparticles. Each plasmonic cross was about 100 nanometers wide and resonated with a specific frequency of light that gave rise to an enhanced localized electromagnetic field. Thanks to this plasmonic effect, the team’s metasurface was a platform for generating high-energy electrons.
“When one laser light pulse hits a plasmonic nanoparticle, it excites the free electrons within it, raising some to high-energy levels that are out of equilibrium,” Schirato said. “That means the electrons are ‘uncomfortable’ and eager to return to a more relaxed state. They return to an equilibrium in a very short time, less than one picosecond.”
Andrea Schirato
Despite the symmetric arrangement of crosses in the metasurface, the nonequilibrium state has asymmetric properties that disappear when the system returns to equilibrium. To exploit this ultrafast phenomenon for polarization control, the researchers used a two-laser setup. Experiments performed by study co-first author Margherita Maiuri at Politecnico’s ultrafast spectroscopy laboratories — and confirmed by the team’s theoretical predictions — used an ultrashort pulse of light from one laser to excite the crosses, allowing them to modulate the polarization of light in a second pulse that arrived less than a picosecond after the first.
“The key point is that we could achieve the control of light with light itself, exploiting ultrafast electronic mechanisms peculiar of plasmonic metasurfaces,” Alabastri said. “By properly designing our nanostructures, we have demonstrated a novel approach that will potentially allow us to optically transmit broadband information encoded in the polarization of light with unprecedented speed.”
Additional co-authors include Politecnico’s Giulio Cerullo and Paolo Laporta and IIT’s Andrea Toma and Silvio Fugattini. The research was funded by the European Union’s Graphene Flagship Core Project 3 and Project METAFAST, the Italian Ministry of Education, University and Research’s Projects of National Interest program and the Welch Foundation.
Radiation of varying frequencies emanate from a leaky waveguide at different angles. This rainbow of frequencies is the basis for a link discovery system for future terahertz data networks.CREDIT Mittleman Lab / Knightly Lab
PROVIDENCE, R.I. [Brown University] -- When someone opens a laptop, a router can quickly locate it and connect it to the local Wi-Fi network. That ability is a basic element of any wireless network known as link discovery, and now a team of researchers has developed a means of doing it with terahertz radiation, the high-frequency waves that could one day make for ultra-fast wireless data transmission.
Because of their high frequency, terahertz waves can carry hundreds of times more data than the microwaves used to carry our data today. But that high frequency also means that terahertz waves propagate differently than microwaves. Whereas microwaves emanate from a source in an omni-directional broadcast, terahertz waves propagate in narrow beams.
"When you're talking about a network that's sending out beams, it raises a whole myriad of questions about how you actually build that network," said Daniel Mittleman, a professor in Brown's School of Engineering. "One of those questions is how does an access point, which you can think of as a router, find out where client devices are in order to aim a beam at them. That's what we're thinking about here."
In a paper published in Nature Communications, researchers from Brown and Rice University showed that a device known as a leaky waveguide can be used for link discovery at terahertz frequencies. The approach enables link discovery to be done passively, and in one shot.
The concept of a leaky waveguide is simple. It's just two metal plates with a space between them where radiation can propagate. One of the plates has a narrow slit cut into it, which allows a little bit of the radiation to leak out. This new research shows the device can be used for link discovery and tracking by exploiting one of its underlying properties: that different frequencies leak out of the slit at different angles.
"We input a wide range of terahertz frequencies into this waveguide in a single pulse, and each one leaks out simultaneously at a different angle," said Yasaman Ghasempour, a graduate student at Rice and co-author on the study. "You can think of it like a rainbow leaking out, with each color represents a unique spectral signature corresponding to an angle."
Now imagine a leaky waveguide placed on an access point. Depending upon where a client device is relative to the access point, it's going to see a different color coming out of the waveguide. The client just sends a signal back to the access point that says, "I saw yellow," and now the access point knows exactly where the client is, and can continue tracking it.
"It is not just about discovering the link once," Yasaman said. "In fact, the direction of transmission needs to be continually adjusted as the client moves. Our technique allows for ultra-fast adaptation which is the key to achieving seamless connectivity."
The setup also uses a leaky waveguide on the client side. On that side, the range of frequencies received through the slit in the waveguide can be used to determine the position of the router relative to the local rotation of the device -- like when someone swivels their chair while using a laptop.
Mittleman says that finding a novel way to make link discovery work in the terahertz realm is important because existing protocols for link discovery in microwaves simply won't work for terahertz signals. Even the protocols that have been developed for burgeoning 5G networks, which are much more directional than standard microwaves, aren't feasible for terahertz. That's because as narrow as 5G beams are, they're still around 10 times wider than the beams in a terahertz network.
"I think some people have assumed that since 5G is somewhat directional, this problem had been solved, but the 5G solution simply isn't scalable," Mittleman said. "A whole new idea is needed. This is one of those fundamental protocol pieces that you need to start building terahertz networks."
OSAKA UNIVERSITY A team of researchers from Osaka University, TU Wien, Nanyang Technological University, Rice University, University of Alberta and Southern Illinois University-Carbondale comes closer to unraveling the physics of quasiparticles in carbon nanotubes.
Carbon nanotubes (CNTs), a model one-dimensional (1D) material made up entirely of carbon atoms, have attracted considerable attention ever since their discovery because of the unique properties arising from quantum confinement effects. CNTs have been labeled as one of the materials for next-generation optoelectronic devices. Critical towards this advancement is understanding how quasiparticles - theoretical particles used to describe observable phenomena in solids - behave and interact with each other in a 1D system. This requires a fundamentally different model compared to a conventional 3D material like silicon as a consequence of the reduced dimensionality in CNTs.
"It was difficult to develop a terahertz radiation device with an external high electric field in a specific direction to CNT," says corresponding author Masayoshi Tonouchi.
By combining different experimental techniques, the team was able to directly probe the creation of free charge carriers in CNTs at different time scales after photoexcitation. Very complex interactions that involve different quasiparticles occur after the initial photoexcitation. These processes change over time, and being able to probe one of the quasiparticles makes it easier to understand the whole process.
Together with state-of-the-art simulations, the team was able to identify two key mechanisms that explain their data and helped them develop a detailed microscopic model describing quasiparticle interactions in a strong electric field in CNTs.
"We proposed a model in which electron-hole bound quasiparticles excited in the high energy E22 exciton band diverge to the low energy band and play a role in ultrafast electrical conduction. This model successfully explained the experimental facts and led to the clarification of the physical properties of CNTs."
Their results shed light on a number of long-standing issues in CNT ultrafast dynamics, moving us closer towards the realization of advanced optoelectronics based on CNTs and other low-dimensional materials.
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The article, "Terahertz Excitonics in Carbon Nanotubes: Exciton Autoionization and Multiplication," was published in ACS Nano Letters at DOI: https://doi.org/10.1021/acs.nanolett.9b05082.
Physicists provide direct evidence of entanglement’s role in quantum criticality
In a new study, U.S. and Austrian physicists have observed quantum entanglement among “billions of billions” of flowing electrons in a quantum critical material.
The research, which appears this week in Science, examined the electronic and magnetic behavior of a “strange metal” compound of ytterbium, rhodium and silicon as it both neared and passed through a critical transition at the boundary between two well-studied quantum phases.
The study at Rice University and Vienna University of Technology (TU Wien) provides the strongest direct evidence to date of entanglement’s role in bringing about quantum criticality, said study co-author Qimiao Si of Rice.
“When we think about quantum entanglement, we think about small things,” Si said. “We don’t associate it with macroscopic objects. But at a quantum critical point, things are so collective that we have this chance to see the effects of entanglement, even in a metallic film that contains billions of billions of quantum mechanical objects.”
Si, a theoretical physicist and director of the Rice Center for Quantum Materials (RCQM), has spent more than two decades studying what happens when materials like strange metals and high-temperature superconductors change quantum phases. Better understanding such materials could open the door to new technologies in computing, communications and more.
The international team overcame several challenges to get the result. TU Wien researchers developed a highly complex materials synthesis technique to produce ultrapure films containing one part ytterbium for every two parts rhodium and silicon (YbRh2Si2). At absolute zero temperature, the material undergoes a transition from one quantum phase that forms a magnetic order to another that does not.
Physicist Silke Bühler-Paschen of the Vienna University of Technology (Photo by Luisa Puiu/TU Wien)
At Rice, study co-lead author Xinwei Li, Junichiro Kono, performed terahertz spectroscopy experiments on the films at temperatures as low as 1.4 Kelvin. The terahertz measurements revealed the optical conductivity of the YbRh2Si2 films as they were cooled to a quantum critical point that marked the transition from one quantum phase to another.
then a graduate student in the lab of co-author and RCQM member
“With strange metals, there is an unusual connection between electrical resistance and temperature,” said corresponding author Silke Bühler-Paschen of TU Wien’s Institute for Solid State Physics. “In contrast to simple metals such as copper or gold, this does not seem to be due to the thermal movement of the atoms, but to quantum fluctuations at the absolute zero temperature.”
To measure optical conductivity, Li shined coherent electromagnetic radiation in the terahertz frequency range on top of the films and analyzed the amount of terahertz rays that passed through as a function of frequency and temperature. The experiments revealed “frequency over temperature scaling,” a telltale sign of quantum criticality, the authors said.
Kono, an engineer and physicist in Rice’s Brown School of Engineering, said the measurements were painstaking for Li, who’s now a postdoctoral researcher at the California Institute of Technology. For example, only a fraction of the terahertz radiation shined onto the sample passed through to the detector, and the important measurement was how much that fraction rose or fell at different temperatures.
Former Rice University graduate student Xinwei Li in 2016 with the terahertz spectrometer he later used to measure entanglement in the conduction electrons flowing through a “strange metal” compound of ytterbium, rhodium and silicon. (Photo by Jeff Fitlow/Rice University)
“Less than 0.1% of the total terahertz radiation was transmitted, and the signal, which was the variation of conductivity as a function of frequency, was a further few percent of that,” Kono said. “It took many hours to take reliable data at each temperature to average over many, many measurements, and it was necessary to take data at many, many temperatures to prove the existence of scaling.
“Xinwei was very, very patient and persistent,” Kono said. “In addition, he carefully processed the huge amounts of data he collected to unfold the scaling law, which was really fascinating to me.”
Making the films was even more challenging. To grow them thin enough to pass terahertz rays, the TU Wien team developed a unique molecular beam epitaxy system and an elaborate growth procedure. Ytterbium, rhodium and silicon were simultaneously evaporated from separate sources in the exact 1-2-2 ratio. Because of the high energy needed to evaporate rhodium and silicon, the system required a custom-made ultrahigh vacuum chamber with two electron-beam evaporators.
“Our wild card was finding the perfect substrate: germanium,” said TU Wien graduate student Lukas Prochaska, a study co-lead author. The germanium was transparent to terahertz, and had “certain atomic distances (that were) practically identical to those between the ytterbium atoms in YbRh2Si2, which explains the excellent quality of the films,” he said.
Si recalled discussing the experiment with Bühler-Paschen more than 15 years ago when they were exploring the means to test a new class of quantum critical point. The hallmark of the quantum critical point that they were advancing with co-workers is that the quantum entanglement between spins and charges is critical.
Former Rice University graduate student Xinwei Li (left) and Professor Junichiro Kono in 2016 with the terahertz spectrometer Li used to measure quantum entanglement in YbRh2Si2. (Photo by Jeff Fitlow/Rice University)
“At a magnetic quantum critical point, conventional wisdom dictates that only the spin sector will be critical,” he said. “But if the charge and spin sectors are quantum-entangled, the charge sector will end up being critical as well.”
At the time, the technology was not available to test the hypothesis, but by 2016, the situation had changed. TU Wien could grow the films, Rice had recently installed a powerful microscope that could scan them for defects, and Kono had the terahertz spectrometer to measure optical conductivity. During Bühler-Paschen’s sabbatical visit to Rice that year, she, Si, Kono and Rice microscopy expert Emilie Ringe received support to pursue the project via an Interdisciplinary Excellence Award from Rice’s newly established Creative Ventures program.
“Conceptually, it was really a dream experiment,” Si said. “Probe the charge sector at the magnetic quantum critical point to see whether it’s critical, whether it has dynamical scaling. If you don’t see anything that’s collective, that’s scaling, the critical point has to belong to some textbook type of description. But, if you see something singular, which in fact we did, then it is very direct and new evidence for the quantum entanglement nature of quantum criticality.”
Si said all the efforts that went into the study were well worth it, because the findings have far-reaching implications.
“Quantum entanglement is the basis for storage and processing of quantum information,” Si said. “At the same time, quantum criticality is believed to drive high-temperature superconductivity. So our findings suggest that the same underlying physics — quantum criticality — can lead to a platform for both quantum information and high-temperature superconductivity. When one contemplates that possibility, one cannot help but marvel at the wonder of nature.”
Si is the Harry C. and Olga K. Wiess Professor in Rice’s Department of Physics and Astronomy. Kono is a professor in Rice’s departments of Electrical and Computer Engineering, Physics and Astronomy, and Materials Science and NanoEngineering and the director of Rice’s Applied Physics Graduate Program. Ringe is now at the University of Cambridge.
Additional co-authors include Maxwell Andrews, Maximilian Bonta, Werner Schrenk, Andreas Limbeck and Gottfried Strasser, all of the TU Wien; Hermann Detz, formerly of TU Wien and currently at Brno University; Elisabeth Bianco, formerly of Rice and currently at Cornell University; Sadegh Yazdi, formerly of Rice and currently at the University of Colorado Boulder; and co-lead author Donald MacFarland, formerly of TU Wien and currently at the University at Buffalo.
The research was supported by the European Research Council, the Army Research Office, the Austrian Science Fund, the European Union’s Horizon 2020 program, the National Science Foundation, the Robert A. Welch Foundation, Los Alamos National Laboratory and Rice University.
RCQM leverages global partnerships and the strengths of more than 20 Rice research groups to address questions related to quantum materials. RCQM is supported by Rice’s offices of the provost and the vice provost for research, the Wiess School of Natural Sciences, the Brown School of Engineering, the Smalley-Curl Institute and the departments of Physics and Astronomy, Electrical and Computer Engineering, and Materials Science and NanoEngineering.
InterceptedA new study shows that it's possible to steal data undetected from terahertz wireless links even though those links involved beam transmissions from transmitter to receiver.Mittleman Lab / Brown University
Terahertz radiation may one day be used in wireless data networks that are many times faster than today's microwave networks. The conventional wisdom in the research community has been that, in addition to greater speed, terahertz data links would also have an inherent immunity to eavesdropping. Unlike microwaves, which travel in wide-angle broadcasts, terahertz waves travel directly from transmitter to receiver in narrow beams. The assumption was that it would be impossible to for an eavesdropper to intercept a terahertz signal without blocking some or all of the beam, which would be easily detected by an intended receiver. But new research finds that a clever eavesdropper can indeed steal terahertz signals undetected. In order for a link to be reliable, the beam's diameter must be slightly larger than the aperture of the receiver. That leaves a sliver of signal available for an attacker to steal without casting a shadow in a receiver. Credit: Mittleman lab / Brown University
Scientists have assumed that future terahertz data links would have an inherent immunity to eavesdropping, but new research shows that’s not necessarily the case.
PROVIDENCE, R.I. [Brown University] — A new study shows that terahertz data links, which may play a role in ultra-high-speed wireless data networks of the future, aren’t as immune to eavesdropping as many researchers have assumed. The research, published in the journal Nature, shows that it is possible for a clever eavesdropper to intercept a signal from a terahertz transmitter without the intrusion being detected at the receiver.
“The conventional wisdom in the terahertz community has been that it’s virtually impossible to spy on a terahertz data link without the attack being noticed,” said Daniel Mittleman, a professor in Brown University’s School of Engineering and a coauthor of the research. “But we show that undetected eavesdropping in the terahertz realm is easier than most people had assumed and that we need to be thinking about security issues as we think about designing network architectures.”
Because of its higher frequency, terahertz radiation can carry up to 100 times more data than the microwaves used in wireless communication today, which makes terahertz an attractive option for use in future wireless networks. Along with enhanced bandwidth, it has also been generally assumed that the way in which high-frequency waves propagate would naturally enhance security. Unlike microwaves, which propagate in wide-angle broadcasts, terahertz waves travel in narrow, very directional beams.
“In microwave communications, an eavesdropper can put an antenna just about anywhere in the broadcast cone and pick up the signal without interfering with the intended receiver,” Mittleman said. “Assuming that the attacker can decode that signal, they can then eavesdrop without being detected. But in terahertz networks, the narrow beams would mean that an eavesdropper would have to place the antenna between the transmitter and receiver. The thought was that there would be no way to do that without blocking some or all of the signal, which would make an eavesdropping attempt easily detectable by the intended receiver.”
Mittleman and colleagues from Brown, Rice University and the University at Buffalo set out to test that notion. They set up a direct line-of-site terahertz data link between a transmitter and receiver, and experimented with devices capable of intercepting signal. They were able show several strategies that could steal signal without being detected — even when the data-carrying beam is very directional, with a cone angle of less than 2 degrees (in contrast to microwave transmission, where the angle is often as large as 120 degrees).
One set of strategies involves placing objects at the very edge of a beam that is capable of scattering a tiny portion of the beam. In order for a data link to be reliable, the diameter of the beam must be slightly larger than the aperture of the receiver. That leaves a sliver of signal for an attacker to work with without casting a detectable shadow on the receiver.
The researchers showed that a flat piece of metal could redirect a portion of the beam to a secondary receiver operated by an attacker. The researchers were able to acquire a usable signal at the second receiver with no significant loss of power at the primary receiver.
The team showed an even more flexible approach (from the attacker’s perspective) by using a metal cylinder in the beam instead of a flat plate.
“Cylinders have the advantage that they scatter light in all directions, giving an attacker more options in setting up a receiver,” said Josep Jornet, an assistant professor of electrical engineering at Buffalo and a study co-author. “And given the physics of terahertz wave propagation, even a very small cylinder can significantly scatter the signal without blocking the line-of-sight path.”
The researchers went on to demonstrate another type of attack involving a lossless beam splitter that would also be difficult, if not impossible, to detect. The beam splitter placed in front of a transmitter would enable an attacker to steal just enough to be useful, yet not so much that it would set off alarm bells among network administrators.
The bottom line, the researchers say, is that while there are inherent security enhancements associated with terahertz links in comparison with lower frequencies, these security improvements are still far from foolproof.
“Securing wireless transmission from eavesdroppers has been a challenge since the days of Marconi,” said Edward Knightly, professor of electrical and computer engineering at Rice University and a study coauthor. “While terahertz bands take a huge leap in this direction, we unfortunately found that a determined adversary can still be effective in intercepting the signal.”
The research was funded in part by the National Science Foundation, the Army Research Office, the Air Force Office of Scientific Research, and the W. M. Keck Foundation. Other coauthors on the paper were Jianjun Ma, Rabi Shrestha and Jacob Adelberg from Brown University; Chia-Yi Yeh and Edward Knightly from Rice University; and Zahed Hossain from Buffalo.
HOUSTON – (Aug. 23, 2018) – After their recent pioneering experiments to couple light and matter to an extreme degree, Rice University scientists decided to look for a similar effect in matter alone. They didn’t expect to find it so soon.
Rice physicist Junichiro Kono, graduate student Xinwei Li and their international colleagues have discovered the first example of Dicke cooperativity in a matter-matter system, a result reported in Science this week.
Rice University scientists observed Dicke cooperativity in a magnetic crystal in which two types of spins, in iron (blue arrows) and erbium (red arrows), interacted with each other. The iron spins were excited to form a wave-like object called a spin wave; the erbium spins precessing in a magnetic field (B) behaved like two-level atoms. Illustration by Xinwei Li
The discovery could help advance the understanding of spintronics and quantum magnetism, Kono said. On the spintronics side, he said the work will lead to faster information processing with lower power consumption and will contribute to the development of spin-based quantum computing. The team’s findings on quantum magnetism will lead to a deeper understanding of the phases of matter induced by many-body interactions at the atomic scale.
Instead of using light to trigger interactions in a quantum well, a system that produced new evidence of ultrastrong light-matter coupling earlier this year, the Kono lab at Rice used a magnetic field to prompt cooperativity among the spins within a crystalline compound made primarily of iron and erbium.
“This is an emerging subject in condensed matter physics,” Kono said. “There’s a long history in atomic and molecular physics of looking for the phenomenon of ultrastrong cooperative coupling. In our case, we’d already found a way to make light and condensed matter interact and hybridize, but what we’re reporting here is more exotic.”
Dicke cooperativity, named for physicist Robert Dicke, happens when incoming radiation causes a collection of atomic dipoles to couple, like gears in a motor that don’t actually touch. Dicke’s early work set the stage for the invention of lasers, the discovery of cosmic background radiation in the universe and the development of lock-in amplifiers used by scientists and engineers.
Xinwei Li, left, and Junichiro Kono of Rice University led an international effort to find the first instance of Dicke cooperativity in a matter-matter system. Photo by Jeff Fitlow
“Dicke was an unusually productive physicist,” Konosaid. “He had many high-impact papers and accomplishments in almost all areas of physics. The particular Dicke phenomenon that’s relevant to our work is related to superradiance, which he introduced in 1954. The idea is that if you have a collection of atoms, or spins, they can work together in light-matter interaction to make spontaneous emission coherent. This was a very strange idea.
“When you stimulate many atoms within a small volume, one atom produces a photon that immediately interacts with another atom in the excited state,” Kono said. “That atom produces another photon. Now you have coherent superposition of two photons.
“This happens between every pair of atoms within the volume and produces macroscopic polarization that eventually leads to a burst of coherent light called superradiance,” he said.
Taking light out of the equation meant the Kono lab had to find another way to excite the material’s dipoles, the compass-like magnetic force inherent in every atom, and prompt them to align. Because the lab is uniquely equipped for such experiments, when the test material showed up, Kono and Li were ready.
“The sample was provided by my colleague (and co-author) Shixun Cao at Shanghai University,” Kono said. Characterization tests with a small or no magnetic field performed by another co-author, Dmitry Turchinovich of the University of Duisburg-Essen, drew little response.
“But Dmitry is a good friend, and he knows we have a special experimental setup that combines terahertz spectroscopy, low temperatures and high magnetic field,” Kono said. “He was curious to know what would happen if we did the measurements.”
“Because we have some experience in this field, we got our initial data, identified some interesting details in it and thought there was something more we could explore in depth,” Li added.
“But we certainly didn’t predict this,” Kono said.
Li said that to show cooperativity, the magnetic components of the compound had to mimic the two essential ingredients in a standard light-atom coupling system where Dicke cooperativity was originally proposed: one a species of spins that can be excited into a wave-like object that simulates the light wave, and another with quantum energy levels that would shift with the applied magnetic field and simulate the atoms.
“Within a single orthoferrite compound, on one side the iron ions can be triggered to form a spin wave at a particular frequency,” Li said. “On the other side, we used the electron paramagnetic resonance of the erbium ions, which forms a two-level quantum structure that interacts with the spin wave.”
While the lab’s powerful magnet tuned the energy levels of the erbium ions, as detected by the terahertz spectroscope, it did not initially show strong interactions with the iron spin wave at room temperature. But the interactions started to appear at lower temperatures, seen in a spectroscopic measurement of coupling strength known as vacuum Rabi splitting.
Chemically doping the erbium with yttrium brought it in line with the observation and showed Dicke cooperativity in the magnetic interactions. “The way the coupling strength increased matches in an excellent manner with Dicke’s early predictions,” Li said. “But here, light is out of the picture and the coupling is matter-matter in nature.”
“The interaction we’re talking about is really atomistic,” Kono said. “We show two types of spin interacting in a single material. That’s a quantum mechanical interaction, rather than the classical mechanics we see in light-matter coupling. This opens new possibilities for not only understanding but also controlling and predicting novel phases of condensed matter.”
Co-authors of the paper are Motoaki Bamba, an associate professor at Osaka University; graduate students Ning Yuan, Maolin Xiang and Kai Xu and professors Zuanming Jin, Wei Ren and Guohong Ma at Shanghai University; Rice alumnus Qi Zhang, a research fellow at Argonne National Laboratory; and Yage Zhao, an undergraduate student at Peking University and former exchange student at Rice. Kono is a professor of electrical and computer engineering, of physics and astronomy, and of materials science and nanoengineering.
The research was supported by the National Science Foundation, the Army Research Office, the PRESTO program of the Japan Science and Technology Agency, the Japan Society for the Promotion of Science’s KAKENHI program, the ImPACT Program of the Government of Japan’s Council for Science, Technology and Innovation, the National Natural Science Foundation of China, German Research Foundation,the European Commission and the Max Planck Society.
