Technology

Researchers at the University of Michigan and City University of New York have recently proposed and experimentally validated a transparent, electronically tunable metasurface. This metasurface, presented in a paper published in Physical Review X, can rotate the polarization of an arbitrarily polarized incident wave without changing its axial ratio.

Metasurfaces are artificial sheet materials textured at a sub-wavelength scale in order to produce tailored electromagnetic responses. In recent years, these materials have enabled unprecedented control over electromagnetic waves, opening up interesting possibilities in numerous areas, including wireless communications, imaging and energy harvesting.

Despite their numerous advantages, most metasurface designs only offer static functionalities. The new metasurface devised by the researchers, on the other hand, has dynamically tunable properties, and could hence be applied in a broader variety of areas.

“A few years ago, our research group introduced a pragmatic approach to realizing metasurfaces with tailored bianisotropic responses,” Zhanni Wu, one of the researchers who carried out the study, told Phys.org. “This approach involves cascading patterned (i.e. anisotropic) metallic surfaces across a subwavelength thickness to achieve targeted electric, magnetic and chiral/omega properties. The technique is amenable to planar micro- and nano-fabrication techniques, allowing realization of metasurfaces from RF to visible wavelengths.”

Over the past few years, the same team of researchers designed and realized several metasurfaces, with various functionalities. Although these ultrathin metasurfaces achieved extreme wavefront control, their functionalities remained static and dependent on their fixed geometrical parameters.

“In this work, we integrated tunable devices, varactor diodes, into a metasurface to achieve dynamic control over the metasurface’s response,” Wu explained. “We demonstrated a transparent metasurface with a tunable chiral response, which can rotate the polarization of an incident wave.”

Manipulating the properties of electromagnetic waves (e.g. amplitude, phase and polarization) typically involves a combination of optical components, such as lenses, polarizers and waveplates. The new metasurface introduced by the researchers has a polarization rotator, which consists of a tunable birefringent structure sandwiched between two ±45° rotated metasurface-based quarter-wave plates.

“A conventional tunable polarization rotator is typically multiple wavelengths in size (tens to hundreds) and consists of two rotated quarter wave plates (linear to circular polarizers) placed on either side of a tunable birefringent medium, like a liquid crystal layer,” Anthony Grbic, another researcher who carried out the study, told Phys.org. “Here, we have replaced this bulky device with a cascade of metasurfaces, resulting in a device of subwavelength thickness and equivalent functionality.”

The approach demonstrated by Wu, Grbic and their colleague Younes Ra’di enables ultra-compact designs and could be applied in microwave polarimetric systems for the characterization or detection of objects. For instance, their metasurface-based polarization rotator could be integrated with an antenna element to develop a compact antenna system for polarimetric microwave imaging.

“Our work paves the way for flat/low profile, dynamically tunable antennas and optical/quasioptical systems,” Grbic said. “One can envision replacing bulky electromagnetic or optical setups requiring combinations of conventional components including lenses, tuning elements, phase shifters, spatial light modulators, waveplates, linear polarizers simply with cascaded ultra-thin, tunable metasurfaces.”

The recent study carried out by Wu, Grbic and Ra’di introduced a novel platform that enables full control of the transmitted wave front. The researchers demonstrated their approach by developing a tunable polarization rotator, yet it could also be used to tailor reflected waves. In the future, their method could hence be applied to the design of a tunable metasurface that not only rotates the polarization of the reflected/transmitted wave, but also steers a beam into different directions.

“Our future research plans also include the development stacked metasurface designs for tunable amplitude control, in addition to the phase/polarization control we have demonstrated to date,” Ra’di told Phys.org. “A further goal of ours is to translate such metasurface designs to optical wavelengths.”

A fascination with movie technology that showed robots perform self-repair through a liquid formula inspired a Purdue University professor to make his own discoveries—which are now helping to lead the way for advancements in self-powering devices such as consumer electronics and defense innovations.

The Purdue team, led by Wenzhuo Wu, the Ravi and Eleanor Talwar Rising Star Assistant Professor of Industrial Engineering, has created wearable technology to convert mechanical energy into electrical energy.

“Our work presents an important step toward the practical realization of self-powered, human-integrated technologies,” Wu said.

The Purdue team invented a liquid-metal-inclusion based triboelectric nanogenerator, called LMI-TENG. Triboelectric energy harvesting transducers—devices which help conserve mechanical energy and turn it into power—are predicted to be a $480 million market by 2028, according to IDTechEx.

The LMI-TENG can harvest and sense the biomechanical signals from the body and use those to help power and direct technological devices. The LMI-TENG consists of a layer of liquid metal embedded functional silicone sandwiched between two Ecoflex layers.

The Purdue technology is featured in the February edition of the Journal of Materials Chemistry A, which named it one of 2019’s HOT papers.

“We realized that liquid represents the ultimate form of anything that can be deformable and morphing into different shapes,” Wu said. “Our technology will enable wearable electronics to take otherwise wasted energy and transform it into energy that can power and control electronic devices and tools used in military defense and consumer applications. Our technology allows the synergistic engineering of TENG components at the material, structural and output levels.”

Wu said the Purdue technology has applications for many self-powered innovations for emerging technologies, such as wearable sensors, pervasive computing, advanced health care, human-machine interfaces, robotics, user interfaces, augmented reality, virtual reality, teleoperation and the Internet of Things.

Researchers at the University of Florence and Istituto dei Sistemi Complessi, in Italy, have recently proved that the invasiveness of quantum measurements might not always be detrimental. In a study published in Physical Review Letters, they showed that this invasive quality can actually be exploited, using quantum measurements to fuel a cooling engine.

Michele Campisi, one of the researchers involved in the study, has been studying quantum phenomena for several years. In his recent work, he investigated whether quantum phenomena can impact the thermodynamics of nanoscopic devices, such as those employed in quantum computers.

“Most colleagues in the field were looking at coherence and entanglement while only few were looking at another at genuine quantum phenomenon, i.e., the quantum measurement process,” Campisi told Phys.org. “Those studies suggested that you need to accompany measurements with feedback control, as in Maxwell’s demon, in order to exploit their potential. I started thinking about it, and eureka—since quantum measurements are very invasive, they are accompanied by energy exchanges, hence can be used to power engines without the need to do feedback control.”

The second law of thermodynamics states that heat naturally flows from hot bodies to cold ones. Past studies found that there are two ways to reverse this natural flow of heat: using work supplied by an external, time-dependent driving force or by implementing a Maxwell demon, which steers the heat via a feedback control loop.

In their study, Campisi and his colleagues showed that there is, in fact, a third method to reverse the flow of heat, which is based on quantum mechanics. This technique entails the use invasive quantum measurements as a fuel that powers refrigeration, without any feedback control. The researchers refer to this mechanism as quantum measurement cooling (QMC).

“The general mathematical framework is standard quantum mechanics, but we had to use a mix of advanced numerical and analytical methods to investigate all facets of quantum measurement cooling,” Lorenzo Buffoni, another researcher involved in the study, told Phys.org. “For example, in order to assess its the robustness to experimental noise we used extensive Monte Carlo sampling of the high-dimensional space of possible measurement projectors, and used machine learning techniques to analyze and visualize the data.”

Campisi and his colleagues illustrated QMC by means of a prototypical two-stroke two-qubit engine. This engine interacts with the measurement apparatus employed by the researchers, as well as with two heat reservoirs set at different temperatures.

“We also embarked on the task of finding the optimal thermodynamic performance by analytical methods, which was very challenging,” Andrea Sofanelli, another researcher who carried out the study, told Phys.org. “We employed Birkhoff theorem to express the so-called transition matrix (containing all relevant information about the energy exchanges in our problem) in terms of permutations, which simplified the problem. But we remained stuck with that until we found a little-known theorem of linear algebra dating back to the early 1990s, which finally led to the solution.”

Campisi, Buffoni, Cuccoli, Solfanelli and their colleague Paola Verrucchi demonstrated that the invasiveness of quantum measurements can be used to fuel a cooling engine via the QMC mechanism they have reported. QMC does not require feedback control, but entanglement must be present in the measurement projectors.

The researchers calculated the probability that QMC will occur when the measurement basis is randomly selected. They found that this probability can be very large compared to the probability of extracting energy (i.e. operating the heat engine), yet it is smaller than the probability of the least important operation (i.e. dumping heat in both baths).

“Showing that measuring a quantum system made by two qubits can produce by itself (i.e. without feedback control) useful thermodynamic effects surely represents the most meaningful outcome of our research,” Alessandro Cuccoli, another researcher involved in the study, told Phys.org. “This follows from looking at the quantum measurement process from a wider perspective, where both the system and its environment, and the energy exchanges accompanying the measurement, are considered.”

According to Cuccoli, the two-quibits thermal engine developed by the researchers could easily be engineered to work as a cooling device. This would, among other things, enable the fabrication of a quantum computer’s processing units to be integrated with auxiliary devices that can keep them at the required low temperature, as both can be achieved using qubits.

“A further insightful observation is that in order to get useful thermodynamic effects, the measurement process has to involve ‘entangled’ states, i.e. peculiarly quantum correlated states of the two qubits, thus revealing the intimate connection between information and energy exchanges,” Cuccoli added. “Deepening our understanding of such relationship in nanoscopic quantum engines is one of the major challenges driving our current and future research in the field of quantum thermodynamics.”

The study carried out by Campisi, Buffoni, Cuccoli, Solfanelli and Verrucchi introduced an entirely new mechanism that can reverse the natural flow of heat, intervening with the second law of thermodynamics, without feedback control requirements. In the future, their findings could have many applications, for instance, aiding the development of devices to cool quantum computers.

The team of researchers involved in this study is part of a collaboration consortium that involves 12 world-class research groups, including experimentalists and theorists from eight E.U. countries. They are currently seeking the resources necessary to support their work in the forthcoming years.

“We are looking forward to collaborating with experimental groups that might be interested in building a functioning quantum-measurement cooler,” Campisi said. “The full understanding and mastering of the energetics of quantum systems and devices is urgently needed, and calls for a joint international effort in order to speed up technological development.”

Read more at: https://phys.org/news/2019-03-quantum-fuel-cooling.html#jCp

Scientists at Aarhus University in Denmark studying the effects of climate change on weather-dependent electricity systems have found a silver lining in Earth’s otherwise fraught future outlook. Temperatures may climb and seas may rise, but the lights (and, undoubtedly, the air conditioning) will still be on in nations with high capacities for wind and solar energy. The research, published March 6 in the journal Joule, suggests that these electricity systems should work almost equally well in both historical and future European climates despite changing weather patterns.

Using data from weather models and climate time series, the team developed models that predict wind turbine and solar panel output for all European countries under the most common global warming scenarios through the year 2100. In order to compare how a European weather-dependent electrical grid functions in these projections with how it functioned historically, the scientists selected five key metrics: need and capacity for dispatchable electricity (which can be stored and used on demand by power grid operators), benefit of electrical transmission, benefit of electrical storage, and variability of electricity production and consumption. These metrics measure the most important aspects of a large-scale renewable-heavy electricity system and are abstract enough that the team could use them to draw general conclusions without focusing on a specific technology mix.

“Most other energy system studies assume a number of technologies and seek to combine them in a cost-optimal way to cover the demand,” says Smail Kozarcanin, a Ph.D. fellow in the Department of Engineering and the first author of the study. “In this study, we seek to understand, for example, how climate change affects the system independent of which technologies are used to cover the demand that remains unmet by wind and solar. To the best of our knowledge, this technology-independent focus in combination with high-resolution data on climate change projections is unique to our study.”

Despite the new weather extremes predicted by future climate scenarios, the study didn’t find a large difference in the key metrics for renewable electrical systems, suggesting that system designs based on historical weather should perform similarly in future climates. Kozarcanin and his colleagues believe this is because current systems are designed to withstand extreme weather events—they simply don’t have to withstand them now as often as they will in the future. The team also notes that European demand for electrified heating and cooling will actually dip slightly as the climate warms, since the demand for air conditioning is much less than that for heating at European latitudes. This relaxed demand will counterbalance the slight decrease in wind and solar energyoutput their models predict.

“Extreme weather might require changes to the renewable generators and other parts of the system,” says Kozarcanin. “For example, future wind turbines may require new types of storm protection and solar panels could need protection against super hailstorms. But our study shows that large-scale infrastructure choices, such as back-up power plant capacity, are relatively unaffected by the level of climate change.”

However, the team does believe that the European electrical grid will still require some tweaking to operate efficiently in the future. Transmission capacities are well developed within most nations, but the massive interconnected electrical system spanning 24 countries across Central Europe will need a boost to effectively transmit renewable energy between nations.

“The main challenge for future grids will most likely be political and societal will to make the investments and proper planning for a grid topology that provides most of the potential benefit from smoothing renewable energy production between countries,” says Kozarcanin.

If humans fail to mitigate the apocalyptic heatwaves and superstorms to come, at least it seems probable that we can hole up in our climate-controlled living rooms and watch HBO Nordic. Indefinitely.

“Everyone is trying to get free energy—from the wind, from the waves, from the sun,” said Dani Levin, a Duke MEMS Ph.D. student in the lab of Professor Earl Dowell.

As an aerospace engineer who specializes in designing structures to avoid destructive vibrations, it was only natural for Levin to approach the quest for sustainable energy by applying what he knows about flutter and limit cycle oscillations, or LCO.

Because of its potential to cause structural failure, flutter is nearly always engineered out of structures. “Usually flutter is not desirable,” said Levin. “But sometimes violent flutter can turn into moderate LCO, which will cause no failure, and this can be useful.” Levin wanted to explore the potential of harnessing the energy generated by flutter, and he took a six-month detour from his dissertation work, with the support of his advisor.

Levin set up an experiment in the aeroelasticity lab’s wind tunnel, using a thin aluminum plate and piezoelectric elements, which generate voltage in response to mechanical stressors like flutter. He found that by increasing the vibrations’ frequency and amplitude, creating more curvature in the vibrational motion, using efficient piezoelectric elements and reducing air speed, he could squeeze the most power from the motion of the plate—sometimes increasing efficiency by more than 200 percent.

Are large-scale flutter farms in our future? Probably not, said Levin. “My vision for this is, maybe you go on a long hike, on a cloudy day. You don’t have any solar power to charge your phone. But you do have a folded generator in your bag that you can take out, let it catch the breeze as you hike, and it can charge your phone. You can go even further and combine solar and wind. It’s scalable and useful. And,” he added jokingly, “it doesn’t involve scary mathematics. We’re using something that is so simple and so basic—at least for an aerospace engineer.”

Levin will present his project, titled “Improving Piezoelectric Energy Harvesting from an Aeroelastic System,” at the International Forum on Aeroelasticity and Structural Dynamics in Savannah, Georgia in June of 2019.

Read more at: https://phys.org/news/2019-03-potential-energy-vibrations.html#jCp

A team of researchers led by the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) has developed a simple method that could turn ordinary semiconducting materials into quantum machines—superthin devices marked by extraordinary electronic behavior. Such an advancement could help to revolutionize a number of industries aiming for energy-efficient electronic systems—and provide a platform for exotic new physics.

The study describing the method, which stacks together 2-D layers of tungsten disulfide and tungsten diselenide to create an intricately patterned material, or superlattice, was published online recently in the journal Nature.

“This is an amazing discovery because we didn’t think of these semiconducting materials as strongly interacting,” said Feng Wang, a condensed matter physicist with Berkeley Lab’s Materials Sciences Division and professor of physics at UC Berkeley. “Now this work has brought these seemingly ordinary semiconductors into the quantum materials space.”

Two-dimensional (2-D) materials, which are just one atom thick, are like nanosized building blocks that can be stacked arbitrarily to form tiny devices. When the lattices of two 2-D materials are similar and well-aligned, a repeating pattern called a moiré superlattice can form.

For the past decade, researchers have been studying ways to combine different 2-D materials, often starting with graphene—a material known for its ability to efficiently conduct heat and electricity. Out of this body of work, other researchers had discovered that moiré superlattices formed with graphene exhibit exotic physics such as superconductivity when the layers are aligned at just the right angle.

The new study, led by Wang, used 2-D samples of semiconducting materials—tungsten disulfide and tungsten diselenide—to show that the twist angle between layers provides a “tuning knob” to turn a 2-D semiconducting system into an exotic quantum material with highly interacting electrons.

Entering a new realm of physics

Co-lead authors Chenhao Jin, a postdoctoral scholar, and Emma Regan, a graduate student researcher, both of whom work under Wang in the Ultrafast Nano-Optics Group at UC Berkeley, fabricated the tungsten disulfide and tungsten diselenide samples using a polymer-based technique to pick up and transfer flakes of the materials, each measuring just tens of microns in diameter, into a stack.

They had fabricated similar samples of the materials for a previous study, but with the two layers stacked at no particular angle. When they measured the optical absorption of a new tungsten disulfide and tungsten diselenide sample for the current study, they were taken completely by surprise.

The absorption of visible light in a tungsten disulfide/tungsten diselenide device is largest when the light has the same energy as the system’s exciton, a quasiparticle that consists of an electron bound to a hole that is common in 2-D semiconductors. (In physics, a hole is a currently vacant state that an electron could occupy.)

For light in the energy range that the researchers were considering, they expected to see one peak in the signal that corresponded to the energy of an exciton.

Instead, they found that the original peak that they expected to see had split into three different peaks representing three distinct exciton states.

What could have increased the number of exciton states in the tungsten disulfide/tungsten device from one to three? Was it the addition of a moiré superlattice?

To find out, their collaborators Aiming Yan and Alex Zettl used a transmission electron microscope (TEM) at Berkeley Lab’s Molecular Foundry, a nanoscale science research facility, to take atomic-resolution images of the tungsten disulfide/tungsten diselenide device to check how the materials’ lattices were aligned.

The TEM images confirmed what they had suspected all along: the materials had indeed formed a moiré superlattice. “We saw beautiful, repeating patterns over the entire sample,” said Regan. “After comparing this experimental observation with a theoretical model, we found that the moiré pattern introduces a large potential energy periodically over the device and could therefore introduce exotic quantum phenomena.”

The researchers next plan to measure how this new quantum system could be applied to optoelectronics, which relates to the use of light in electronics; valleytronics, a field that could extend the limits of Moore’s law by miniaturizing electronic components; and superconductivity, which would allow electrons to flow in devices with virtually no resistance.