Combining two thin-film materials yields surprising room-temperature magnetism.
David L. Chandler | MIT News Office
May 9, 2016
A new and unexpected magnetic effect has taken researchers by surprise, and could open up a new pathway to advanced electronic devices and even robust quantum computer architecture.
The finding is based on a family of materials called topological insulators (TIs) that has drawn much interest in recent years. The novel electronic properties of TIs might ultimately lead to new generations of electronic, spintronic, or quantum computing devices. The materials behave like ordinary insulators throughout their interiors, blocking electrons from flowing, but their outermost surfaces are nearly perfect conductors, allowing electrons to move freely. The confinement of electrons to this vanishingly thin surface makes then behave in unique ways.
But harnessing the materials’ promise still faces numerous obstacles, one of which is to find a way of combining a TI with a material that has controllable magnetic properties. Now, researchers at MIT and elsewhere say they have found a way to overcome that hurdle.
The team at MIT, led by Jagadeesh Moodera of the Department of Physics and postdoc Ferhat Katmis, was able to bond together several molecular layers of a topological insulator material called bismuth selenide (Bi2Se3) with an ultrathin layer of a magnetic material, europium sulfide (EuS). The resulting bilayer material retains all the exotic electronic properties of a TI and the full magnetization capabilities of the EuS.
But the big surprise was the stability of that effect. While EuS itself is known to retain its ability to hold a magnetic state only at extremely low temperatures, just 17 degrees above absolute zero (17 Kelvin), the combined material keeps those characteristics all the way up to ordinary room temperature. That could make all the difference for developing devices that are practical to operate, and could open up new avenues of device design as well as research into a new area of basic physical phenomena.
The findings are being reported in the journal Nature, in a paper by Katmis, Moodera, and 10 others at MIT, and a multinational, multidisciplinary team from Oak Ridge, Argonne National Laboratories, and institutions in Germany, France, and India.
The room-temperature magnetic effect seen in this work, Moodera says, was something that “wasn’t in anybody’s wildest expectations. This is what astonished us.” Research like this, he says, is still so near the frontiers of scientific knowledge that the phenomena are impossible to predict. “You can’t tell what you’re going to see next week or what’s going to happen” in the next experiment, he says.
In particular, novel combinations of two materials with very different properties “is an area with very little depth of research.” And getting clear and repeatable results depends on a high degree of precision in the preparation of the surfaces and joining of the two materials; any contamination or imperfections at the interface between the two – even down to the level of individual atomic layer – can throw off the results, Moodera says. “What happens, happens where they meet,” he says, and the careful and persistent effort of Katmis in making these materials was key to the new discovery.
The finding could be a step toward new kinds of magnetic interactions at the interfaces between materials, with stability that could result in magnetic memory devices which could store information at the level of individual molecules, the team says.
The effect, which the researchers call proximity-induced magnetism, could also enable a new variety of “spintronic” devices based on a property of electrons called spin, rather than on their electrical charge. It might also provide the first practical way of producing a kind of particle called Majorana fermions, predicted by physicists but not yet observed convincingly. That in turn could help in the development of quantum computers, they say.
“A nice thing about this is that it shows both very fundamental physics and also takes us forward to many possible applications,” Katmis says. He says the effect is somewhat similar to unexpected findings a decade ago in the interfaces between some oxide materials, which has triggered a decade of intensive research.
This new finding, coupled with other recent quantum behavior observed in TIs, can lead to many possibilities for future electronics and spintronics, the team says.
“This beautiful work from Moodera’s group is a very exciting demonstration that the whole is greater than the sum of its parts,” says Philip Kim, a professor of physics at Harvard University, who was not involved in this work. “Topological insulators and magnetic insulators are two completely dissimilar materials. Yet they produce very unusual emergent effects at their atomically clean interface,” he adds. “The enhanced interfacial magnetism shown in this work can be very relevant to building up novel spintronics devices that can process information with low energy consumption.”
The team also included associate professor of physics Pablo Jarillo-Herrero and postdoc Peng Wei at MIT, and researchers at the Institute for Theoretical Physics in Bochum and the Institute for Theoretical Solid State Physics in Dresden, both in Germany; the Ecole Normale Superieure in Paris; and the Institute of Nuclear Physics, in Kolkata, India. The work was supported by the National Science Foundation, Office of Naval Research, and the U.S. Department of Energy.
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All-optical control of plasmons can enable optical switches with high speeds, small footprints and high on/off ratios. Here Guo et al., demonstrate ultrafast plasmon modulation in the near-infrared (NIR) to mid-infrared (MIR) range by intraband pumping of indium tin oxide nanorod arrays (ITO-NRAs). They observe redshifts of localized surface plasmon resonances arising from a change of the plasma frequency of ITO, which is governed by the conduction band non-parabolicity. They generalize the plasma frequency for non-parabolic bands, quantitatively model the fluence-dependent plasma frequency shifts, and show that different from noble metals, the lower electron density in ITO enables a remarkable change of electron distributions, yielding a significant plasma frequency modulation and concomitant large transient bleaches and induced absorptions, which can be tuned spectrally by tailoring the ITO-NRA geometry. The low electron heat capacity explains the sub-picosecond kinetics that is much faster than noble metals. Their work demonstrates a new scheme to control infrared plasmons for optical switching, telecommunications and sensing.
The news was sent to the administrator by Nasrin Asgari.
PUBLISHED ONLINE date of the paper: 22 FEBRUARY 2016
Source: DOI: 10.1038/NPHOTON.2016.14
Fig.1. Schematic of the formation of BH-Ag/Pt NPs at different stages of
galvanic replacement process
Daqian Ma et al., successfully fabricated a non-enzymatic glucose sensor by immobilization of bimetallic hollow Ag/Pt nanoparticles (BH-Ag/Pt NPs) using the galvanic replacement reaction onto the surface of the pretreated pure Au electrode. The morphology and composition of the BH-Ag/Pt NPs were investigated by high-resolution transmission electron microscopy (HRTEM), scanning transmission electron microscopy (STEM), energy dispersive X-ray spectroscopy (EDX), and X-ray diffraction (XRD), which proved the formation of bimetallic hollow Ag/Pt nanoparticles. The electroactive surface area and interface property of the Au electrode modified by BH-Ag/Pt NPs were measured by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The associated calculated values were 0.210 cm2 and 11.30 Ω cm2, which were distinctly higher than those of the pure Au electrode. The electrocatalytic properties of the modified electrode toward glucose oxidation were evaluated by CV and differential pulse voltammetry (DPV). The results showed that the modified electrode had a high electrocatalytic activity toward glucose oxidation, a linear response to the glucose concentrations ranging from 1 to 12 mM covering the physiological level of 3–8 mM with a current sensitivity of 7 μAmM−1 and a low detection limit of 0.013 mM. Moreover, the modified electrode also showed ideal reproducibility, long-term stability, and high selectivity. It also showed good glucometer test values for real samples. Therefore, both of the facile preparation method and the excellent properties of the Au electrode modified by BH-Ag/Pt NPs could potentially be implemented to develop novel non-enzymatic glucose sensors.
Daqian Ma, Xiaona Tang, Meiqing Guo, Huiran Lu & Xinhua Xu – Ionics, Springer- 2014
Computer-generated holography is a widely used technology for various applications, e.g., from authentication and optical data storage, to interferometry, particle trapping, and phase conjugation.1–4 In these applications, complex waveforms are radiated efficiently at small angles from the holographic elements. Alternatively, simple grating lobes—with specific resonant conditions—can be used to project the waveforms at larger angles.5–7 To achieve wide-angle projection of computer-generated holograms, a steep phase gradient between adjacent pixels is required. To attain such a gradient, however, a small number of pixels in each period is needed and the projected hologram is therefore inefficient.8–10 Efficient projection of complex waveforms at large angles thus remains a challenge.11, 12
An alternative approach for the generation of complex reflective patterns involves the use of nano-antenna-based metasurfaces. Nano-antennas, which are nanometer-sized metallic structures, resonate at optical frequencies and are essentially a scaled-down counterpart of conventional radio-frequency antennas.13–16 In some recent studies, the use of nano-antennas for holography has been demonstrated.17–21 These previous investigations, however, have mostly focused on optical transmission metasurfaces. This has limited the measured efficiency levels to below 10%.
In this work, we propose and demonstrate the use of a nano-antenna reflectarray for efficient, broadband, and wide-angle holography applications.22 We use our reflectarray, which is composed of optical nano-antenna elements in a coupled dipole-patch configuration, to control the phase of the scattered light. Before we are able to realize our nano-antenna-based hologram approach, we have to determine the phase map that corresponds to the desired output beam. We thus implement the Gerchberg-Saxton algorithm23, 24 for this purpose. We have chosen the logo of Tel Aviv University as the pattern for the demonstration of our technique. During the demonstration, we projected this logo at angles of 20 and 45°, relative to the incident beam. The required phase map and the corresponding optical output are shown in Figure 1, as well as an illustration of the holography concept.
The next step in our approach is to design nano-antennas that scatter light with the desired phase. The antennas are chosen so that the continuous phase is quantized into six discrete values between 0 and 300° (in 60° increments). To span the phase completely, we find that it is advantageous to use unit cells that comprise two antenna elements with different geometries. The combined spectral response of the two elements provides more degrees of freedom for the design and facilitates 2π-phase spanning. We used the technique of electron-beam lithography to fabricate our nano-antenna arrays (see Figure 2).
To achieve a high efficiency with our technique, the reflectivity of the individual elements should differ only in phase and a constant amplitude should be maintained. By properly selecting the dimensions of our antenna elements—see Figure 3(b)—it is possible to attain a scattered wave that possesses any required phase response, while retaining a uniform amplitude. By varying the dimensions of these elements, we can alter the combined antenna response, which in turn changes the phase of the reflected wave. We conduct the optimization of the nano-antennas over a supercell consisting of the six phase pixels, organized in sequence from 0 to 300°, as shown in Figure 3(a). We simulate the supercell in an infinite 2D array, which enables a computationally efficient optimization of the elements. It is also possible to modify the element dimensions in the supercell. We can thus optimize the phase response and obtain the final element dimensions.
The scattering efficiency of our resultant hologram is illustrated in Figure 4. These measurement results indicate that the efficiency remains high over a spectral range of 200nm. This broadband response is caused by the phase response of our designed nano-antennas, which is strongly wavelength independent. We also find that the image projected by the hologram remains unchanged. For comparison, the theoretical efficiencies of the phase-quantized hologram projected at 20 and 45° are 60 and 55%, respectively. Our measured efficiency is therefore only slightly lower than the theoretical predictions. This difference arises from fabrication errors and optimization tolerances.
We have demonstrated a new wide-angle, highly efficient optical holography approach in which we use a reflectarray of optical nano-antenna elements to control the phase of the scattered light. In our methodology, we use the Gerchberg-Saxton algorithm to determine the phase map that is required to project our chosen pattern at angles of 20 and 45°, relative to the surface normal. We found that the measured efficiency of the projected hologram is between 40 and 50% over a broad wavelength range. To further improve the efficiency of our holographic technique, we need to develop new methods to eliminate phase and fabrication errors. Moreover, by incorporating an active tuning mechanism, it may be possible to extend our approach and thus realize active holographic displays and communication devices.
By taking advantage of the thermal gradient that is generated in plasmonic systems and by using an a.c. field, plasmonic tweezers can have a large radius of action and can trap and manipulate single nano-objects.
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DUBLIN, Dec. 8, 2015 — A sensor that exploits plasmonics to gauge nanoscale distortions in lightwaves could yield more powerful tools for metrology and chemical sensing, as well as sharper microscopes.
The method detects wavefront aberrations indirectly by measuring changes in the reflectivity of gold films. It may be the first to use plasmons to address a classical optics problem, according to its developers at University College Dublin.
As light travels through water, the atmosphere and even human tissue its wavefront becomes distorted, blurring images and reducing resolution. It’s possible to correct for these distortions with adaptive optics by precisely measuring the shape of the wavefront.
Such measurements — albeit on relatively large scales — are used in astronomy to correct for atmospheric distortion.
Conventional wavefront sensors work by either mechanically sampling wavefronts with microlenses or other devices or measuring interference patterns. The latter approach requires the extra step of ensuring that the interacting light waves are in phase — meaning their waveforms overlap precisely.
Cartesian wavefront derivatives can be determined by monitoring intensity variations across the reflected beam of light used to excite surface plasmon polaritons in the Kretschmann configuration. Courtesy of Optica/The Optical Society.
Now, by observing how efficiently incoming light creates surface plasmon polaritons (SPPs) on gold film, it’s possible to derive previously undetectable nanoscale distortions in the wavefronts.
SPPs arise when light meets an electrically conducting material, causing electrons to oscillate in a wavelike pulse that travels across the material’s surface. Any changes in the angle of incidence — as would occur from a distortion in the wavefront — affects the way the SPPs are formed. This directly affects how much light is reflected back from the surface.
“Since these polaritons are perfectly coupled to the light that forms them, any changes in their behavior would indicate a change in the waveform of light,” said Brian Vohnsen, a senior lecturer at University College Dublin. “We make use of the attenuation of the signal from the gold surface to simply convert the wavefront shape — or slope — into an intensity difference in a beam of light.”
This change is captured with cameras that are sensitive to minute changes in intensity.
To fully reconstruct the wavefront, the system requires two separate measurements made at 90° to one another. It is then possible to calculate the tiny changes in the actual wavefront based on the orthogonal intensity data points. The speed of the measurement is limited only by the speed of the cameras.
This type of sensor may find applications in the quality inspection of planar materials, films and coatings, the researchers said. It could also replace some wavefront sensors used in astronomy, microscopy and vision science.
The researchers are working to overcome two limitations in the current setup. The first is the requirement for simultaneous measurement of wavefront changes with two cameras. The second is improving the method by which the SPPs are “excited” on the surface of the gold film.
The results were published in Optica (doi: 10.1364/optica.2.001024 [open access]).
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The International Conference on Nanophotonics (ICNP) will be held from 21-25 March 2016 at Academia Sinica, Taipei, Taiwan.
This conference is a unique event where the latest advances in optics and photonics both in nano- and micro-scale will be reported and discussed. This conference primarily aims to explore novel ideas in nanophotonic science and technology that might enable technological breakthroughs in high impact areas such as biomedical and life sciences; information processing; communications; energy harvesting and storage; environment and conservation. The comprehensive coverage of conference themes, including microscopy and nanoscopy, silicon photonics; quantum optics; metamaterials; plasmonics; transformation optics; materials for micro- and nano-photonics; nanofabrication; and photonic devices, ensures researchers in this exciting field always have the opportunity to report their work and exchange information with fellow co-workers.
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