David L. Chandler | MIT News Office
July 14, 2016
A new MIT study could open up new areas of technology based on types of light emission that had been thought to be “forbidden,” or at least so unlikely as to be practically unattainable. The new approach, the researchers say, could cause certain kinds of interactions between light and matter, which would normally take billions of years to happen, to take place instead within billionths of a second, under certain special conditions.
The findings, based on a theoretical analysis, are reported today in the journal Science in a paper by MIT doctoral student Nicholas Rivera, Department of Physics Professor Marin Soljačić, Francis Wright Davis Professor of Physics John Joannopoulos, and postdocs Ido Kaminer and Bo Zhen.
Interactions between light and matter, described by the laws of quantum electrodynamics, are the basis of a wide range of technologies, including lasers, LEDs, and atomic clocks. But from a theoretical standpoint, “Most light-matter interaction processes are ‘forbidden’ by electronic selection rules, which limits the number of transitions between energy levels we have access to,” Soljačić explains.
For example, spectrograms, which are used to analyze the elemental composition of materials, show a few bright lines against a mostly dark background. The bright lines represent the specific “allowed” energy level transitions in the atoms of that element that can be accompanied by the release of a photon (a particle of light). In the dark regions, which make up most of the spectrum, emission at those energy levels is “forbidden.”
With this new study, Kaminer says, “we demonstrate theoretically that these constraints can be lifted” using confined waves within atomically thin, 2-D materials. “We show that some of the transitions which normally take the age of the universe to happen could be made to happen within nanoseconds. Because of this, many of the dark regions of a spectrogram become bright once an atom is placed near a 2-D material.”
Electrons in an atom have discrete energy levels, and when they hop from one level to another they give off a photon of light, a process called spontaneous emission. But the atom itself is much smaller than the wavelength of the light that gets emitted — about 1/1,000 to 1/10,000 as big — substantially impairing the interactions between the two.
The trick is, in effect, to “shrink” the light so it better matches the scale of the atom, as the researchers show in their study. The key to enabling a whole range of interactions, specifically transitions in atomic states that relate to absorbing or emitting light, is the use of a two-dimensional material called graphene, in which light can interact with matter in the form of plasmons, a type of electromagnetic oscillation in the material.
These plasmons, which resemble photons but have wavelengths hundreds of times shorter, are very narrowly confined in the graphene, in a way that makes some kinds of interactions with that matter many orders of magnitude more likely than they would be in ordinary materials. This enables a variety of phenomena normally considered unattainable, such as the simultaneous emission of multiple plasmons, or two-step light-emitting transitions between energy levels, the team says.
This method can enable the simultaneous emission of two photons that are “entangled,” meaning they share the same quantum state even when separated. Such generation of entangled photons is an important element in quantum devices, such as those that might be used for cryptography.
Making use of these forbidden transitions could open up the ability to tailor the optical properties of materials in ways that had not been thought possible, Rivera says. “By altering these rules” about the relationship between light and matter, “it can open new doors to reshaping the optical properties of materials.”
Kaminer predicts that this work “will serve as a founding piece for the next generation of studies on light-matter interactions” and could lead to “further theoretical and experimental advances in many fields which rely on light-matter interactions, including atomic, molecular and optical physics, photonics, chemistry, optoelectronics, and many others.”
Beyond its scientific implications, he says, “this study has possible applications across multiple disciplines, since in principle it has potential to enable the full use of the periodic table for optical applications.” This could potentially lead to applications in spectroscopy and sensing devices, ultrathin solar cells, new kinds of materials to absorb solar energy, organic LEDs with higher efficiencies, and photon sources for possible quantum computing devices.
“From the standpoint of fundamental science, this work lays the groundwork for a subfield that just a few years ago was difficult to imagine and until now was largely unexplored,” Soljačić says.
“Two-dimensional materials confine fields to a surface and motion to a plane, making possible many effects that are orders-of-magnitude too weak to appear in a bulk volume,” says Jason Fleischer, an associate professor of electrical engineering at Princeton University, who was not involved in this research. This work, he says, “systematically explores how 2-D materials improve light-matter interactions, laying a theoretical foundation for faster electronic transitions, enhanced sensing, and better emission, including the compact generation of broadband and quantum light.”
The work was partly supported by the Army Research Office through the Institute for Soldier Nanotechnologies at MIT, and by the U.S. Department of Energy.
The link was sent to us by Ms. Asghari.
Evangelia Vezouviou et al., have reported a near infrared holographic glucose sensor. Real-time glucose monitoring has been beneficial in reducing health complications associated with diabetes as well as a decrease in mortality. This report describes a novel holographic platform, fabricated via laser ablation on chitosan hydrogel with gold nanoparticles with a replaying in visible and near IR. The sensor responded with a 12nm and 7nm shift in waveleng that glucose concentrations in the 0.70 mM range and in the visible and near IR, respectively, at pH 7.4 and an ionic strength of 154 mM. The sensor did not respond to potential interferences found in the interstitial fluid, such as fructose, vitamin C and lactate, at the irrespective normal concentrations and was stable to fluctuations in temperature, pH and ionic strength. The characteristics of this sensor suggest that it may be applicable for use as an implanted device for the real time monitoring of glucose concentrations in the interstitial fluid using near IR as the interrogating medium.
Holographic fabrication. A photosensitive emulsion (1) coated on a glass slide (2) is placed inside a tray with a 7° spacer (3) and facing towards a reflecting mirror (4).Upon exposure to the laser irradiation (5), the gold nanoparticle grains are arranged in fringes (6) align in parallel to the surface of the polymer, and are separated by half the distance of the wavelength they were exposed to. The diagram is not to scale.
Evangelia Vezouviou, Christopher R. Lowe – 2015 – Biosensors and Bioelectronics 68 ,371–381.
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If you want to measure the temperature at any point on an object’s surface, a thin coating of programmable material can do the job, say computer scientists.
The world is full of complex structures such as bridges, roads, wind turbines, power stations, and so on, that have to be carefully monitored to ensure their integrity.
Today, much of this work has to be done by engineers on the spot. That’s not so easy for objects that span hundreds, or even thousands, of kilometers, such as roads, or remote structures such as offshore wind turbines.
So a way of doing this remotely would be hugely valuable. Clearly it requires some kind of independent sensor that can measure the required property such as temperature or acidity, or cracking, and so on.
And indeed there are numerous gadgets for doing this. For example, optical fibers attached to or embedded in objects can measure the forces acting on it and sensors attached to these fibers can monitor temperature, acidity, and so on.
But these kinds of sensors do not provide global coverage—they cannot tell you the temperature at any point on the object. For that you need something more ambitious.
The dream would be to have a smart coating that does this job. This would be a “programmable material” that entirely coats an object in a thin layer. It would contain tiny particulate sensors that gather information about the surface, such as its temperature, and communicate it to their nearest neighbors.
While mathematicians have long pondered the properties of programmable materials, one question has stumped them. Is it possible to use a smart coating to determine the temperature at any point on an arbitrary object, even though the sensors have no knowledge of its overall geometry?
Today, we get an answer to this question thanks to the work of Zahra Derakhshandeh at Arizona State University in Tempe and a few pals. They’ve developed a series of algorithms that provide the mathematical framework that allows these particles to solve this problem.
To make this work, the particulate sensors and the coating must have certain properties. Derakhshandeh and co say the sensors must be able to move within the surface and to make, and break, communication bonds with their nearest neighbors. The object must have a geometry that allows a uniform coating.
Under those conditions, Derakhshandeh and co say that their framework functions as a universal coating algorithm for programmable matter. The particles need only have limited memory and communicate only over short distances and are entirely anonymous—in other words they are all equivalent.
That’s curious work that could one day lead to some useful applications in remote monitoring.
There is still work to be done, however. Given the task of measuring some property of the material at a specific point, one important problem is how quickly the algorithm can do this. To find out, the team suggests testing the algorithm in a simulation or with real programmable matter. It will be interesting to see how they get on.
Another important problem will be the energy efficiency of this kind of programmable matter. What kind of communications overhead does the coating problem impose and could the energy for this conceivably be harvested from the environment?
It’s still early days for programmable matter and for a universal coating. But the savings that Derakhshandeh and co’s algorithms might allow are considerable, given the cost of monitoring and maintaining off shore wind turbines, for example. That alone should guarantee further interest in this topic for the future.
Ref: http://arxiv.org/abs/1601.01008 : Universal Coating for Programmable Matter
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Tugchin et al., study experimentally the excitation of the radially polarized conical surface plasmon polariton (SPP) in a fully metal-coated conically tapered M-profile fiber which works as a “plasmonic tip” for the scanning near-field optical microscope (SNOM). This structure extends the Kretschmann configuration to the conical geometry. In this plasmonic tip, the radially polarized waveguide mode, propagating inside the fiber, resonantly excites the radially polarized SPP on the metal surface, which consequently gets confined at the apex where the field oscillates longitudinally along the tip axis. We also demonstrate the reverse process, where a longitudinal field excites the radially polarized SPP mode which then resonantly excites the radially polarized waveguide mode. This plasmonic tip combines the advantageous properties of near-field optical probes. Though, it has the shape of an apertureless SNOM tip, it can simplify the detection/excitation procedure and suppresses the background signal by its fiber-based design. Unlike the sharp apertureless SNOM tips that detects only the longitudinal field component or aperture SNOM tips that detect mostly the transversal component, the plasmonic tip detects both longitudinal and transversal field in collection mode and backward-scattering mode, respectively. The plasmonic tip, with further improvements, can become an advanced tool in SNOM due to its ability for background-free near-field detection, ease of operation, and higher conversion efficiency from far-field to near-field than conventional tips.
The link of published paper can be found here:
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.
The news was sent to us by Mr. Mosleh.
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.
To have access to this paper, please visit:
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]).