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While solar cell technology is currently being used by many industrial and government entities, it remains prohibitively expensive to many individuals who would like to utilize it. There is a need for cheaper, more efficient solar cells than the traditional silicon solar cells so that more people may have access to this technology. One of the current popular topics in photovoltaic technology research centers around the use of organic-inorganic halide perovskites as solar cells because of the high power conversion efficiency and the low-cost fabrication.

Perovskites are a type of crystalline material that can be formed using a wide variety of different chemical combinations. Of the many different perovskites formulations that can be used in solar cells, the methylammonium lead iodide perovskite (MAPbI3) has been the most widely studied. Solar cells made of this material have been able to reach efficiencies exceeding 20% and are cheaper to manufacture than silicon. However, their short lifespans have prevented them from becoming a viable silicon solar cell alternative. In order to help create better solar cells in the future, members of the Energy Materials and Surface Sciences Unit at the Okinawa Institute of Science and Technology Graduate University (OIST) have been investigating the cause of rapid degradation of these perovskite solar cells (PSCs).

Dr. Shenghao Wang, first author of the publication in Nature Energy, suggests that the degradation of MAPbI3 perovskites may not be a fixable issue. His research reveals that iodide-based perovskites will universally produce a gaseous form of iodine, I2, during operation, which in turn causes further degradation of perovskite. While many researchers have pointed to other sources, such as moisture, atmospheric oxygen and heat as the cause of MAPbI3 degradation, the fact that these solar cells continue to degrade even in the absence of these factors led Wang to believe that a property intrinsic to these PSCs was causing the breakdown of material.

“We found that these PSCs are self-exposed to I2 vapor at the onset of degradation, which led to accelerated decomposition of the MAPbI3 perovskite material into PbI2.” Wang explained, “Because of the relatively high vapor pressure of I2, it can quickly permeate the rest of the perovskite material causing damage of the whole PSC.

This research does not rule out the probability of using perovskites in solar cells, however. Professor Yabing Qi, leader of the Energy Materials and Surface Sciences Unit and corresponding author of this work, expounds “our experimental results strongly suggest that it is necessary to develop new materials with a reduced concentration of iodine or a reinforced structure that can suppress iodine-induced degradation, in addition to desirable photovoltaic properties”.

These researchers at OIST are continuing to investigate different types of perovskite materials in order to find more efficient, cost-effective, and long lifespan perovskite material suitable for use. Their ultimate goal is to make solar cells that are affordable, efficient and stable so that they will be more accessible to the general population. Hopefully, better, cheaper solar cells will entice more people to utilize this technology.

When most living creatures get hurt, they can self-heal and recover from the injury. But, when damage occurs to inanimate objects, they don’t have that same ability and typically either lose functionality or have their useful lifecycle reduced. Researchers at the Beckman Institute for Advanced Science and Technology are working to change that.

For more than 15 years, Jeff Moore, a professor of chemistry, Nancy Sottos, a professor of materials science and engineering, and Scott White, a professor of aerospace engineering, have been collaborating in the Autonomous Materials Systems Group. Their work focuses on creating synthetic materials that can react to their environment, recover from damage, and even self-destruct once their usefulness has come to an end.

The trio of Beckman researchers are pioneers in what is now a dynamic and growing field. Their work on self-healing polymers was first presented in the journal Nature more than a decade-and-a-half ago. Prior to that, there had been just a few papers published on the subject of autonomous polymers. In the years since, research in the field has exploded, with hundreds of papers published.

Now, in a sweeping perspective article published this month in the journal Nature, the researchers, along with Beckman Postdoctoral Fellows Jason Patrick and Maxwell Robb, review the state-of-the-art autonomous polymers and lay out future directions for the field.

“What we’ve tried to capture for the first time is a vision of polymers as multifunctional entities that can manage their well-being,” Moore said.

The article is an overview of how their work has evolved from the development of self-healing polymers to a concentration on “life cycle control of polymers” — what he called “the healthy aging of materials.” He described the autonomous function of materials this way: “Live long, be fit, die fast, and leave no mess behind. … We want the materials to live as long as they can in a healthy state and, when the time comes, be able to trigger the inevitable from a functional state to recoverable materials resources.”

In the paper, the researchers identified five landscape-altering developments: self-protection, self-reporting, self-healing, regeneration, and controlled degradation.

Much of their work revolves around microcapsules, which are small, fluid-filled spheres that can be integrated into various material systems. The capsules contain a healing agent that is released automatically when exposed to a specific environmental change, such as physical damage or excessive temperature.

“You have capsules that remain stable in the material until the environment causes a stress that causes them to rupture,” explained Sottos. “A lot of different external stimuli can open up the capsules. You can have a thermal trigger, a mechanical trigger, and we’ve worked a lot on chemical triggers. They open up, release their contents, and the science is in what comes out and reacts.”

By developing new chemistries and ways to integrate microcapsules over the years, the researchers have created polymers that can do everything from re-filling minor damage in paints and coatings (self-protecting), changing color when undergoing stress (self-reporting), and re-bonding cracks or restoring electrical conductivity (self-healing).

The AMS Group also developed a way to efficiently fabricate vascular networks within polymers. These networks, which can include multiple channels that run throughout a material, are able to deliver healing agents multiple times, change thermal or magnetic properties, and facilitate other useful chemical interactions in a material.

A major development in their self-healing work focuses on repairing large-scale damage through the process of regeneration.

“Ballistic impacts, drilling holes in sheets of plastic, and these sorts of things, where a significant mass is lost … traditional self-healing has no way of dealing with that problem at all,” White said. “The materials that would be used to heal that hole would simply fall out, bleed out under gravity.”

So White and his collaborators came up with a two-channel healing system. When damage occurs on a large scale, a gel-like substance fills the space and builds upon itself, keeping the healing agents in place until they harden.

Their most recent work is concerned with how to deal with material systems when they have reached the end of their useful life. This work involves making materials that can self-destruct when a specific environmental signal is given (triggered transience). The researchers believe that triggers such as high temperature, water, ultraviolet light, and many others may one day be used to make obsolete devices degrade quickly so that they can be reused or recycled, thus reducing electronic waste and boosting sustainability.

Autonomous polymers are beginning to make their way into the commercial sector. Commercialization efforts have produced materials such as wear-resistant mobile device cases and automotive paints that can self-repair minor scratches. And more self-healing products are slowly coming to market including a microcapsule-based powder coating produced by the Champaign-based start-up company Autonomic Materials Inc.

While the practical application of many of these techniques still face challenges, Moore, Sottos, White, and their colleagues continue to work toward the creation of smart materials that can function independently, self-heal, and disintegrate once they are no longer useful, offering the eventual promise of safer, more efficient, and longer-lasting products that require fewer resources and produce less waste.

The next time you place your coffee order, imagine slapping onto your to-go cup a sticker that acts as an electronic decal, letting you know the precise temperature of your triple-venti no-foam latte. Someday, the high-tech stamping that produces such a sticker might also bring us food packaging that displays a digital countdown to warn of spoiling produce, or even a window pane that shows the day’s forecast, based on measurements of the weather conditions outside.

Engineers at MIT have invented a fast, precise printing process that may make such electronic surfaces an inexpensive reality. In a paper published today in Science Advances, the researchers report that they have fabricated a stamp made from forests of carbon nanotubes that is able to print electronic inks onto rigid and flexible surfaces.

A. John Hart, the Mitsui Career Development Associate Professor in Contemporary Technology and Mechanical Engineering at MIT, says the team’s stamping process should be able to print transistors small enough to control individual pixels in high-resolution displays and touchscreens. The new printing technique may also offer a relatively cheap, fast way to manufacture electronic surfaces for as-yet-unknown applications.

“There is a huge need for printing of electronic devices that are extremely inexpensive but provide simple computations and interactive functions,” Hart says. “Our new printing process is an enabling technology for high-performance, fully printed electronics, including transistors, optically functional surfaces, and ubiquitous sensors.”

Sanha Kim, a postdoc in MIT’s departments of Mechanical Engineering and Chemical Engineering, is the lead author, and Hart is the senior author. Their co-authors are mechanical engineering graduate students Hossein Sojoudi, Hangbo Zhao, and Dhanushkodi Mariappan; Gareth McKinley, the School of Engineering Professor of Teaching Innovation; and Karen Gleason, professor of chemical engineering and MIT’s associate provost.

A stamp from tiny pen quills

There have been other attempts in recent years to print electronic surfaces using inkjet printing and rubber stamping techniques, but with fuzzy results. Because such techniques are difficult to control at very small scales, they tend to produce “coffee ring” patterns where ink spills over the borders, or uneven prints that can lead to incomplete circuits.

“There are critical limitations to existing printing processes in the control they have over the feature size and thickness of the layer that’s printed,” Hart says. “For something like a transistor or thin film with particular electrical or optical properties, those characteristics are very important.”

Hart and his team sought to print electronics much more precisely, by designing “nanoporous” stamps. (Imagine a stamp that’s more spongy than rubber and shrunk to the size of a pinky fingernail, with patterned features that are much smaller than the width of a human hair.) They reasoned that the stamp should be porous, to allow a solution of nanoparticles, or “ink,” to flow uniformly through the stamp and onto whatever surface is to be printed. Designed in this way, the stamp should achieve much higher resolution than conventional rubber stamp printing, referred to as flexography.

Kim and Hart hit upon the perfect material to create their highly detailed stamp: carbon nanotubes — strong, microscopic sheets of carbon atoms, arranged in cylinders. Hart’s group has specialized in growing forests of vertically aligned nanotubes in carefully controlled patterns that can be engineered into highly detailed stamps.

“It’s somewhat serendipitous that the solution to high-resolution printing of electronics leverages our background in making carbon nanotubes for many years,” Hart says. “The forests of carbon nanotubes can transfer ink onto a surface like massive numbers of tiny pen quills.”

Printing circuits, roll by roll

To make their stamps, the researchers used the group’s previously developed techniques to grow the carbon nanotubes on a surface of silicon in various patterns, including honeycomb-like hexagons and flower-shaped designs. They coated the nanotubes with a thin polymer layer (developed by Gleason’s group) to ensure the ink would penetrate throughout the nanotube forest and the nanotubes would not shrink after the ink was stamped. Then they infused the stamp with a small volume of electronic ink containing nanoparticles such as silver, zinc oxide, or semiconductor quantum dots.

The key to printing tiny, precise, high-resolution patterns is in the amount of pressure applied to stamp the ink. The team developed a model to predict the amount of force necessary to stamp an even layer of ink onto a substrate, given the roughness of both the stamp and the substrate, and the concentration of nanoparticles in the ink.

To scale up the process, Mariappan built a printing machine, including a motorized roller, and attached to it various flexible substrates. The researchers fixed each stamp onto a platform attached to a spring, which they used to control the force used to press the stamp against the substrate.

“This would be a continuous industrial process, where you would have a stamp, and a roller on which you’d have a substrate you want to print on, like a spool of plastic film or specialized paper for electronics,” Hart says. “We found, limited by the motor we used in the printing system, we could print at 200 millimeters per second, continuously, which is already competitive with the rates of industrial printing technologies. This, combined with a tenfold improvement in the printing resolution that we demonstrated, is encouraging.”

After stamping ink patterns of various designs, the team tested the printed patterns’ electrical conductivity. After annealing, or heating, the designs after stamping — a common step in activating electronic features — the printed patterns were indeed highly conductive, and could serve, for example, as high-performance transparent electrodes.

Going forward, Hart and his team plan to pursue the possibility of fully printed electronics.

“Another exciting next step is the integration of our printing technologies with 2-D materials, such as graphene, which together could enable new, ultrathin electronic and energy conversion devices,” Hart says.

At this week’s IEEE IEDM conference, imec, the research and innovation hub in nano-electronics and digital technologies showed for the first time a silicon (Si)-passivated germanium (Ge) nMOS gate stack with dramatically reduced interface defect density (DIT) reaching the same level as a Si gate stack and with high mobility and reduced positive bias temperature instability (PBTI). These promising results pave the way to Ge-based finFETs and gate all-around devices, as promising options for 5nm and beyond logic devices.

Today’s results were achieved by band engineering using an interface dipole at high-k/SiO2 interface, and a H2 high-pressure anneal (HPA) finalizing the process flow. The interface dipole was formed on SiO2 layer by depositing a Lanthanum (La)SiO layer by atomic layer deposition (ALD), which is a 3D-compatible process. While a high DIT has been the leading concern for Si-passivated Ge nFET, it was dramatically reduced, for the first time, from 2×1012 cm-2eV-1 down to 5×1010 cm-2eV-1 around midgap using a combination of the LaSiO insertion and a H2 HPA. Consequently, electron mobility was increased (approximately 50 percent at peak) while PBTI reliability was improved thanks to the interface dipole-induced band engineering.

At IEEE IEDM, imec also presents a model for heterostructure interface resistivity (Rhi) analysis for highly doped semiconductors. Using this novel model, imec predicted that high-doping Si:P in a TiSix/Si:P/n-Ge contact stack helps to overcome the high contact resistance problem in Ge nMOS. With development of an advanced low-temperature Si:P epitaxy technique, imec demonstrated a TiSix/Si:P/n-Ge contact stack with record-low contact resistivity for n-Ge.

“Dedicated to push the boundaries of Moore’s Law, Ge-based devices are a key focus area or our research,” stated An Steegen, Executive Vice President Semiconductor Technology and Systems. “These breakthrough achievements underscore our dedication to understanding the fundamental roadblocks that need to be overcome in order for Ge-based devices to become a viable solution for 5nm and beyond.”

This work was performed in collaboration with ASM, Poongsan and Nanyang Technological University. Imec’s research into advanced logic scaling is performed in cooperation with imec’s key partners in its core CMOS programs including GlobalFoundries, Huawei, Intel, Micron, Qualcomm, Samsung, SK Hynix, Sony and TSMC.

In electronics, lower power consumption leads to operation cost savings, environmental benefits and the convenience advantages from longer running devices. While progress in energy efficiencies has been reported with alternative materials such as SiC and GaN, energy-savings in the standard inexpensive and widely used silicon devices are still keenly sought. K Tsutsui at Tokyo Institute of Technology and colleagues in Japan have now shown that by scaling down size parameters in all three dimensions their device they can achieve significant energy savings.

Tsutsui and colleagues studied silicon insulated gate bipolar transistors (IGBTs), a fast-operating switch that features in a number of every day appliances. While the efficiency of IGBTs is good, reducing the ON resistance, or the voltage from collector to emitter required for saturation (Vce(sat)), could help increase the energy efficiency of these devices further.

Previous investigations have highlighted that increases in the “injection enhancement (IE) effect”, which give rise to more charge carriers, leads to a reduction in Vce(sat). Although this has been achieved by reducing the mesa width in the device structure, the mesa resistance was thereby increased as well. Reducing the mesa height could help counter the increased resistance but is prone to impeding the (IE) effect. Instead the researchers reduced the mesa width, gate length, and the oxide thickness in the MOSFET to increase the IE effect and so reduce Vce(sat) from 1.70 to 1.26 V. With these alterations the researchers also used a reduced gate voltage, which has advantages for CMOS integration.

They conclude, “It was experimentally confirmed for the first time that significant Vce(sat) reduction can be achieved by scaling the IGBT both in the lateral and vertical dimensions with a decrease in the gate voltage.”

A simple solution-based electrical doping technique could help reduce the cost of polymer solar cells and organic electronic devices, potentially expanding the applications for these technologies. By enabling production of efficient single-layer solar cells, the new process could help move organic photovoltaics into a new generation of wearable devices and enable small-scale distributed power generation.

polymer-solar_2021

Developed by researchers at the Georgia Institute of Technology and colleagues from three other institutions, the technique provides a new way of inducing p-type electrical doping in organic semiconductor films. The process involves briefly immersing the films in a solution at room temperature, and would replace a more complex technique that requires vacuum processing.

“Our hope is that this will be a game-changer for organic photovoltaics by further simplifying the process for fabricating polymer-based solar cells,” said Bernard Kippelen, director of Georgia Tech’s Center for Organic Photonics and Electronics and a professor in the School of Electrical and Computer Engineering. “We believe this technique is likely to impact many other device platforms in areas such as organic printed electronics, sensors, photodetectors and light-emitting diodes.”

Sponsored by the Office of Naval Research, the work was reported December 5 in the journal Nature Materials. The research also involved scientists from the University of California at Santa Barbara, Kyushu University in Japan, and the Eindhoven University of Technology in The Netherlands.

The technique consists of immersing thin films of organic semiconductors and their blends in polyoxometalate (PMA and PTA) solutions in nitromethane for a brief time – on the order of minutes. The diffusion of the dopant molecules into the films during immersion leads to efficient p-type electrical doping over a limited depth of 10 to 20 nanometers from the surface of the film. The p-doped regions show increased electrical conductivity and high work function, reduced solubility in the processing solvent, and improved photo-oxidation stability in air.

This new method provides a simpler alternative to air-sensitive molybdenum oxide layers used in the most efficient polymer solar cells that are generally processed using expensive vacuum equipment. When applied to polymer solar cells, the new doping method provided efficient hole collection. For the first time, single-layer polymer solar cells were demonstrated by combining this new method with spontaneous vertical phase separation of amine-containing polymers that leads to efficient electron collection at the opposing electrode. The geometry of these new devices is unique as the functions of hole and electron collection are built into the light-absorbing active layer, resulting in the simplest single-layer geometry with few interfaces.

“The realization of single-layer photovoltaics with our approach enables both electrodes in the device to be made out of low-cost conductive materials,” said Canek Fuentes-Hernandez, a senior research scientist in Kippelen’s research group. “This offers a dramatic simplification of a device geometry, and it improves the photo-oxidation stability of the donor polymer. Although lifetime and cost analysis studies are needed to assess the full impact of these innovations, they are certainly very exciting developments on the road to transform organic photovoltaics into a commercial technology.”

By simplifying the production of organic solar cells, the new processing technique could allow fabrication of solar cells in areas of Africa and Latin America that lack capital-intensive manufacturing capabilities, said Felipe Larrain, a Ph.D. student in Kippelen’s lab.

“Our goal is to further simplify the fabrication of organic solar cells to the point at which every material required to fabricate them may be included in a single kit that is offered to the public,” Larrain said. “The solar cell product may be different if you are able to provide people with a solution that would allow them to make their own solar cells. It could one day enable people to power themselves and be independent of the grid.”

Organic solar cells have been studied in many academic and industrial laboratories for several decades, and have experienced a continuous and steady improvement in their power conversion efficiency with laboratory values reaching 13 percent – compared to around 20 percent for commercial silicon-based cells. Though polymer-based cells are currently less efficient, they require less energy to produce than silicon cells and can be more easily recycled at the end of their lifetime.

“Being able to process solar cells entirely at room temperature using this simple solution-based technique could pave the way for a scalable and vacuum-free method of device fabrication, while significantly reducing the time and cost associated with it,” said Vladimir Kolesov, a Ph.D. researcher and the paper’s lead author.

Beyond solar cells, the doping technique could be more broadly used in other areas of organic electronics, noted Ph.D. researcher Wen-Fang Chou. “With its simplicity, this is truly a promising technology offering adjustable conductivity of semiconductors that could be applied to various organic electronics, and could have huge impact on the industry for mass production.”

Also at Georgia Tech, the research involved professors Samuel Graham and Seth Marder, both from the Center for Organic Photonics and Electronics. Beyond Georgia Tech, the project also involved Naoya Aizawa from Kyushu University; Ming Wang, Guillermo Bazan and Thuc-Quyen Nguyen from the University of California Santa Barbara, and Alberto Perrotta from Eindhoven University of Technology.

University of Texas at Dallas physicists have published new findings examining the electrical properties of materials that could be harnessed for next-generation transistors and electronics.

Dr. Fan Zhang, assistant professor of physics, and senior physics student Armin Khamoshi recently published their research on transition metal dichalcogenides, or TMDs, in the journal Nature Communications. Zhang is a co-corresponding author, and Khamoshi is a co-lead author of the paper, which also includes collaborating scientists at Hong Kong University of Science and Technology.

In recent years, scientists and engineers have become interested in TMDs in part because they are superior in many ways to graphene, a one-atom thick, two-dimensional sheet of carbon atoms arranged in a lattice. Since it was first isolated in 2004, graphene has been investigated for its potential to replace conventional semiconductors in transistors, shrinking them even further in size. Graphene is an exceptional conductor, a material in which electrons move easily, with high mobility.

“It was thought that graphene could be used in transistors, but in transistors, you need to be able to switch the electric current on and off,” Zhang said. “With graphene, however, the current cannot be easily switched off.”

Beyond Graphene

In their search for alternatives, scientists and engineers have turned to TMDs, which also can be made into thin, two-dimensional sheets, or monolayers, just a few molecules thick.

“TMDs have something graphene does not have — an energy gap that allows the flow of electrons to be controlled, for the current to be switched on and off,” Khamoshi said. “This gap makes TMDs ideal for use in transistors. TMDs are also very good absorbers of circularly polarized light, so they could be used in detectors. For these reasons, these materials have become a very popular topic of research.”

One of the challenges is to optimize and increase electron mobility in TMD materials, a key factor if they are to be developed for use in transistors, Khamoshi said.

In their most recent project, Zhang and Khamoshi provided the theoretical work to guide the Hong Kong group on the layer-by-layer construction of a TMD device and on the use of magnetic fields to study how electrons travel through the device. Each monolayer of TMD is three molecules thick, and the layers were sandwiched between two sheets of boron nitride molecules.

The behavior of electrons controls the behavior of these materials,” Zhang said. “We want to make use of highly mobile electrons, but it is very challenging. Our collaborators in Hong Kong made significant progress in that direction by devising a way to significantly increase electron mobility.”

The team discovered that how electrons behave in the TMDs depends on whether an even or odd number of TMD layers were used.

“This layer-dependent behavior is a very surprising finding,” Zhang said. “It doesn’t matter how many layers you have, but rather, whether there are an odd or even number of layers.”

Electron Physics

Because the TMD materials operate on the scale of individual atoms and electrons, the researchers incorporated quantum physics into their theories and observations. Unlike classical physics, which describes the behavior of large-scale objects that we can see and touch, quantum physics governs the realm of very small particles, including electrons.

On the size scale of everyday electrical devices, electrons flowing through wires behave like a stream of particles. In the quantum world, however, electrons behave like waves, and the electrical transverse conductance of the two-dimensional material in the presence of a magnetic field is no longer like a stream — it changes in discrete steps, Zhang said. The phenomenon is called quantum Hall conductance.

“Quantum Hall conductance might change one step by one step, or two steps by two steps, and so on,” he said. “We found that if we used an even number of TMD layers in our device, there was a 12-step quantum conductance. If we applied a strong enough magnetic field to it, it would change by six steps at a time.”

Using an odd number of layers combined with a low magnetic field also resulted in a 6-step quantum Hall conductance in the TMDs, but under stronger magnetic fields, it became a 3-step by 3-step phenomenon.

“The type of quantum Hall conductance we predicted and observed in our TMD devices has never been found in any other material,” Zhang said. “These results not only decipher the intrinsic properties of TMD materials, but also demonstrate that we achieved high electron mobility in the devices. This gives us hope that we can one day use TMDs for transistors.”

A collaborative effort between research groups at the Technical University of Freiberg and the University of Siegen in Germany demonstrates that the physical properties of SrTiO3, or strontium titanate, in its single crystal form can be changed by a relatively simple electrical treatment. SrTi03 is a mineral often studied for its superconducting properties.

The treatment, described this week in Applied Physics Letters, from AIP Publishing, creates the effect known as piezoelectricity, where electricity results from mechanical stress, in the material which did not originally see piezoelectric effects. This could be extremely important as our technologically-oriented society makes ever-growing demands for new materials and unusual properties.

Crystalline materials are made of atoms and electrons, which arrange themselves in periodic patterns. The atomic structure of a crystal is similar to a piece of a cross-stitching pattern, but the scale is about ten million times smaller. While a cross-stitching technique might be tricky at first, once you learn the pattern, you just repeat the same stitches to fill the available space. Nature works much the same way in building crystals: it “learns” how to connect atoms with each other in a so-called unit cell and then repeats this building block to fill the space making a crystal lattice.

Looking at a crystal structure is somewhat like looking at fabric through a magnifying glass. Using a technique called X-ray diffraction, researchers apply external stimuli (e.g. stretch or an electric voltage) to a crystal and see how different connections (atomic “stitches”) respond.

“The idea for this work was born when I was giving a colloquium talk in TU Freiberg, presenting our new technique for time-resolved X-ray diffraction and investigating piezoelectric material. Our colleagues in Freiberg had been investigating artificially created near-surface volumes of SrTiO3 crystals, with properties different from the normal bulk SrTiO3,” said Semën Gorfman, a University of Siegen physicist.

The Siegen research team had developed unique experimental equipment to investigate crystal structures under a periodically varying field using X-ray diffraction that is mobile and can connect to any available instrument, such as a home-lab X-ray diffractometer or a synchrotron beamline.

“Since the measurements are non-routine, this experimental equipment makes our research truly unique and original,” Gorfman said. “It turned out that the technique developed at Siegen, was ideally matched to the research direction that the Freiberg team was working on, so we came up with the hypothesis to be tested (piezoelectricity in field-modified near surface phase of SrTiO3 crystal), and a suggested experimental method (stroboscopic time-resolved X-ray diffraction), performed the experiment and got results.”

This work shows that new physical properties can be created artificially, reporting the piezoelectric effect in the artificially designed new phase of SrTiO3, a material that is not piezoelectric under normal conditions.

“We believe that physical properties of migration field induced polar phase in SrTiO3 opens a new and interesting chapter for research, Gorfman said. “The challenge now is to make the effect practical so that it can be used for devices.”

A multi-institutional research collaboration has created a novel approach for fabricating three-dimensional micro-optics through the shape-defined formation of porous silicon (PSi), with broad impacts in integrated optoelectronics, imaging, and photovoltaics.

Working with colleagues at Stanford and The Dow Chemical Company, researchers at the University of Illinois at Urbana-Champaign fabricated 3-D birefringent gradient refractive index (GRIN) micro-optics by electrochemically etching preformed Si micro-structures, like square columns, PSi structures with defined refractive index profiles.

“The emergence and growth of transformation optics over the past decade has revitalized interest in using GRIN optics to control light propagation,” explained Paul Braun, the Ivan Racheff Professor of Materials Science and Engineering at Illinois. “In this work, we have figured out how to couple the starting shape of the silicon micro-structure and the etch conditions to realize a unique set of desirable optical qualities. For example, these elements exhibit novel polarization-dependent optical functions, including splitting and focusing, expanding the use of porous silicon for a wide range of integrated photonics applications.

“The key is that the optical properties are a function of the etch current,” Braun said. “If you change the etch current, you change the refractive index. We also think that the fact that we can create the structures in silicon is important, as silicon is important for photovoltaic, imaging, and integrated optics applications.

“Our demonstration using a three-dimensional, lithographically-defined silicon platform not only displayed the power of GRIN optics, but it also illustrated it in a promising form factor and material for integration within photonic integrated circuits,” stated Neil Krueger, a former PhD student in Braun’s research group and first author of the paper, “Porous Silicon Gradient Refractive Index Micro-Optics,” appearing in Nano Letters.

“The real novelty of our work is that we are doing this in a three-dimensional optical element,” added Krueger, who has recently joined Honeywell Aerospace as a Scientist in Advanced Technology. “This gives added control over the behavior of our structures given that light follows curvilinear optical paths in optically inhomogeneous media such as GRIN elements. The birefringent nature of these structures is an added bonus because coupled birefringent/GRIN effects provide an opportunity for a GRIN element to perform distinct, polarization-selective operations.”

According to the researchers, PSi was initially studied due to its visible luminescence at room temperature, but more recently, as this and other reports have shown, has proven to be a versatile optical material, as its nanoscale porosity (and thus refractive index) can be modulated during its electrochemical fabrication.

“The beauty of this 3D fabrication process is that it is fast and scalable,” commented Weijun Zhou at Dow. “Large scale, nanostructured GRIN components can be readily made to enable a variety of new industry applications such as advanced imaging, microscopy, and beam shaping.”

“Because the etching process enables modulation of the refractive index, this approach makes it possible to decouple the optical performance and the physical shape of the optical element,” Braun added. “Thus, for example, a lens can be formed without having to conform to the shape that we think of for a lens, opening up new opportunities in the design of integrated silicon optics.”

A team of physicists from ITMO University, MIPT, and The University of Texas at Austin have developed an unconventional nanoantenna that scatters light in a particular direction depending on the intensity of incident radiation. The research findings will help with the development of flexible optical information processing in telecommunication systems.

Fig. 1. An artist's rendering of nonlinear light scattering by a dimer of two silicon particles with a variable radiation pattern. Credit: Image courtesy of the press office of MIPT

Fig. 1. An artist’s rendering of nonlinear light scattering by a dimer of two silicon particles with a variable radiation pattern. Credit: Image courtesy of the press office of MIPT

Photons–the carriers of electromagnetic radiation–have neither mass nor electric charge. This means that light is relatively hard to control, unlike, for example, electrons: their flow in electronic circuits can be controlled by applying a constant electric field. However, such devices as nanoantennas enable a certain degree of control over the propagation of electromagnetic waves.

One area that requires the “advanced” light manipulation is the development of optical computers. In these devices, the information is carried not by electrons, but by photons. Using light instead of charged particles has the potential to greatly improve the speed of transmitting and processing information. To make these computers a reality, we need specific nanoantennas with characteristics that can be manipulated in some way–by applying a constant electric or magnetic field, for instance, or by varying the intensity of incident light.

In the paper published in Laser & Photonics Reviews, the researchers designed a novel nonlinear nanoantenna that can change the direction of light scattering depending on the intensity of the incident wave (Fig. 1). At the heart of the proposed nanoantenna are silicon nanoparticles, which generate electron plasma under harsh laser radiation. The authors previously demonstrated the possibilities of using these nanoparticles for the nonlinear and ultrafast control of light. The researchers then managed to manipulate portions of light radiation scattered forward and backward. Now, by changing the intensity of incident light, they have found a way to turn a scattered light beam in the desired direction.

To rotate the radiation pattern of the nanoantenna, the authors used the mechanism of plasma excitation in silicon. The nanoantenna is a dimer–two silicon nanospheres of unequal diameters. Irradiated with a weak laser beam, this antenna scatters the light sideways due to its asymmetric shape. The diameters of the two nanoparticles are chosen so that one particle is resonant at the wavelength of the laser light. Irradiated with an intense laser pulse, electron plasma is generated in the resonant particle which causes changes in the optical properties of the particle. The other particle remains nonresonant, and the powerful laser field has little effect on it. Generally speaking, by accurately choosing the relative size of both particles in combination with the parameters of the incident beam (duration and intensity), it is possible to make the size of the particles virtually the same, which enables the antenna to bounce the light beam forward.

“Existing optical nanoantennas can control light in a fairly wide range. However, this ability is usually embedded in their geometry and the materials they are made of, so it is not possible to configure these characteristics at any time,” says Denis Baranov, a postgraduate student at MIPT and the lead author of the paper. “The properties of our nanoantenna, however, can be dynamically modified. When we illuminate it with a weak laser impulse, we get one result, but with a strong impulse, the outcome is completely different.”

The scientists performed numerical modeling of the light scattering mechanism. The simulation showed that when the nanoantenna is illuminated with a weak laser beam, the light scatters sideways. However, if the nanoantenna is illuminated with an intense laser impulse, that leads to the generation of electron plasma within the device and the scattering pattern rotates by 20 degrees (red line). This provides an opportunity to deflect weak and strong incident impulses in different directions.

Sergey Makarov, a senior researcher at the Department of Nanophotonics and Metamaterials at ITMO University concludes: “In this study, we focused on the development of a nanoscale optical chip measuring less than 200×200×500 nanometers. This is much less than the wavelength of a photon, which carries the information. The new device will allow us to change the direction of light propagation at a much better rate compared to electronic analogues. Our device will be able to distribute a signal into two optical channels within a very short space of time, which is extremely important for modern telecommunication systems.”

Today, information is transmitted via optical fibers at speeds of up to hundreds of Gbit/s. However, even modern electronic devices process these signals quite slowly: at speeds of only a few Gbit/s for a single element. The proposed nonlinear optical nanoantenna can solve this problem, as it operates at 250 Gbit/s. This paves the way for ultrafast processing of optical information. The nonlinear antenna developed by the researchers provides more opportunities to control light at nanoscale, which is what is required in order to successfully develop photonic computers and other similar devices.