Tag Archives: letter-wafer-tech

A new method to improve semiconductor fiber optics may lead to a material structure that might one day revolutionize the global transmission of data, according to an interdisciplinary team of researchers.

Researchers are working with semiconductor optical fibers, which hold significant advantages over silica-based fiber optics, the current technology used for transmitting nearly all digital data. Silica — glass — fibers can only transmit electronic data converted to light data. This requires external electronic devices that are expensive and consume enormous amounts of electricity. Semiconductor fibers, however, can transmit both light and electronic data and might also be able to complete the conversion from electrical to optical data on the fly during transmission, improving delivery speed.

Amorphous silicon core is inside a 1.7-micron inner-diameter glass capillary. Credit: Penn State

Amorphous silicon core is inside a 1.7-micron inner-diameter glass capillary. Credit: Penn State

Think of these conversions as exit ramps on the information superhighway, said Venkatraman Gopalan, professor of materials science and engineering, Penn State. The fewer the exits the data takes, the faster the information travels. Call it “fly-by optoelectronics,” he said.

In 2006, researchers, led by John Badding, professor of chemistry, physics, and materials science and engineering, first developed silicon fibers by embedding silicon and other semiconductor materials into silica-fiber capillaries. The fibers, comprised of a series of crystals, were limited in their ability to transmit data because imperfections, such as grain boundaries at the surfaces where the many crystals within the fiber core bonded together, forced portions of the light to scatter, disrupting the transmission.

A method designed by Xiaoyu Ji, doctoral candidate in materials science and engineering, improves on the polycrystalline core of the fiber by melting a high-purity amorphous silicon core deposited inside a 1.7-micron inner-diameter glass capillary using a scanning laser, allowing for formation of silicon single crystals that were more than 2,000 times as long as they were thick. This method transforms the core from a polycrystal with many imperfections to a single crystal with few imperfections that transmits light much more efficiently.

That process, detailed in a trio of articles published in ACS Photonics, Advanced Optical Materials, and Applied Physics Letters early this year, demonstrates a new methodology to improve data transfer by eliminating imperfections in the fiber core that can be made of various materials. Gopalan said equipment constraints kept the crystals from being longer.

Because of the ultra-small core, Ji was able to melt and refine the crystal structure of the core material at temperatures of about 750 to 930 degrees Fahrenheit, lower than a typical fiber-drawing process for silicon core fibers. The lower temperatures and the short heating time that can be controlled by the laser power and the laser scanning speed also prevented the silica capillary, which has different thermal properties, from softening and contaminating the core.

“High purity is fundamentally important for high performance when dealing with materials designated for optical or electrical use,” said Ji.

The important takeaway, said Gopalan, is that this new method lays out the methodology for how a host of materials can be embedded into fiber optics and how voids and imperfections can be reduced to increase light-transfer efficiency, necessary steps to advancing the science from its infancy.

“Glass technology has taken us this far,” said Gopalan. “The ambitious idea that Badding and my group had about 10 years ago was that glass is great, but can we do more by using the numerous electronically and optically active materials other than plain glass. That’s when we began trying to embed semiconductors into glass fiber.”

Like fiber-optic cable, which took decades to become a reliable data-delivery device, decades of work likely remains to create commercially viable, semiconductor fiber networks. It took 10 years for researchers to reach polycrystalline fibers to specifications that are far better, but are still not competitive with traditional fiber-optic cable.

“Xiaoyu has been able to start from nicely deposited amorphous silicon and germanium core and use a laser to crystallize them, so that the whole semiconductor fiber core is one nice single crystal with no boundaries,” said Gopalan. “This improved light and electronic transfer. Now we can make some real devices, not just for communications, but also for endoscopy, imaging, fiber lasers and many more.”

Gopalan said he is not only in the business of creating commercially viable materials. He is interested in dreaming big and taking the long view on new technologies. Perhaps one day, every new home constructed might have a semiconductor fiber, bringing faster internet to it.

“This is why we got into this in the first place,” said Gopalan. “Badding’s group was able to figure out how to put silicon and germanium and metals and other semiconductors into the fiber, and this method improves on that.”

Tiny “black holes” on a silicon wafer make for a new type of photodetector that could move more data at lower cost around the world or across a datacenter. The technology, developed by electrical engineers at the University of California, Davis, and W&WSens Devices, Inc. of Los Altos, Calif., a Silicon Valley startup, is described in a paper published April 3 in the journal Nature Photonics.

“We’re trying to take advantage of silicon for something silicon cannot usually do,” said Saif Islam, professor of electrical and computer engineering at UC Davis, who co-lead the project together with the collaborators at W&WSens Devices, Inc. Existing high-speed photodetector devices use materials such as gallium arsenide. “If we don’t need to add non-silicon components and can monolithically integrate with electronics into a single silicon chip, the receivers become much cheaper.”

The new detector uses tapered holes in a silicon wafer to divert photons sideways, preserving the speed of thin-layer silicon and the efficiency of a thicker layer. So far, Islam’s group has built an experimental photodetector and solar cell using the new technology. The photodetector can convert data from optical to electronics at 20 gigabytes per second (or 25 billion bits per second, more than 200 times faster than your cable modem) with a quantum efficiency of 50 percent, the fastest yet reported for a device of this efficiency.

Datacenters need fast connections

The growth of datacenters that power the internet “cloud” has created a demand for devices to move large amounts of data, very fast, over short distances of a few yards to hundreds of yards. Such connections could also be used for high-speed home connections, Islam said.

When computer engineers want to move large amounts of data very fast, whether across the world or across a data center, they use fiber-optic cables that transmit data as pulses of light. But these signals need to be converted to electronic pulses at the receiving end by a photodetector. You can use silicon as a photodetector – incoming photons generate a flow of electrons. But there’s a tradeoff between speed and efficiency. To capture most of the photons, the piece of silicon needs to be thick, and that makes it relatively slow. Make the silicon thinner so it works faster, and too many photons get lost.

Instead, circuit designers have used materials such as gallium arsenide and indium phosphide to make high-speed, high-efficiency photodetectors. Gallium arsenide, for example, is about ten times as efficient as a silicon at the same scale and wavelength. But it is significantly more expensive and cannot be monolithically integrated with silicon electronics.

Tapered holes as light traps

Islam’s group began by experimenting with ways to increase the efficiency of silicon by adding tiny pillars or columns, then holes to the silicon wafer. After two years of experiments, they settled on a pattern of holes that taper towards the bottom.

“We came up with a technology that bends the incoming light laterally through thin silicon,” Islam said.

The idea is that photons enter the holes and get pulled sideways into the silicon. The wafer itself is about two microns thick, but because they move sideways, the photons travel through 30 to 40 microns of silicon, like the ripple of waves on a pond when a pebble is dropped into the water.

The holes-based device can also potentially work with a wider range of wavelengths of light than current technology, Islam said.

An innovative new technique to produce the quickest, smallest, highest-capacity memories for flexible and transparent applications could pave the way for a future golden age of electronics.

Engineering experts from the University of Exeter have developed innovative new memory using a hybrid of graphene oxide and titanium oxide. Their devices are low cost and eco-friendly to produce, are also perfectly suited for use in flexible electronic devices such as ‘bendable’ mobile phone, computer and television screens, and even ‘intelligent’ clothing.

Crucially, these devices may also have the potential to offer a cheaper and more adaptable alternative to ‘flash memory’, which is currently used in many common devices such as memory cards, graphics cards and USB computer drives.

The research team insist that these innovative new devices have the potential to revolutionise not only how data is stored, but also take flexible electronics to a new age in terms of speed, efficiency and power.

The research is published in the leading scientific journal ACS Nano.

Professor David Wright, an Electronic Engineering expert from the University of Exeter and lead author of the paper said: “Using graphene oxide to produce memory devices has been reported before, but they were typically very large, slow, and aimed at the ‘cheap and cheerful’ end of the electronics goods market.

“Our hybrid graphene oxide-titanium oxide memory is, in contrast, just 50 nanometres long and 8 nanometres thick and can be written to and read from in less than five nanoseconds — with one nanometre being one billionth of a metre and one nanosecond a billionth of a second.”

Professor Craciun, a co-author of the work, added: “Being able to improve data storage is the backbone of tomorrow’s knowledge economy, as well as industry on a global scale. Our work offers the opportunity to completely transform graphene-oxide memory technology, and the potential and possibilities it offers.”

A new way to grow narrow ribbons of graphene, a lightweight and strong structure of single-atom-thick carbon atoms linked into hexagons, may address a shortcoming that has prevented the material from achieving its full potential in electronic applications. Graphene nanoribbons, mere billionths of a meter wide, exhibit different electronic properties than two-dimensional sheets of the material.

This graphene nanoribbon was made bottom-up from a molecular precursor. Nanoribbon width and edge effects influence electronic behavior. Credit: Oak Ridge National Laboratory, U.S. Dept. of Energy; scanning tunneling microscopy by Chuanxu Ma and An-Ping Li

This graphene nanoribbon was made bottom-up from a molecular precursor. Nanoribbon width and edge effects influence electronic behavior. Credit: Oak Ridge National Laboratory, U.S. Dept. of Energy; scanning tunneling microscopy by Chuanxu Ma and An-Ping Li

“Confinement changes graphene’s behavior,” said An-Ping Li, a physicist at the Department of Energy’s Oak Ridge National Laboratory. Graphene in sheets is an excellent electrical conductor, but narrowing graphene can turn the material into a semiconductor if the ribbons are made with a specific edge shape.

Previous efforts to make graphene nanoribbons employed a metal substrate that hindered the ribbons’ useful electronic properties.

Now, scientists at ORNL and North Carolina State University report in the journal Nature Communications that they are the first to grow graphene nanoribbons without a metal substrate. Instead, they injected charge carriers that promote a chemical reaction that converts a polymer precursor into a graphene nanoribbon. At selected sites, this new technique can create interfaces between materials with different electronic properties. Such interfaces are the basis of semiconductor electronic devices from integrated circuits and transistors to light-emitting diodes and solar cells.

“Graphene is wonderful, but it has limits,” said Li. “In wide sheets, it doesn’t have an energy gap–an energy range in a solid where no electronic states can exist. That means you cannot turn it on or off.”

When a voltage is applied to a sheet of graphene in a device, electrons flow freely as they do in metals, severely limiting graphene’s application in digital electronics.

“When graphene becomes very narrow, it creates an energy gap,” Li said. “The narrower the ribbon is, the wider is the energy gap.”

In very narrow graphene nanoribbons, with a width of a nanometer or even less, how structures terminate at the edge of the ribbon is important too. For example, cutting graphene along the side of a hexagon creates an edge that resembles an armchair; this material can act like a semiconductor. Excising triangles from graphene creates a zigzag edge–and a material with metallic behavior.

To grow graphene nanoribbons with controlled width and edge structure from polymer precursors, previous researchers had used a metal substrate to catalyze a chemical reaction. However, the metal substrate suppresses useful edge states and shrinks the desired band gap.

Li and colleagues set out to get rid of this troublesome metal substrate. At the Center for Nanophase Materials Sciences, a DOE Office of Science User Facility at ORNL, they used the tip of a scanning tunneling microscope to inject either negative charge carriers (electrons) or positive charge carriers (“holes”) to try to trigger the key chemical reaction. They discovered that only holes triggered it. They were subsequently able to make a ribbon that was only seven carbon atoms wide–less than one nanometer wide–with edges in the armchair conformation.

“We figured out the fundamental mechanism, that is, how charge injection can lower the reaction barrier to promote this chemical reaction,” Li said. Moving the tip along the polymer chain, the researchers could select where they triggered this reaction and convert one hexagon of the graphene lattice at a time.

Next, the researchers will make heterojunctions with different precursor molecules and explore functionalities. They are also eager to see how long electrons can travel in these ribbons before scattering, and will compare it with a graphene nanoribbon made another way and known to conduct electrons extremely well. Using electrons like photons could provide the basis for a new electronic device that could carry current with virtually no resistance, even at room temperature.

“It’s a way to tailor physical properties for energy applications,” Li said. “This is an excellent example of direct writing. You can direct the transformation process at the molecular or atomic level.” Plus, the process could be scaled up and automated.

Researchers at North Carolina State University have developed a technique for converting positively charged (p-type) reduced graphene oxide (rGO) into negatively charged (n-type) rGO, creating a layered material that can be used to develop rGO-based transistors for use in electronic devices.

“Graphene is extremely conductive, but is not a semiconductor; graphene oxide has a bandgap like a semiconductor, but does not conduct well at all — so we created rGO,” says Jay Narayan, the John C. Fan Distinguished Chair Professor of Materials Science and Engineering at NC State and corresponding author of a paper describing the work. “But rGO is p-type, and we needed to find a way to make n-type rGO. And now we have it for next-generation, two-dimensional electronic devices.”

Specifically, Narayan and Anagh Bhaumik — a Ph.D. student in his lab — demonstrated two things in this study. First, they were able to integrate rGO onto sapphire and silicon wafers — across the entire wafer.

Second, the researchers used high-powered laser pulses to disrupt chemical groups at regular intervals across the wafer. This disruption moved electrons from one group to another, effectively converting p-type rGO to n-type rGO. The entire process is done at room temperature and pressure using high-power nanosecond laser pulses, and is completed in less than one-fifth of a microsecond. The laser radiation annealing provides a high degree of spatial and depth control for creating the n-type regions needed to create p-n junction-based two-dimensional electronic devices.

The end result is a wafer with a layer of n-type rGO on the surface and a layer of p-type rGO underneath.

This is critical, because the p-n junction, where the two types meet, is what makes the material useful for transistor applications.

Princeton researchers have discovered a new form of the simple compound GeSe that has surprisingly escaped detection until now. This so-called beta-GeSe compound has a ring type structure like graphene and its monolayer form could have similarly valuable properties for electronic applications, according to the study published in the Journal of the American Chemical Society.

Graphene has been hailed as a two-dimensional wonder material for electronics but its lack of a band gap has hindered its development for devices. As such, a closely related material, black phosphorus, has been receiving intense research attention because it has a small band gap and a high charge carrier mobility, and can easily be reduced to nanometer thicknesses. The researchers calculated that GeSe is highly analogous to black phosphorus and can be considered a pseudo-group-V element.

This is the building blocks of graphene, black phosphorus, α-GeSe, and β-GeSe. Credit: Cava lab

This is the building blocks of graphene, black phosphorus, α-GeSe, and β-GeSe. Credit: Cava lab

Under extreme pressure, black phosphorus is transformed into a simple cubic form, so the team wondered if the same could be done to GeSe and heated the abundant alpha-GeSe form of the compound to 1200 °C under 6 GPa of pressure or 60,000 times atmospheric pressure.

“What we found was not only a new kind of GeSe–which is already unconventional by itself in that you rarely find new binary compounds anymore–but that it has this uncommon ‘boat’ conformation that we were amazed by,” said first author of the study Fabian von Rohr, a postdoctoral researcher in the laboratory of Robert Cava, the Russell Wellman Moore Professor of Chemistry.

beta-GeSe’s rare “boat” form is likely stabilized by the slightly smaller distance between its layers, while black phosphorus and alpha-GeSe exist in standard “chair” conformations. The difference in structures gives rise to the compounds’ different electronic properties. The researchers found that beta-GeSe possesses a band gap size in between that of black phosphorus and alpha-GeSe, which could prove promising for future applications. GeSe is also an attractive material for electronics because it’s robust under ambient conditions while black phosphorus is reactive to both air and water.

For several years, a team of researchers at The University of Texas at Dallas has investigated various materials in search of those whose electrical properties might make them suitable for small, energy-efficient transistors to power next-generation electronic devices.

They recently found one such material, but it was nothing anyone expected.

In an article published online March 10 in the journal Advanced Materials, Dr. Moon Kim and his colleagues describe a material that, when heated to about 450 degrees Celsius, transforms from an atomically thin, two-dimensional sheet into an array of one-dimensional nanowires, each just a few atoms wide.

An image caught in mid-transformation looks like a tiny United States flag, and with false colors added, is arguably the world’s smallest image of Old Glory, Kim said.

This tiny US flag -- just a few nanometers wide and invisible to the naked eye -- is arguably the world's smallest image of Old Glory, according to its creators at the University of Texas at Dallas. In an experiment, the nanoflag pattern emerged unexpectedly as sheets of the "stripe" material -- molybdenum ditelluride -- were heated to about 450 degrees Celsius, at which point its atoms began to rearrange and form new structures -- the 'stars' in this false-color image. Each star consists of six central atoms of molybdenum surrounded by six atoms of tellurium. Stacked on top of one another, the stars form nanowires that might power advanced electronics. The transformation from stripes to stars is reported in the journal Advanced Materials. Credit: University of Texas at Dallas

This tiny US flag — just a few nanometers wide and invisible to the naked eye — is arguably the world’s smallest image of Old Glory, according to its creators at the University of Texas at Dallas. In an experiment, the nanoflag pattern emerged unexpectedly as sheets of the “stripe” material — molybdenum ditelluride — were heated to about 450 degrees Celsius, at which point its atoms began to rearrange and form new structures — the ‘stars’ in this false-color image. Each star consists of six central atoms of molybdenum surrounded by six atoms of tellurium. Stacked on top of one another, the stars form nanowires that might power advanced electronics. The transformation from stripes to stars is reported in the journal Advanced Materials. Credit: University of Texas at Dallas

“The phase transition we observed, this new structure, was not predicted by theory,” said Kim, the Louis Beecherl Jr. Distinguished Professor of materials science and engineering at UT Dallas.

Because the nanowires are semiconductors, they might be used as switching devices, just as silicon is used in today’s transistors to turn electric current on and off in electronic devices.

“These nanowires are about 10 times smaller than the smallest silicon wires, and, if used in future technology, would result in powerful energy-efficient devices,” Kim said. The lead authors of the study are Hui Zhu and Qingxiao Wang, graduate students in materials science and engineering in the Erik Jonsson School of Engineering and Computer Science.

Just a Phase?

When certain materials are subjected to changes in external conditions, such as temperature or pressure, they can undergo a phase transition. A familiar example is when liquid water is cooled to form a solid (ice), or heated to form a gas (steam).

For many materials, however, a phase transition means something a little different. As external temperature and pressure change, these materials’ atoms rearrange and redistribute to make a material with a different structure and composition. These changes can affect the new material’s properties, such as how electrons move through it. For scientists interested in new applications for materials, understanding such transitions is paramount.

In most cases, a type of graphic called a phase diagram helps researchers predict structural and property changes in a material when it undergoes a phase transition.

But nothing predicted what Kim’s team observed as it conducted experiments on a material called molybdenum ditelluride.

Nanoflags and Nanoflowers

Using a transmission electron microscope, the researchers started with atomically thin, two-dimensional sheets of molybdenum ditelluride, a material made up of one layer of molybdenum atoms and two layers of tellurium atoms. The material belongs to a class called transition metal dichalcogenides (TMDs), which show promise in replacing silicon in transistors.

“We wanted to understand the thermal stability of this particular material,” Kim said. “We thought it was a good candidate for next-generation nanoelectronics. Out of curiosity, we set out to see whether it would be stable above room temperature.”

When they increased the temperature to above 450 degrees Celsius, two things happened.

“First, we saw a new pattern begin to emerge that was aesthetically pleasing to the eye,” Kim said. Across the surface of the sample, the repeating rows, or stripes, of molybdenum ditelluride layers began to transform into shapes that looked like tiny six-pointed stars, or flowers with six petals.

The material was transitioning into hexa-molybdenum hexa-telluride, a one-dimensional wire-like structure. The cross section of the new material is a structure consisting of six central atoms of molybdenum surrounded by six atoms of tellurium.

As the phase transition progressed, part of the sample was still “stripes” and part had become “stars.” The team thought the pattern looked like a United States flag. They made a false-color version with a blue field behind the stars and half of the stripes colored red, to make a “nanoflag.”

Not in the textbooks

“Then, when we examined the material more closely, we found that the transition we were seeing from ‘stripes’ to ‘stars’ was not in any of the phase diagrams,” Kim said. “Normally, when you heat up particular materials, you expect to see a different kind of material emerge as predicted by a phase diagram. But in this case, something unusual happened — it formed a whole new phase.”

Each individual nanowire is a semiconductor, which means that electric current moving through the wire can be switched on and off, Kim said. When many of the individual nanowires are grouped together in bulk they behave more like a metal, which easily conducts current.

“We would want to use the nanowires one at a time because we are pushing the size of a transistor as small as possible,” Kim said. “Currently, the smallest transistor size is about 10 times larger than our nanowire. Each of ours is smaller than 1 nanometer in diameter, which is essentially an atomic-scale wire.

“Before we can put this discovery to use and make an actual device, we have many more studies to do, including determining how to separate out the individual nanowires, and overcoming technical challenges to manufacturing and mass production,” Kim said. “But this is a start.”

Synopsys, Inc. (Nasdaq:  SNPS) today announced that TSMC has certified the complete suite of products in the Synopsys Galaxy Design Platform for the most current version of 12-nanometer (nm) FinFET process technology. This 12nm certification brings with it the broad body of design collateral, including routing rules, physical verification runsets, signoff-accurate extraction technology files, SPICE correlated timing and interoperable process design kits (iPDKs) for this latest FinFET process. Synopsys Custom Compiler design solution support is enabled through an iPDK.

To accelerate access to this power-efficient, high-density process, IC Compiler II place-and-route system has been enabled to support new standard cell architectures seamlessly co-existing with 16FFC intellectual property (IP). Recent collaborations have resulted in enhancements to IC Compiler II’s core placement and legalization engines ensuring maximum utilization while minimizing placement fragmentation and cell displacement. The 12nm ready iPDK enables designers to use Custom Compiler’s layout assistant features to shorten time in creating FinFET layouts.

“This power-efficient, high-density node offers a broad set of opportunities to our customers, enabling them to deliver highly differentiated products,” said Suk Lee, TSMC senior director, Design Infrastructure Marketing Division. “Our ongoing collaboration with Synopsys is helping expedite designer access to 12-nm process technology.”

“The long-standing collaboration between Synopsys and TSMC continues to be key in bringing accelerated access to new process technology nodes,” said Bijan Kiani, vice president of product marketing for the Design Group at Synopsys. “With the Galaxy Design Platform certified for 12nm readiness, our mutual customers are enabled to speed up development and deployment to accelerate their time-to-market.”

Metamaterials don’t exist in nature, but their ability to make ultra-thin lenses and ultra-efficient cell phone antennas, bend light to keep satellites cooler and let photovoltaics absorb more energy mean they offer a world of possibilities.

Formed by nanostructures that act as “atoms,” arranged on a substrate to alter light’s path in ways no ordinary material can achieve, these surrogate substances can manipulate an incoming light beam to enable the creation of more efficient versions of ubiquitous, valuable devices — optical filters, lasers, frequency converters and devices that steer beams, for example.

But extensive commercial use of metamaterials has been restrained by the limitations imposed by the materials comprising them. Metal-based metamaterials are “lossy” (lose energy) at shorter wavelengths and can operate effectively only at low frequencies, such as the radio frequencies used by radar, before being overwhelmed by their own absorption. Silicon doesn’t emit light and can transmit it only in a limited wavelength range because of its narrow working range (bandgap). So neither class of material can create a metamaterial that will operate in the infrared and optical ranges, where most military and commercial applications would take place.

This three-resonator-thick III-V metasurface of cylindrical resonators illustrates three possible uses: The left light beam changes color as it passes through the metasurfaces, signifying that nonlinear harmonic generation is taking place that converts the light beam to a shorter wavelength. The blue trace in the middle shows a train of pulses passing through the surface. As they pass, the pulse width decreases due to pulse compression, which requires that the phase of the transmitted optical wave vary with the wavelength. The multilayer metasurfaces are able to achieve the correct phase variation -- something not possible with single layer metasurfaces. The beam on the right signifies that these metasurfaces can act as efficient emitters of light. Click on the thumbnail for a high-resolution image. Credit: (Illustration courtesy of Sandia National Laboratories)

This three-resonator-thick III-V metasurface of cylindrical resonators illustrates three possible uses: The left light beam changes color as it passes through the metasurfaces, signifying that nonlinear harmonic generation is taking place that converts the light beam to a shorter wavelength. The blue trace in the middle shows a train of pulses passing through the surface. As they pass, the pulse width decreases due to pulse compression, which requires that the phase of the transmitted optical wave vary with the wavelength. The multilayer metasurfaces are able to achieve the correct phase variation — something not possible with single layer metasurfaces. The beam on the right signifies that these metasurfaces can act as efficient emitters of light. Click on the thumbnail for a high-resolution image. Credit: (Illustration courtesy of Sandia National Laboratories)

Optical metamaterials enter the arena

Sandia National Laboratories researchers are helping lead the way to the use of III-V semiconductors as the building blocks of metamaterials. (III-V refers to elements in those columns in the periodic table.) Sandia researchers have published technical papers, including three in the past year, on work featuring materials like gallium-arsenide and aluminum-arsenide, which are more efficient than metals for optical metamaterial applications, with wider bandgap ranges than silicon. The work is promising enough to have been featured on the covers of two technical journals.

“There is very little work worldwide on all-dielectric metamaterials using III-V semiconductors,” said Sandia researcher Igal Brener, who leads the Sandia work with researchers Mike Sinclair and Sheng Liu. “Our advantage is Sandia’s vast access to III-V technology, both in growth and processing, so we can move pretty fast.”

Shinier than gold

The new Sandia dielectric materials — a kind of electrical insulator — offer more than just efficiency. They lose little incoming energy and can even be fabricated in multiple layers to form complex, three-dimensional meta-atoms that reflect more light than shiny gold surfaces, usually considered the ultimate in infrared reflectivity. The III-V materials also emit photons when excited — something that silicon, which can reflect, transmit and absorb — can’t do.

Another advantage is their highly variable outputs, across the color spectrum so they might be used to extend the wavelength range of lasers or for generating “entangled photons” for quantum computing.

Sandia’s approach also is attractive for its relatively simple method of forming the artificial atoms, known as resonators, that are the guts of the metamaterial.

Created under the supervision of Liu, the meta-atoms are a few hundred nanometers in diameter and made of many actual atoms. One of Liu’s improvements was to oxidize these tiny groupings around their perimeters to create layered coatings with a low index of refraction, rather than use a more expensive, time-consuming “flip-chip” bonding process. The complexity of previous methods was an obstacle to cost- and time-efficiency. Other Sandia researchers had used a variant of his simplification previously to make lasers, but not metamaterials, he said.

The oxidized, low-index surface surrounds the high-index core “like in wintertime, you have a coat surrounding you,” Liu said. “To confine light, you need a high refractive-index contrast.” Put another way, interior light bumping into the low-indexed oxide surface is herded back by the refractive difference so it travels along the high-index core.

Liu’s Sandia colleague Gordon Keeler achieved controlled oxidation simply by putting III-V materials in a hot oven and flowing water vapor over the sample. “It will oxidize at a certain rate,” Liu says. “The more material, the longer it takes.”

The man-made meta-atoms are sculpted in place during a lithographic process that permits researchers to make any pattern they chose for the placement of the metamaterial components. “We use simulations to direct us,” Liu said. Spacing is determined to some extent by the size of the manmade atoms.

Fractured cubic nanostructures store unusually large amounts of energy

The researchers experimented with cylindrical and cubic nanostructures, reducing the symmetry of the latter to achieve even better properties.

“Cylinders are much easier to fabricate and typically can be used for conventional metasurfaces,” said Brener. “But broken-symmetry cubes are crucial to obtain very sharp resonances. That’s the key issue of the paper.”

The idea of intentionally reducing the symmetry of a cubic resonator nanostructure originated five or six years ago, said Sinclair, with a serendipitous design that happened to break the intentionally symmetrical shape of the meta-atoms when the team tried to mimic a particular manufacturing flaw.

“During a Laboratory Directed Research and Development [LDRD] Metamaterials Grand Challenge, when we were first fabricating cubic resonators in our effort to see if we could get beyond microwaves into infrared and optical metamaterials, we were playing with the shape of resonators to try to simulate the effect of lithography errors. In one simulation, we happened to cut a corner of the cube and all of a sudden very sharp reflection bands appeared,” Sinclair said.

Prior to that discovery, dielectric resonator metamaterials only showed broad bands that didn’t trap much energy. The researchers found the new sharp resonances allowed greater energy storage — beneficial for efficient frequency conversion, and perhaps even for light emission and lasing.

Exploration of the crimped resonator had to wait for a later project, sponsored by the Department of Energy’s Office of Science. Salvatore Campione, building on previous work by Lorena Basilio, Larry Warne and William Langston — all of Sandia — used electromagnetic simulations to unravel precisely how the cubes trap light. Sandia’s Willie Luk measured the cubes’ reflective properties. Another LDRD grant currently supports research into metamaterial lasing.

“We feel we’ve created a pretty flexible platform for a lot of different kinds of devices,” Sinclair said.

The ongoing work is aided by Sandia’s John Reno, nationally known for growing extremely precise crystalline structures, who contributed the III-V wafers.

Three patents on aspects of the work have been submitted.

Russian physicists, with their colleagues from Europe through changing the light parameters, learned to generate quasiparticles – excitons, which were fully controllable and also helped to record information at room temperature. These particles act as a transitional form between photons and electrons so the researchers believe that with excitons, they will be able to create compact optoelectronic devices for rapid recording and processing an optical signal. The proposed method is based on use of a special class of materials called metal-organic frameworks. The study appeared in Advanced Materials.

The way of how the light with different wavelengths influences on a MOF crystal: different types of excitons are showed in red and blue (left). Image of crystals (right). Credit: ITMO University

The way of how the light with different wavelengths influences on a MOF crystal: different types of excitons are showed in red and blue (left). Image of crystals (right). Credit: ITMO University

To simplify the description of complex effects in quantum mechanics, scientists have introduced a concept of quasiparticles. One of them which is called exciton is an “electron – hole” pair, which provides energy transfer between photons and electrons. According to the scientific community, this mediation of quasiparticles will help to combine optics with electronics to create a fundamentally new class of equipment – more compact and energy efficient. However, all exciton demo devices either operate only at low temperature, or are difficult to manufacture which inhibits their mass adoption.

In the new study, the scientists from ITMO University in Saint Petersburg, Leipzig University in Germany and Eindhoven University of Technology in the Netherlands could generate excitons at room temperature by changing the light parameters. The authors also managed to control the quasiparticles with ultra-high sensitivity of about hundreds of femtoseconds (10-13 s). Finally, they developed an easy method for data recording with excitons. This all became possible through the use of an individual class of materials called metal-organic frameworks.

Metal-organic frameworks (MOF) synthesized at ITMO University, have a layered structure. Between the layers, there is a physical attraction called van der Waals force. To prevent the plates from uncontrollably coming together, the interlayer space is filled with an organic liquid, which fixes the framework to be three-dimensional.

In such crystals, the researchers learned to bring two types of excitons individually: intralayer and interlayer. The first arise when a photon absorbed by the crystal turns into an electron-hole pair inside a layer, but the second appear when an electron and a hole belong to neighboring layers. In some time, both kinds of quasiparticles disintegrate, re-radiating the energy as a photon. But excitons can move around the crystal while they exist.

The life time of intralayer excitons is relatively short, but their high density and agility allow one to use these quasiparticles to generate light in LEDs and lasers, for instance. Interlayer excitons are more stable, but slow-moving, so the researchers propose them to be used for the data recording. Both types of excitons fit processing of an optical signal, according to the physicists.

The innovative approach for information recording concerns the changing a distance between crystal layers to switch “on” and “off” the interlayer excitons. Valentin Milichko, the first author of the paper, associate professor of Department of Nanophotonics and Metamaterials at ITMO University, comments: “We locally heated the crystal with a laser. In the place of exposure, the layers stuck together and the luminescence of excitons disappeared while the rest of the crystal continued shining. This could mean that we recorded 1 bit of information, and the record, in the form of a dark spot, was kept for many days. To delete the data, it was enough to put the MOF into the same organic liquid that supports layers. In this case, the crystal itself is not affected, but the recorded information (the dark spot) disappears.”

The authors believe that in the future the new material will help to bring processing of an optical signal to the usual pattern of zeros and ones: “In fact, we can influence the exciton behavior in the crystal, changing the light intensity. At weak irradiation, excitons are accumulated (in ‘1’ state), but if the laser power increases, the concentration of quasiparticles grows so much that they can instantly disintegrate (in ‘0’ state),” says Valentin Milichko.

Typically, excitons occur in dielectric and semiconductor crystals, but the scientists could create these quasiparticles and get control over them in a completely different class of materials, which never was used for this. The MOF crystal combines organic components with inorganic that gives it additional properties not available for materials of a single nature. Thus, the organic term allows one to generate excitons at room temperature, but inorganic provides their efficient transfer around the crystal.