The future of photonics using quantum dots – researchers are trying to integrate photonics into silicon devices.


QDots for Photonics 180327141726_1_540x360One type of laser that’s particularly suited for quantum dots is a mode-locked laser, which passively generates ultrashort pulses less than one picosecond in duration.
Credit: Peter Allen

The future of photonics using quantum dots

Thousands of miles of fiber-optic cables crisscross the globe and package everything from financial data to cat videos into light. But when the signal arrives at your local data center, it runs into a silicon bottleneck. Instead of light, computers run on electrons moving through silicon-based chips — which, despite huge advances, are still less efficient than photonics.

To break through this bottleneck, researchers are trying to integrate photonics into silicon devices. They’ve been developing lasers — a crucial component of photonic circuits — that work seamlessly on silicon. In a paper appearing this week in APL Photonics, from AIP Publishing, researchers from the University of California, Santa Barbara write that the future of silicon-based lasers may be in tiny, atom like structures called quantum dots.

Such lasers could save a lot of energy. Replacing the electronic components that connect devices with photonic components could cut energy use by 20 to 75 percent, Justin Norman, a graduate student at UC Santa Barbara, said. “It’s a substantial cut to global energy consumption just by having a way to integrate lasers and photonic circuits with silicon.”

Silicon, however, does not have the right properties for lasers. Researchers have instead turned to a class of materials from Groups III and V of the periodic table because these materials can be integrated with silicon.

Initially, the researchers struggled to find a functional integration method, but ultimately ended up using quantum dots because they can be grown directly on silicon, Norman said. Quantum dots are semiconductor particles only a few nanometers wide — small enough that they behave like individual atoms. When driven with electrical current, electrons and positively charged holes become confined in the dots and recombine to emit light — a property that can be exploited to make lasers.

The researchers made their III-V quantum-dot lasers using a technique called molecular beam epitaxy. They deposit the III-V material onto the silicon substrate, and its atoms self-assemble into a crystalline structure. But the crystal structure of silicon differs from III-V materials, leading to defects that allow electrons and holes to escape, degrading performance. Fortunately, because quantum dots are packed together at high densities — more than 50 billion dots per square centimeter — they capture electrons and holes before the particles are lost.

These lasers have many other advantages, Norman said. For example, quantum dots are more stable in photonic circuits because they have localized atomlike energy states. They can also run on less power because they don’t need as much electric current. Moreover, they can operate at higher temperatures and be scaled down to smaller sizes.

In just the last year, researchers have made considerable progress thanks to advances in material growth, Norman said. Now, the lasers operate at 35 degrees Celsius without much degradation and the researchers report that the lifetime could be up to 10 million hours.

They are now testing lasers that can operate at 60 to 80 degrees Celsius, the more typical temperature range of a data center or supercomputer. They’re also working on designing epitaxial waveguides and other photonic components, Norman said. “Suddenly,” he said, “we’ve made so much progress that things are looking a little more near term.”

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Materials provided by American Institute of PhysicsNote: Content may be edited for style and length.


Journal Reference:

  1. S. A. Kazazis, E. Papadomanolaki, M. Androulidaki, M. Kayambaki, E. Iliopoulos. Optical properties of InGaN thin films in the entire composition rangeJournal of Applied Physics, 2018; 123 (12): 125101 DOI: 10.1063/1.5020988
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MIT: “Ironing out” graphene’s wrinkles to make ‘Pristine Graphene’ ~ Promising Successor to Silicon (at the ‘Speed of Light’)


MIT-Wrinkle Single-Domain_0.jpg

Researchers at MIT have found a way to make graphene with fewer wrinkles, and to iron out the wrinkles that do appear. They found each wafer exhibited uniform performance, meaning that electrons flowed freely across each wafer, at similar speeds, even across previously wrinkled regions.

New technique produces highly conductive graphene wafers.

” … electrons can blitz through graphene at velocities approaching the speed of light, far faster than they can travel through silicon and other semiconducting materials.

” … Graphene, therefore, has been touted as a promising successor to silicon, with the potential to enable faster, more efficient electronic and photonic devices.”

” … pristine graphene — a single, perfectly flat, ultrathin sheet of carbon atoms, precisely aligned and linked together like chickenwire — is extremely difficult.”

” … which can derail an electron’s bullet-train journey, significantly limiting graphene’s electrical performance.”

From an electron’s point of view, graphene must be a hair-raising thrill ride. For years, scientists have observed that electrons can blitz through graphene at velocities approaching the speed of light, far faster than they can travel through silicon and other semiconducting materials.

Graphene, therefore, has been touted as a promising successor to silicon, with the potential to enable faster, more efficient electronic and photonic devices.

But manufacturing pristine graphene — a single, perfectly flat, ultrathin sheet of carbon atoms, precisely aligned and linked together like chickenwire — is extremely difficult. Conventional fabrication processes often generate wrinkles, which can derail an electron’s bullet-train journey, significantly limiting graphene’s electrical performance.

Now engineers at MIT have found a way to make graphene with fewer wrinkles, and to iron out the wrinkles that do appear. After fabricating and then flattening out the graphene, the researchers tested its electrical conductivity. They found each wafer exhibited uniform performance, meaning that electrons flowed freely across each wafer, at similar speeds, even across previously wrinkled regions.

In a paper published today in the Proceedings of the National Academy of Sciences, the researchers report that their techniques successfully produce wafer-scale, “single-domain” graphene — single layers of graphene that are uniform in both atomic arrangement and electronic performance.

“For graphene to play as a main semiconductor material for industry, it has to be single-domain, so that if you make millions of devices on it, the performance of the devices is the same in any location,” says Jeehwan Kim, the Class of 1947 Career Development Assistant Professor in the departments of Mechanical Engineering and Materials Science and Engineering at MIT. “Now we can really produce single-domain graphene at wafer scale.”graphene wonder material images

Kim’s co-authors include Sanghoon Bae, Samuel Cruz, and Yunjo grapehene electronics imagesKim from MIT, along with researchers from IBM, the University of California at Los Angeles, and Kyungpook National University in South Korea.

A patchwork of wrinkles

The most common way to make graphene involves chemical vapor deposition, or CVD, a process in which carbon atoms are deposited onto a crystalline substrate such as copper foil. Once the copper foil is evenly coated with a single layer of carbon atoms, scientists submerge the entire thing in acid to etch away the copper. What remains is a single sheet of graphene, which researchers then pull out from the acid.

The CVD process can produce relatively large, macroscropic wrinkles in graphene, due to the roughness of the underlying copper itself and the process of pulling the graphene out from the acid. The alignment of carbon atoms is not uniform across the graphene, creating a “polycrystalline” state in which graphene resembles an uneven, patchwork terrain, preventing electrons from flowing at uniform rates.

In 2013, while working at IBM, Kim and his colleagues developed a method to fabricate wafers of single-crystalline graphene, in which the orientation of carbon atoms is exactly the same throughout a wafer.

Rather than using CVD, his team produced single-crystalline graphene from a silicon carbide wafer with an atomically smooth surface, albeit with tiny, step-like wrinkles on the order of several nanometers. They then used a thin sheet of nickel to peel off the topmost graphene from the silicon carbide wafer, in a process called layer-resolved graphene transfer.

Ironing charges

In their new paper, Kim and his colleagues discovered that the layer-resolved graphene transfer irons out the steps and tiny wrinkles in silicon carbide-fabricated graphene. Before transferring the layer of graphene onto a silicon wafer, the team oxidized the silicon, creating a layer of silicon dioxide that naturally exhibits electrostatic charges. When the researchers then deposited the graphene, the silicon dioxide effectively pulled graphene’s carbon atoms down onto the wafer, flattening out its steps and wrinkles.

Kim says this ironing method would not work on CVD-fabricated graphene, as the wrinkles generated through CVD are much larger, on the order of several microns.

“The CVD process creates wrinkles that are too high to be ironed out,” Kim notes. “For silicon carbide graphene, the wrinkles are just a few nanometers high, short enough to be flattened out.”

To test whether the flattened, single-crystalline graphene wafers were single-domain, the researchers fabricated tiny transistors on multiple sites on each wafer, including across previously wrinkled regions.

“We measured electron mobility throughout the wafers, and their performance was comparable,” Kim says. “What’s more, this mobility in ironed graphene is two times faster. So now we really have single-domain graphene, and its electrical quality is much higher [than graphene-attached silicon carbide].”

Kim says that while there are still challenges to adapting graphene for use in electronics, the group’s results give researchers a blueprint for how to reliably manufacture pristine, single-domain, wrinkle-free graphene at wafer scale.

“If you want to make any electronic device using graphene, you need to work with single-domain graphene,” Kim says. “There’s still a long way to go to make an operational transistor out of graphene. But we can now show the community guidelines for how you can make single-crystalline, single-domain graphene.”

Photonic Optical Fiber with ‘Einstein’s Theory’ effect


opticalfibreCoreless optical fibre: If a photonic crystal fibre is twisted, it does not require a core with a different refractive index to trap light at its centre. Credit: Science 2016/MPI for the Science of Light

Researchers at the Max Planck Institute for the Science of Light in Erlangen have discovered a new mechanism for guiding light in photonic crystal fibre (PCF). PCF is a hair-thin glass fibre with a regular array of hollow channels running along its length. When helically twisted, this spiralling array of hollow channels acts on light rays in an analogous manner to the bending of light rays when they travel through the gravitationally curved space around a star, as described by the general theory of relativity.

Optical fibres act as pipes for . And just as the inside of a pipe is enclosed by a wall, optical fibres normally have a light-guiding core, whose glass has a higher refractive index than the glass of the enclosing outer cladding. The difference in the refractive index causes the light to be reflected at the cladding interface and trapped in the core like water in a pipe. A team headed by Philip Russell, Director at the Max Planck Institute for the Science of Light, is the first to succeed in guiding light in a PCF with no core.

Photonic crystals give butterflies their colour and can also guide light

A typical photonic crystal consists of a piece of glass with holes arranged in regular periodic pattern throughout its volume. Since glass and the air have different refractive indices, the refractive index has a periodic structure. This is the reason these materials are called crystals—their atoms form an ordered, three-dimensional lattice as found in crystalline salt or silicon, for example. In a conventional crystal, the precise design of the 3-D structure determines the behaviour of electrons, resulting for example in electrical insulators, conductors and semiconductors.

In a similar manner, the optical properties of a photonic crystal depend on the periodic 3-D microstructure, which is responsible for the shimmering colours of some butterfly wings, for example. Being able to control the optical properties of materials is useful in a wide variety of applications. The photonic crystal fibres developed by Philip Russell and his team at the Erlangen-based Max Planck Institute can be used to filter specific wavelengths out of the visible spectrum or to produce very white light, for example.

As is the case with all optical fibres used in telecommunications, all conventional photonic crystal fibres have a core and cladding each with different refractive indices or optical properties. In PCF, the air-filled channels already give the glass a different from the one it would have if completely solid.

The holes define the space in a photonic crystal fibre

“We are the first to succeed in guiding light through a coreless fibre,” says Gordon Wong from the Max Planck Institute for the Science of Light in Erlangen. The researchers working in Philip Russell’s team have fabricated a photonic crystal fibre whose complete cross-section is closely packed with a large number of air-filled channels, each around one thousandth of a millimetre in diameter, which extend along its whole length.

While the core of a conventional PCF is solid glass, the cross-sectional view of the new optical fibre resembles a sieve. The holes have regular separations and are arranged so that every hole is surrounded by a regular hexagon of neighbouring holes. “This structure defines the space in the fibre,” explains Ramin Beravat, lead author of the publication. The holes can be thought of as distance markers. The interior of the fibre then has a kind of artificial spatial structure which is formed by the regular lattice of holes.

“We have now fabricated the fibre in a twisted form,” continues Beravat. The twisting causes the hollow channels to wind around the length of the fibre in helical lines. The researchers then transmitted laser light through the fibre. In the case of the regular, coreless cross-section, one would actually expect the light to distribute itself between the holes of the sieve as evenly as their pattern determines, i.e. at the edge just as much as in the centre. Instead, the physicists discovered something surprising: the light was concentrated in the central region, where the core of a conventional optical fibre is located.

In a twisted PCF, the light follows the shortest path in the interior of the fibre

“The effect is analogous to the curvature of space in Einstein’s general theory of relativity,” explains Wong. This predicts that a heavy mass such as the Sun will distort the space surrounding it – or more precisely, distort spacetime, i.e. the combination of the three spatial dimensions with the fourth dimension, time – like a sheet of rubber into which a lead sphere is placed. Light follows this curvature. The shortest path between two points is then no longer a straight line, but a curve. During a solar eclipse, stars which should really be hidden behind the Sun thus become visible. Physicists call these shortest connecting paths “geodesics”.

“By twisting the fibre, the ‘space’ in our photonic crystal fibre becomes twisted as well,” says Wong. This leads to helical geodesic lines along which light travels. This can be intuitively understood by taking into account the fact that light always takes the shortest route through a medium. The glass strands between the air-filled channels describe spirals, which define possible paths for the . The path through the wide spirals at the edge of the fibre is longer than that through the more closely wound spirals in its centre, however, resulting in curved ray paths that at a certain radius are reflected by a effect back towards the fibre axis.

A twisted PCF as a large-scale environmental sensor

The more the fibre is twisted, the narrower is the space within which the light concentrated. In analogy to Einstein’s theory, this corresponds to a stronger gravitational force and thus a greater deflection of the light. The Erlangen-based researchers write that they have created a “topological channel” for the light (topology is concerned with the properties of space which are conserved under continuous distortion).

The researchers emphasize that their work is basic research. They are one of the very few research groups working in this field anywhere in the world. Nevertheless, they can think of several applications for their discovery. A twisted fibre which is less twisted at certain intervals, for example, will allow a portion of the light to escape to the outside. Light could then interact with the environment at these defined locations. “This could be used for sensors which measure the absorption of a medium, for instance.” A network of these fibres could collect data over large areas as an environmental sensor.

Explore further: A photonic crystal fibre generates light from the ultraviolet to the mid-infrared

More information: R. Beravat et al. Twist-induced guidance in coreless photonic crystal fiber: A helical channel for light, Science Advances (2016). DOI: 10.1126/sciadv.1601421

 

Photonic crystals increase solar efficiency: Video: U of Toronto: Dr. Sajeev John


photonic-crystals-image1Gains are derived from nanowires and light trapping for better energy conversion.

Sajeev John is a University Professor at the University of Toronto and Government of Canada Research Chair holder. He received his bachelor’s degree in physics in 1979 from the Massachusetts Institute of Technology and his PhD in physics at Harvard in 1984. His PhD work introduced the theory of classical wave localization and in particular the localization of light in three-dimensional strongly scattering dielectrics.sajeev-john-u-of-t-maxresdefault

Watch the Video Here

His groundbreaking work in the field of light localization that enables light to be controlled at the microscopic level has earned him an international reputation. He is a pioneering theoretician in photonic band gap (PBG) materials. This new class of optical materials presents exciting possibilities in the fields of physics, chemistry, engineering and medicine. PBG materials could eventually be used for optical communications/information processing, clinical medicine, lighting and solar energy harvesting.

John has received numerous awards, including the King Faisal International Prize in Science (2001), the IEEE Nanotechnology Pioneer Award (2008), and the Killam Prize in Natural Sciences (2014) from the Canada Council for the Arts. He is a Fellow of the American Physical Society, OSA, the Royal Society of Canada, and a member of the Max-Planck Society of Germany.

 

Sajeev John current research (Univ. of Toronto)

 

 

Nanotubes “Line-Up” to Form Films for Flexible Electonics: Video


Rice logo_rice3 Rice University researchers discover way to make highly aligned, wafer-scale films 

A simple filtration process helped Rice University researchers create flexible, wafer-scale films of highly aligned and closely packed carbon nanotubes.

Scientists at Rice, with support from Los Alamos National Laboratory, have made inch-wide films of densely packed, chirality-enriched single-walled carbon nanotubes through a process revealed today in Nature Nanotechnology.

In the right solution of nanotubes and under the right conditions, the tubes assemble themselves by the millions into long rows that are aligned better than once thought possible, the researchers reported.

The thin films offer possibilities for making flexible electronic and photonic (light-manipulating) devices, said Rice physicist Junichiro Kono, whose lab led the study. Think of a bendable computer chip, rather than a brittle silicon one, and the potential becomes clear, he said.

“Once we have centimeter-sized crystals consisting of single-chirality nanotubes, that’s it,” Kono said. “That’s the holy grail for this field. For the last 20 years, people have been looking for this.”Rice scanning 040616 0215.WAFERS-5-rn-26x5r2v

A scanning electron microscope image shows highly aligned and closely packed carbon nanotubes gathered into a film by researchers at Rice. Courtesy of the Kono Lab

 

The Rice lab is closing in, he said, but the films reported in the current paper are “chirality-enriched” rather than single-chirality. A carbon nanotube is a cylinder of graphene, with its atoms arranged in hexagons. How the hexagons are turned sets the tube’s chirality, and that determines its electronic properties. Some are semiconducting like silicon, and others are metallic conductors.

A film of perfectly aligned, single-chirality nanotubes would have specific electronic properties. Controlling the chirality would allow for tunable films, Kono said, but nanotubes grow in batches of random types.

For now, the Rice researchers use a simple process developed at the National Institute of Standards and Technology to separate nanotubes by chirality. While not perfect, it was good enough to let the researchers make enriched films with nanotubes of different types and diameters and then make terahertz polarizers and electronic transistors.

Rice CNT Inventors 040616 0215.WAFERS-1-rn-168jkgm

Rice graduate students Xiaowei He, left, and Weilu Gao, center, and Professor Junichiro Kono show a film of highly aligned carbon nanotubes made in Kono’s lab. Photo by Jeff Fitlow

The Rice lab discovered the filtration technique in late 2013 when graduate students and lead authors Xiaowei He and Weilu Gao inadvertently added a bit too much water to a nanotube-surfactant suspension before feeding it through a filter assisted by vacuum. (Surfactants keep nanotubes in a solution from clumping.)

The film that formed on the paper filter bore further investigation. “Weilu checked the film with a scanning electron microscope and saw something strange,” He said. Rather than drop randomly onto the paper like pickup sticks, the nanotubes – millions of them – had come together in tight, aligned rows.

“That first picture gave us a clue we might have something totally different,” He said. A year and more than 100 films later, the students and their colleagues had refined their technique to make nanotube wafers up to an inch wide (limited only by the size of their equipment) and of any thickness, from a few to hundreds of nanometers.

Further experiments revealed that each element mattered: the type of filter paper, the vacuum pressure and the concentration of nanotubes and surfactant. Nanotubes of any chirality and diameter worked, but each required adjustments to the other elements to optimize the alignment.

The films can be separated from the paper and washed and dried for use, the researchers said.

They suspect multiwalled carbon nanotubes and non-carbon nanotubes like boron nitride would work as well.

Co-author Wade Adams, a senior faculty fellow at Rice who specializes in polymer science, said the discovery is a step forward in a long quest for aligned structures.

“They formed what is called a monodomain in liquid crystal technology, in which all the rigid molecules line up in the same direction,” Adams said. “It’s astonishing. (The late Rice Nobel laureate) Rick Smalley and I worked very hard for years to make a single crystal of nanotubes, but these students have actually done it in a way neither of us ever imagined.”

Why do the nanotubes line up? Kono said the team is still investigating the mechanics of nucleation — that is, how the first few nanotubes on the paper come together. “We think the nanotubes fall randomly at first, but they can still slide around on the paper,” he said. “Van der Waals force brings them together, and they naturally seek their lowest-energy state, which is in alignment.” Because the nanotubes vary in length, the researchers suspect the overhangs force other tubes to line up as they join the array.

The researchers found their completed films could be patterned with standard lithography techniques. That’s yet another plus for manufacturers, said Kono, who started hearing buzz about the discovery months before the paper’s release.

“I gave an invited talk about our work at a carbon nanotube conference, and many people are already trying to reproduce our results,” he said. “I got so much enthusiastic response right after my talk. Everybody asked for the recipe.”

Co-authors are Rice graduate students Qi Zhang, Sidong Lei and John Robinson and postdoctoral researcher Bo Li; Lijuan Xie of Zhejiang University, who has a complimentary appointment at Rice.

Rice alumnus Erik Haroz and Stephen Doorn of Los Alamos National Laboratory; Robert Vajtai, a faculty research fellow at Rice; Pulickel Ajayan, chair of Rice’s Department of Materials Science and NanoEngineering, the Benjamin M. and Mary Greenwood Anderson Professor in Engineering and a professor of chemistry; and the late Robert Hauge, distinguished faculty fellow in chemistry and in materials science and nanoengineering at Rice.

Adams is a senior faculty fellow in materials science and nanoengineering. Kono is a Rice professor of electrical and computer engineering, of physics and astronomy and of materials science and nanoengineering.

The Department of Energy and the Robert A. Welch Foundation supported the research.

 

University of Rochester: Defects on an atomically thin semiconductor can produce light-emitting quantum dots: Applications in Integrated Photonics


Silicon Photonics id39403Researchers at the University of Rochester have shown that defects on an atomically thin semiconductor can produce light-emitting quantum dots. The quantum dots serve as a source of single photons and could be useful for the integration of quantum photonics with solid-state electronics — a combination known as integrated photonics.

Scientists have become interested in integrated solid-state devices for quantum information processing uses. Quantum dots in atomically thin semiconductors could not only provide a framework to explore the fundamental physics of how they interact, but also enable nanophotonics applications, the researchers say.

Quantum dots are often referred to as artificial atoms. They are artificially engineered or naturally occurring defects in solids that are being studied for a wide range of applications. Nick Vamivakas, assistant professor of optics at the University of Rochester and senior author on the paper, adds that atomically thin, 2D materials, such as graphene, have also generated interest among scientists who want to explore their potential for optoelectronics. However, until now, optically active quantum dots have not been observed in 2D materials.

In a paper published in Nature Nanotechnology this week, the Rochester researchers show how tungsten diselenide (WSe2) can be fashioned into an atomically thin semiconductor that serves as a platform for solid-state quantum dots. Perhaps most importantly the defects that create the dots do not inhibit the electrical or optical performance of the semiconductor and they can be controlled by applying electric and magnetic fields.

Vamivakas explains that the brightness of the quantum dot emission can be controlled by applying the voltage. He adds that the next step is to use voltage to “tune the color” of the emitted photons, which can make it possible to integrate these quantum dots with nanophotonic devices.

A key advantage is how much easier it is to create quantum dots in atomically thin tungsten diselenide compared to producing quantum dots in more traditional materials like indium arsenide.

“We start with a black crystal and then we peel layers of it off until we have an extremely thin later left, an atomically thin sheet of tungsten diselenide,” said Vamivakas.1-nano devices howtomakemob

The researchers take two of these atomically thin sheets and lay one over the other one. At the point where they overlap, a quantum dot is created. The overlap creates a defect in the otherwise smooth 2D sheet of semiconductor material. The extremely thin semiconductors are much easier to integrate with other electronics.

The quantum dots in tungsten diselenide also possess an intrinsic quantum degree of freedom — the electron spin. This is a desirable property as the spin can both act as a store of quantum information as well as provide a probe of the local quantum dot environment.

“What makes tungsten diselenide extremely versatile is that the color of the single photons emitted by the quantum dots is correlated with the quantum dot spin,” said first author Chitraleema Chakraborty. Chakraborty added that the ease with which the spins and photons interact with one another should make these systems ideal for quantum information applications as well as nanoscale metrology.


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The above story is based on materials provided by University of Rochester. Note: Materials may be edited for content and length.