The ultra-thin semiconductor, which is about 100,000 times thinner than a human hair, stretches across the top of the photonic cavity.

University of Washington scientists have built a new nanometer-sized laser -- using the thinnest semiconductor available today -- that is energy efficient, easy to build and compatible with existing electronics.

Lasers play essential roles in countless technologies, from medical therapies to metal cutters to electronic gadgets. But to meet modern needs in computation, communications, imaging and sensing, scientists are striving to create ever-smaller laser systems that also consume less energy.

The UW nanolaser, developed in collaboration with Stanford University, uses a tungsten-based semiconductor only three atoms thick as the "gain material" that emits light. The technology is described in a paper published in the March 16 online edition of Nature.

"This is a recently discovered, new type of semiconductor which is very thin and emits light efficiently," said Sanfeng Wu, lead author and a UW doctoral candidate in physics. "Researchers are making transistors, light-emitting diodes, and solar cells based on this material because of its properties. And now, nanolasers."

Nanolasers -- which are so small they can't be seen with the eye -- have the potential to be used in a wide range of applications from next-generation computing to implantable microchips that monitor health problems. But nanolasers so far haven't strayed far from the research lab.

Other nanolaser designs use gain materials that are either much thicker or that are embedded in the structure of the cavity that captures light. That makes them difficult to build and to integrate with modern electrical circuits and computing technologies.

The UW version, instead, uses a flat sheet that can be placed directly on top of a commonly used optical cavity, a tiny cave that confines and intensifies light. The ultrathin nature of the semiconductor -- made from a single layer of a tungsten-based molecule -- yields efficient coordination between the two key components of the laser.

The UW nanolaser requires only 27 nanowatts to kickstart its beam, which means it is very energy efficient.

Other advantages of the UW team's nanolaser are that it can be easily fabricated, and it can potentially work with silicon components common in modern electronics. Using a separate atomic sheet as the gain material offers versatility and the opportunity to more easily manipulate its properties.

"You can think of it as the difference between a cell phone where the SIM card is embedded into the phone versus one that's removable," said co-author Arka Majumdar, UW assistant professor of electrical engineering and of physics.

"When you're working with other materials, your gain medium is embedded and you can't change it. In our nanolasers, you can take the monolayer out or put it back, and it's much easier to change around," he said.

The researchers hope this and other recent innovations will enable them to produce an electrically-driven nanolaser that could open the door to using light, rather than electrons, to transfer information between computer chips and boards.

The current process can cause systems to overheat and wastes power, so companies such as Facebook, Oracle, HP, Google and Intel with massive data centers are keenly interested in more energy-efficient solutions.

Using photons rather than electrons to transfer that information would consume less energy and could enable next-generation computing that breaks current bandwidth and power limitations. The recently proven UW nanolaser technology is one step toward making optical computing and short distance optical communication a reality.

"We all want to make devices run faster with less energy consumption, so we need new technologies," said co-author Xiaodong Xu, UW associate professor of materials science and engineering and of physics. "The real innovation in this new approach of ours, compared to the old nanolasers, is that we're able to have scalability and more controls."

Still, there's more work to be done in the near future, Xu said. Next steps include investigating photon statistics to establish the coherent properties of the laser's light.

Physicists from Forschungszentrum Jülich have developed a criterion with which scientists can seek suitable substrate materials for graphene in a targeted way. Interactions with the substrate material often lead to a loss of the amazing properties that characterize this special form of carbon. Together with partners at other institutions, the scientists were able to demonstrate that the influence exerted by the substrate on the electronic properties of graphene can be estimated by means of a simple structural parameter. The related publication was chosen as the Editor's Suggestion of the journal Physical Review Letters.

Harder than diamond, tougher than steel and many times more conductive than silicon—these and further extraordinary properties are the reason why graphene is intensively studied worldwide. The material is only one atomic layer thick. Its use, however, is so far mostly limited to laboratory experiments. One of the major tasks on the way to practical applications is the search for suitable substrate materials without which the extremely thin material is of little use.

“We simply wanted to find an accessible parameter which can be used to compare different substrates directly,” reports Dr. François Bocquet. “The decisive criterion turned out to be the atomic distance between the graphene layer and the underlying substrate,” explains the physicist and Helmholtz postdoc at Jülich's Peter Grünberg Institute (PGI-3).

Considering the van der Waals radius—a known value for the size of atoms in their free state—the strength of the interaction can be calculated directly from the distance. SuperComputer simulations performed by scientists from the Berlin Fritz Haber Institute of the Max Planck Society confirm this result.

Highly precise measurements with X-rays

At the Diamond synchrotron radiation source in Didcot, Oxfordshire, UK,François Bocquet and his colleagues used X-rays to measure the distance between graphene and its substrate at a precision down to the picometre range. One picometre corresponds to one thousandth of a nanometre, i.e. one billionth of a millimetre. Length differences much smaller than the atomic diameter can thus be determined.

The scientists used silicon carbide with hydrogen applied to its surface as a sample. Scientists from the Max Planck Institute for Solid State Research in Stuttgart only developed the specially prepared semiconductor material a few years ago for use as a substrate material for graphene. In contrast to the usual metallic substrates, a graphene layer deposited on this material is practically interaction-free and thus retains its extraordinary electrical properties.

“With the emergence of this new class of substrates, it was time for a new criterion with which even very weak interactions can be detected precisely,” explains the director at the Jülich Peter Grünberg Institute, Prof. Stefan Tautz, who heads the subinstitute Functional Nanostructures at Surfaces (PGI-3). “With the techniques available so far, for example photoelectron spectroscopy, the degree of interaction with the substrate could only be deduced indirectly. Bonds as weak as these could hardly be detected.”

This illustration shows the high performance photodetector which uses few layer black phosphorus (red atoms) to sense light in the waveguide (green material). Graphene (gray atoms) is also used to tune the performance.

Phosphorus, a highly reactive element commonly found in match heads, tracer bullets, and fertilizers, can be turned into a stable crystalline form known as black phosphorus. In a new study, researchers from the University of Minnesota used an ultrathin black phosphorus film, only 20 layers of atoms, to demonstrate high-speed data communication on nanoscale optical circuits.

The devices showed vast improvement in efficiency over comparable devices using the earlier "wonder material" graphene.

The work by University of Minnesota Department of Electrical and Computer Engineering Professors Mo Li and Steven Koester and graduate students Nathan Youngblood and Che Chen was published today in Nature Photonics--a leading journal in the field of optics and photonics.

As consumers demand electronic devices that are faster and smaller, electronics makers cram more processor cores on a single chip, but getting all those processors to communicate with each other has been a key challenge for researchers. The goal is to find materials that will allow high-speed, on-chip communication using light.

While the existence of black phosphorus has been known for more than a century, only in the past year has its potential as a semiconductor been realized. Due to its unique properties, black phosphorus can be used to detect light very effectively, making it desirable for optical applications. For the first time, the University of Minnesota team created intricate optical circuits in silicon and then laid thin flakes of black phosphorus over these structures using facilities at the University's Minnesota Nano Center.

"After the discovery of graphene, new two-dimensional materials continue to emerge with novel optoelectronic properties," said Professor Li, who led the research team. "Because these materials are two-dimensional, it makes perfect sense to place them on chips with flat optical integrated circuits to allow maximal interaction with light and optimally utilize their novel properties."

The University of Minnesota team demonstrated that the performance of the black phosphorus photodetectors even rivals that of comparable devices made of germanium--considered the gold standard in on-chip photodetection. Germanium, however, is difficult to grow on silicon optical circuits, while black phosphorus and other two-dimensional materials can be grown separately and transferred onto any material, making them much more versatile.

The team also showed that the devices could be used for real-world applications by sending high-speed optical data over fibers and recovering it using the black phosphorus photodetectors. The group demonstrated data speeds up to three billion bits per second, which is equivalent to downloading a typical HD movie in about 30 seconds.

"Even though we have already demonstrated high speed operation with our devices, we expect higher transfer rates through further optimization," said Nathan Youngblood, the lead author of the study. "Since we are the first to demonstrate a high speed photodetector using black phosphorus, more work still needs to be done to determine the theoretical limits for a fully optimized device."

Bridging the gap

While black phosphorus has much in common with graphene--another two-dimensional material--the materials have significant differences, the most important of which is the existence of an energy gap, often referred to as a "band gap."

Materials with a band gap, known as "semiconductors," are a special group of materials that only conduct electricity when the electrons in that material absorb enough energy for them to "jump" the band gap. This energy can be provided through heat, light, and other means.

While graphene has proven useful for a wide variety of applications, its main limitation is its lack of a band gap. This means that graphene always conducts a significant amount of electricity, and this "leakage" makes graphene devices inefficient. In essence, the device is "on" and leaking electricity all the time.

Black phosphorus, on the other hand, has a widely-tunable band gap that varies depending on how many layers are stacked together. This means that black phosphorus can be tuned to absorb light in the visible range but also in the infrared. This large degree of tunability makes black phosphorus a unique material that can be used for a wide range of applications--from chemical sensing to optical communication.

Additionally, black phosphorus is a so-called "direct-band" semiconductor, meaning it has the potential to efficiently convert electrical signals back into light. Combined with its high performance photodetection abilities, black phosphorus could also be used to generate light in an optical circuit, making it a one-stop solution for on-chip optical communication.

"It is really exciting to think of a single material that can be used to send and receive data optically and is not limited to a specific substrate or wavelength," Youngblood said. "This could have huge potential for high-speed communication between CPU cores which is a bottleneck in computing industry right now."

Fast growing potential

The past several years have seen a flurry of two-dimensional material discoveries, first with graphene, more recently with transition metal dichalcogenides (TMDs) such as molybdenum disulphide (MoS2), and now black phosphorus. All of the previous two-dimensional materials have serious trade offs, but black phosphorus provides the "best of both worlds" with a tunable band gap and high-speed capability.

"Black phosphorus is an extremely versatile material," said Professor Steven Koester, who contributed to the project. "It makes great transistors and photodetectors, and has the potential for light emission and other novel devices, making it an ideal platform for a new type of adaptable electronics technology." 

This illustration depicts a copper nanowire coated with graphene - an ultrathin layer of carbon - which lowers resistance and heating, suggesting potential applications in computer chips and flexible displays.

A new process for coating copper nanowires with graphene - an ultrathin layer of carbon – lowers resistance and heating, suggesting potential applications in computer chips and flexible displays.

"Highly conductive copper nanowires are essential for efficient data transfer and heat conduction in many applications like high-performance semiconductor chips and transparent displays," said doctoral student Ruchit Mehta, working with Zhihong Chen, an associate professor of electrical and computer engineering at Purdue University.

Now, researchers have developed a technique for encapsulating the wires with graphene and have shown that the hybrid wires are capable of 15 percent faster data transmission while lowering peak temperature by 27 percent compared with uncoated copper nanowires.

"This is compelling evidence for improved speed and thermal management by adapting the copper-graphene hybrid technology in future silicon chips and flexible electronic applications," he said.

Findings are detailed in a research paper published online in February in the journal Nano Letters. The paper was authored by Mehta, doctoral student Sunny Chugh and Chen.

Researchers and industry are trying to create smaller wires to increase the "packing density" of electronic components in chips. However, while smaller wires are needed to increase performance and capacity, scaling down the size of the wires reduces electrical and thermal conductivity, which can lead to overheating and damage. The graphene coating prevents the copper wires from oxidizing, preserving low resistance and reducing the amount of heating.

"If the surface is covered with oxide then a lot of the electrical and thermal conductive properties of copper are lost," Mehta said. "This is very important because you want as much current as possible going through these wires to increase speed, but they cannot take too much current because they will melt. But if the copper has good electrical and thermal conductivity you can push more current through it."

The hybrid wires are promising for transparent and flexible displays because they could be used in sparse numbers, maintaining transparency, and they are bendable.

"Copper wires usually aren't good for these displays because they eventually oxidize and stop working," Mehta said. "If you can prevent the oxidation, they potentially become a good fit."

Until now it has been difficult to coat copper nanowires with graphene because the process requires chemical vapor deposition at temperatures of about 1,000 degrees Celsius, which degrades copper thin films and small-dimension wires. The researchers have developed a new process that can be performed at about 650 degrees Celsius, preserving the small wires intact, using a procedure called plasma-enhanced chemical vapor deposition. Wires were tested in two width sizes: 180 nanometers - or more than 500 times thinner than a human hair - and 280 nanometers.

New approach to distributing computations could make multicore chips much faster.

Computer chips' clocks have stopped getting faster. To keep delivering performance improvements, chipmakers are instead giving chips more processing units, or cores, which can execute computations in parallel.

But the ways in which a chip carves up computations can make a big difference to performance. In a 2013 paper, Daniel Sanchez, the TIBCO Founders Assistant Professor in MIT's Department of Electrical Engineering and Computer Science, and his student, Nathan Beckmann, described a system that cleverly distributes data around multicore chips' memory banks, improving execution times by 18 percent on average while actually increasing energy efficiency.

This month, at the Institute of Electrical and Electronics Engineers' International Symposium on High-Performance Computer Architecture, members of Sanchez's group have been nominated for a best-paper award for an extension of the system that controls the distribution of not only data but computations as well. In simulations involving a 64-core chip, the system increased computational speeds by 46 percent while reducing power consumption by 36 percent.

"Now that the way to improve performance is to add more cores and move to larger-scale parallel systems, we've really seen that the key bottleneck is communication and memory accesses," Sanchez says. "A large part of what we did in the previous project was to place data close to computation. But what we've seen is that how you place that computation has a significant effect on how well you can place data nearby."

Disentanglement

The problem of jointly allocating computations and data is very similar to one of the canonical problems in chip design, known as "place and route." The place-and-route problem begins with the specification of a set of logic circuits, and the goal is to arrange them on the chip so as to minimize the distances between circuit elements that work in concert.

This problem is what's known as NP-hard, meaning that as far as anyone knows, for even moderately sized chips, all the computers in the world couldn't find the optimal solution in the lifetime of the universe. But chipmakers have developed a number of algorithms that, while not absolutely optimal, seem to work well in practice.

Adapted to the problem of allocating computations and data in a 64-core chip, these algorithms will arrive at a solution in the space of several hours. Sanchez, Beckmann, and Po-An Tsai, another student in Sanchez's group, developed their own algorithm, which finds a solution that is more than 99 percent as efficient as that produced by standard place-and-route algorithms. But it does so in milliseconds.

"What we do is we first place the data roughly," Sanchez says. "You spread the data around in such a way that you don't have a lot of [memory] banks overcommitted or all the data in a region of the chip. Then you figure out how to place the [computational] threads so that they're close to the data, and then you refine the placement of the data given the placement of the threads. By doing that three-step solution, you disentangle the problem."

In principle, Beckmann adds, that process could be repeated, with computations again reallocated to accommodate data placement and vice versa. "But we achieved 1 percent, so we stopped," he says. "That's what it came down to, really."

Keeping tabs

The MIT researchers' system monitors the chip's behavior and reallocates data and threads every 25 milliseconds. That sounds fast, but it's enough time for a computer chip to perform 50 million operations.

During that span, the monitor randomly samples the requests that different cores are sending to memory, and it stores the requested memory locations, in an abbreviated form, in its own memory circuit.

Every core on a chip has its own cache -- a local, high-speed memory bank where it stores frequently used data. On the basis of its samples, the monitor estimates how much cache space each core will require, and it tracks which cores are accessing which data.

The monitor does take up about 1 percent of the chip's area, which could otherwise be allocated to additional computational circuits. But Sanchez believes that chipmakers would consider that a small price to pay for significant performance improvements.

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