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Flexible Optical Metasurfaces Promise "Smart" Contact Lenses

The name of the game in optical metasurfaces is shortening the wavelengths of light. This yields devices that can manipulate light for information processing and also reduce the bulk of the devices based on traditional optics.

Metasurfaces have been pretty good at offering small, flat features, but the integrated metallic resonators they use to filter light according to specified frequencies have lacked efficiency. It was thought that dielectric resonators were an attractive alternative, but they present another problem: though fairly efficient, their frequency-filtering is hard to fine-tune.

Now researchers at RMIT University and the University of Adelaide have delivered the best of both worlds with a dielectric resonator that can be mechanically tuned. The property that makes these mechanically tunable dielectric resonators especially attractive is that they are embedded in a biocompatible polymer that renders them flexible. 

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Graphene's Role as a Superconductor Just Got Better

Graphene is an amazing conductor. The transport of electrons through graphene nanoribbons has even surpassed what scientists thought were the theoretical limits for the material—so much so that electrons moving through it seem to behave almost like photons.

Graphene’s amazing properties as a conductor has inspired some researchers to explore whether the single-atom-thick sheets of carbon could also be made into superconductors. Last year, an international research team from Canada and Germany was able to demonstrate that graphene can be made to behave that way when it’s doped with lithium atoms.

Now researchers in Japan (from Tohoku University and the University of Tokyo) have developed a new method for coaxing graphene to behave as a superconductor that has some important and distinctive differences from the previous research by the Canadian and German researchers.

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Cheap Plasmonic Interferometer Could Enable Prickless Glucose Monitor

Since 2012, IEEE Spectrum has been covering Domenico Pacifici at Brown University as he works to improve the capabilities of plasmonic interferometers. One major application would be glucose monitors that enable diabetics to check glucose levels through saliva instead of blood—no finger pricking necessary.

In his latest research, Pacifici and his team have developed a way to get a plasmonic interferometer to take measurements without the need for a coherent light source.

To have a coherent light, the light waves have to run in parallel, possess the same wavelength, and travel in-phase, which means the peaks and valleys of light waves are in alignment. Producing this kind of light demands expensive and bulky equipment. By eliminating that need, Pacifici’s team has created a far smaller and less expensive way to operate these devices.

“It has always been assumed that coherent light was necessary for plasmonic interferometry,” said Pacifici, in the press release. “But we were able to disprove that assumption.”

In research published in the Nature journal Scientific Reports, Pacifici and his team embedded light emitters in the form of fluorescent atoms directly into the sub-wavelength cavities of the plasmonic interferometers. The result was that even when the source light had very low coherence, the internal emitters could get the interferometer to operate as though the light was coming from a coherent light source.

Plasmonic interferometers operate on the same principles of plasmonics as every other plasmonic device does. When photons of light hit a metal surface, they rattle the electrons in the metal so much that they generate waves of electrons known as surface plasmons.

A plasmonic interferometer exploits this phenomenon by its very architecture, essentially a piece of metal that has a hole—or cavity—at its center and around that hole is carved co-centric grooves. The cavity is around 300 nanometers in diameter and the co-centric grooves are measured in microns.

When the light hits the surface of this device, some of the photons go into the cavity at the center of it while others hit the outer grooves and scatter. The scattered photons excite the electrons on the metal surface to the point where they become waves of surface plasmons. Just like waves on water, the waves move along the surface of the metal until they go into cavity at the center. Here they interfere with the photons that were originally drawn into the cavity generating an interference pattern: you can measure how the light weakens and strengthens coming out of the cavity.

By embedding fluorescent atoms in the cavity of the device, Pacifici’s team made the cavity produce its own surface plasmons. In this way, surface plasmons move out of the cavity onto the surface and then bounce off the co-centric grooves and back into the cavity. When these surface plasmons come in contact with the fluorescent atoms that were its source, it creates an interference with the directly transmitted photon. In this arrangement, the photons in the cavity and the plasmons are coming from the same emitter, so they are naturally coherent and interference occurs even though the emitters (the fluorescent atoms) are excited by incoherent light.

“The important thing here is that this is a self-interference process,” Pacifici said in the release. “It doesn’t matter that you’re using incoherent light to excite the emitters, you still get a coherent process.”

In addition to being able to use incoherent light sources, the architecture provides additional benefits, such as greater accuracy and the internal emitters mean that more delicate samples can be tested.

While this work is really just a test of concept, Pacifici believes that this is such a fundamentally different way for these devices to operate that it represents a significant breakthrough in the field.

A chip with 2-D dimensional tin oxide (SnO)

Tin Oxide: The First Stable p-type 2-D Semiconductor Material

Researchers at the University of Utah have developed the first stable intrinsic p-type (carrying positive charges) 2-D semiconductor material, tin oxide. If this semiconductor is mated with a n-type (carrying electrons) 2-D semiconductor in a transistor, it opens up the possibility of building power-saving two-dimensional complementary logic circuits like the ones in microprocessors today.

"Now we have everything—we have p-type 2-D semiconductors and n-type 2-D semiconductors," said Ashutosh Tiwari, an associate professor at the University of Utah and the leader of the research, in a press release. "Now things will move forward much more quickly."

The potential for 2-D materials—such as graphene and molybdenum disulfide—as an alternative to the three-dimensional silicon, raises hopes for smaller, faster, lower-power transistors.

However, the path to success for these 2-D materials in transistors has not always been clear, whether it be issues such as graphene not being a natural semiconductor or the charge carrier traps that compromise molybdenum disulfide. But possibly the biggest issue has been that all previous 2-D materials have only been stable n-type semiconductors. It has been possible to dope other 2-D materials, such as molybdenum disulfide and tungsten diselenide, to behave as p-type. However, this new tin oxide represents the first intrinsic p-type semiconductor in a 2D material.

In research described in the journal Advanced Electronic Materials, the Utah University researchers overcame this limitation by layering 2-D tin oxide (SnO) onto a sapphire-and-silicon-dioxide (SiO2) substrate. The researchers were then able to fabricate a few field-effect transistors (FETs) from it.

With this development, the attractive properties of 2-D materials in transistors are more fully exploitable. For instance, in 2-D materials charge transport is basically confined to a single plane, meaning electrical and thermal transport properties that can be much better than those of bulk silicon. That could mean chips that consume less power and throw-off less heat.

In an e-mail interview, Tiwari said that the next step in their research will be to build a CMOS (complementary metal oxide semiconductor) using their new p-type 2-D semiconductor.

This post was corrected on 18 February to indicate that some 2-D materials could act as p-type semiconductors when doped.

A silicon nanowire transistor used as a qubit

Quantum Computing With Ordinary CMOS Transistors

Future quantum computers might not be all that different from the one you’re using now. An international team of researchers have created a the most fundamental part of a quantum computer—the quantum bit, or qubit—using only a CMOS transistor that is not much different from those in today’s microprocessors.

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Electronic Qubit Integrated Into Solid-State Switch

Essential for so-called quantum networks and quantum computers is something called a quantum bit, or qubit, which would replace the traditional bits that are stored or transmitted in today’s computers and optical networks. It consists of an ion with an unpaired electron that has two spin states—up or down, or a 0 and 1 in a binary system. But under some conditions, these qubits can be made to have both the 0 and 1 state simultaneously—a quantum state.

One of the problems researchers have faced in incorporating qubits in optical networks is getting the photons to strongly interact with the qubits in a solid-state device. The aim has been to develop something akin to an electro-optic modulator that uses electronic signals to modulate properties of light in today’s optical networks.

Researchers at the University of Maryland have developed a novel design that may have achieved that aim by “combining the light-trapping of photonic crystals with the electron-trapping of quantum dots.

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Introducing One of the Best Thin-film Transistors Ever

Wide-band gap metal-oxide thin-film transistors (TFTs) have never been quite as popular as the ubiquitous metal-oxide semiconductor field-effect transistors (MOSFET). One of the main issues with TFTs has been that they are limited to n-type semiconductor materials that can only move negative charges through them, limiting their electrical output.

While different architectures have been investigated to overcome this, the problem has remained that there just haven’t been p-type wide-band inorganic semiconductor materials that have done the trick. The result has been that TFTs have been limited to low-power applications, such as display screens.

Now researchers at the University of Alberta in Canada say they have come up with a design that will take nearly any wide-band metal-oxide n-type semiconductor used in thin-film transistors and create a p-type channel, or inversion layer, through which positive charges can travel without the need of some new semiconductor material. As proof of their design, the Canadian researchers have created a TFT capable of conducting both electrons (negative charges) and holes (positive charges) resulting in far greater electrical output.

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Nanotube-Based Tunneling Field Effect Transistor Offers Semiconductor-Free Switching

Researchers at Michigan Technological University (MTU) have developed a method for producing a tunneling field effect transistor (TFET) that overcomes a key obstacle to their adoption: the need to be operated at cryogenic temperatures. Eliminating semiconductor materials in the design of their nanotube-based TFET has allowed the MTU researchers to fabricate a device that can operate at room temperatures and offers the added feature of flexibility.

In research explained in an article published in the Nature journal Scientific Reports, the MTU researchers, led by Yoke Khin Yap, used iron quantum dots in combination with functionalized boron nitride nanotubes (BNNTs) to create flexible tunneling channels in TFET devices.

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Nanomembrane May Bring Rechargeable Lithium-Metal Batteries Back

Researchers at Cornell University, in Ithaca, N.Y., may have found a way to bring the long-moribund prospects of rechargeable lithium-metal batteries back from the dead with a novel nanostructured membrane that could make the batteries both safe and efficient.

In research described in the journal Nature Communications, the Cornell researchers looked anew at the problem that has been plaguing the prospects of rechargeable lithium-metal batteries: growths, referred to as dendrites, that over time branch out of the anode, into the electrolyte, and eventually expand to the point where they actually bridge the gap between the two electrodes and cause the battery to short out—or even worse.

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Cheap Cubic-Boron Nitride Could Enable Next Gen Smart Grid

Researchers at North Carolina State University (NCSU), led by Jay Narayan, have developed a new method for converting hexagonal-boron nitride (h-BN) directly into cubic-boron nitride (c-BN) that is faster and less expensive than previous processes and promises to make a material that is more viable for high-power electronics, transistors and solid-state devices.

Boron nitride (BN) comes in four basic forms. Two of these forms of boron nitride, namely hexagonal-boron nitride (h-BN) and cubic-boron nitride (c-BN), represent the two most attractive forms of BN for electronic applications because their structures and properties are quite similar to both graphite and diamonds. Like diamonds, c-BN has very good thermal properties for integrated circuits as well as high-frequency power capabilities that compare favorably with silicon.

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IEEE Spectrum’s nanotechnology blog, featuring news and analysis about the development, applications, and future of science and technology at the nanoscale.

Dexter Johnson
Madrid, Spain
Rachel Courtland
New York City
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