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In this false color image from a scanning electron microscope, single photons travel through a pink waveguide atop a blue surface made of silicon dioxide.

A Mix of Nanomaterials Leads to a New Quantum Photonic Circuit Architecture

Scientists at the National Institute of Standards and Technology (NIST) have developed an underlying architecture for a new class of quantum photonic circuits—chip-based devices that leverage the quantum properties of light to process and communicate information.

In research described in the journal Nature Communications, the NIST researchers and their collaborators in China and the UK developed a class of devices that are composed of networks of low-loss waveguides and single-photon sources, all on a single chip. The resulting quantum circuit architecture could have an impact on photonic quantum computing and simulation, as well as metrology and communications.

Operating the device consists of producing streams of single photons, launching them into a network of waveguides and beamsplitters in which they are allowed to interfere with each other, and then detecting them at the network’s outputs.

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Illustration of a lightbulb with a single yellow arrow pointing to the bulb from the left, and green, orange, blue and red arrows exiting it from the right.

Nanodevice Maps the Angle of Light to Color and Vice Versa

An international team of researchers has added a small twist to typical light filters. They’ve fabricated a nanoscale device that divides incident light into its different component colors based on the direction the light is coming from. Their device that could drastically alter how color displays are designed and even improve the efficiency of solar cells.

To accomplish this trick, the researchers have moved beyond the periodic architectures of a generation of tunable optical devices. In other words, they’ve switched from making filters with equally spaced grooves to an aperiodic architecture wherein the grooves are not equally spaced.

Researchers at the U.S. National Institute of Standards and Technology (NIST), in collaboration with Syracuse University and Nanjing University in China, have discovered that when incident light strikes these aperiodic grooves (nanoscale trenches on an otherwise opaque metal film) it triggers the creation of surface waves, referred to as plasmons.

In research described in the journal Nature Communications, the scientists were able to engineer the structure so that plasmons generated by individual grooves meet at the device’s central through slit with the same phase (constructive interference), but at multiple user-defined angles and wavelengths. The result: an angle-dependent color filtering response.

The angular response of an equivalent device with uniformly spaced grooves is limited by the grooves’ uniformity. In diffraction gratings, for example, the groove dimensions are restricted by the diffraction limit. But because the wavelength of plasmons is much smaller than the incident light, the researchers are able to sidestep the diffraction limit and shrink the groove dimensions so that they are much smaller than the wavelength of incident light. 

“Plasmons, because of their shorter wavelengths, help us shrink the device footprint to the micron scale while at the same time allowing us to engineer light-matter interaction at the nanoscale,” explained NIST physicist and co-author of the paper Amit Agrawal.  “By incorporating engineered aperiodicity into the device, we are able to get multi-functional response: angle-to-color, or color-to-angle mapping.”

Currently, displays and consumer cameras typically use what is referred to as Bayer color filters. One super-pixel in a display is in fact a combination of three sub-pixels, one for each RGB color. Any combination thereof will provide the desired color for that super-pixel. However, this comes at a loss of resolution by a factor of three.

Because the footprint of the device developed by Agrawal and his colleagues (Matthew Davis and Wenqi Zhu of NIST and the University of Maryland, along with NIST physicist Henri Lezecmatches) matches the size of individual pixels (10 micrometers or so), Agrawal envisions placing these color filtering devices on top of individual CCD pixels, and performing color filtering by changing the angle of incidence using a micromirror-array based device.

Another alternative Agrawal suggests is using the device in reverse. In this way, one can direct the RGB colors coming at different angles to three pixels forming a super-pixel (same as an LCD).

While this all sounds very appealing, Agrawal is quick to caution that the efficiency of the devices at this point is still pretty low because of scattering and absorption losses in the metal film. So, although there is some work to be done to improve it, Agrawal believes that the incorporation of such devices on top of an array of CCD pixels or on top of multi-junction solar cells, for example, should be pretty straightforward.

“There is already a precedent for it,” said Agrawal  “Since these devices are angle sensitive (in the case of forward illumination), there will be some engineering required to illuminate them at various angles with micromirror arrays. However, at this point it is a matter of making an effort to increase the efficiency.”

The key to this efficiency-raising effort will be working with higher quality metal films than can be created using traditional physical vapor deposition techniques. It will also call for further optimization of the design process, according to Agrawal.

While the researchers grapple with those issues, Agrawal notes that the approach can also be easily extended to other regions of the electromagnetic spectrum, such as mid-infrared and the terahertz, where there are more choices for metal, and the losses are significantly smaller.

Agrawal adds: “In terms of efficiency, these devices should work remarkably well at those wavelengths and are especially suitable for applications such as sensing. Some biological and chemical molecules have spectral signatures at those wavelengths, so it makes logical sense for us to extend the operation of these devices to longer wavelengths.”

A photo of a flexible terahertz detector which looks like a transparent square with a pattern of boxes and grids etched onto it in gold.

Graphene Takes Us One Step Closer to Flexible Terahertz Electronics

About five years ago, graphene-based photodetectors moved beyond detecting the visible and near-infrared range of the electromagnetic spectrum to push into the terahertz range. This is a big deal because terahertz radiation penetrates materials that block visible and mid-infrared light. Detecting it opened up a range of potential applications in medical diagnostics, process control, and even intelligent vehicles.

Also about five years ago, we saw research that combined graphene and quantum dots to create flexible photodetectors. While these photodetectors are great for detecting waves along much of the spectrum, they couldn’t capture terahertz radiation.

Now researchers at Chalmers University in Sweden have combined flexibility and terahertz detection into one flexible, graphene-based detector that could lead to new products such as wearable terahertz sensors for medical diagnosis.

In research published in the journal Applied Physics Letters, the Chalmers researchers developed a field-effect transistor (FET) built up on a plastic substrate in which the channel is made from graphene. The resulting flexible device can detect signals in the range of 330 to 500 GHz.

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An artificially coloured view of a micro-cityscape created out of Si.

New Laser Technique Promises Photonic Devices Inside of Silicon

When a laser beam modifies a material, those modifications can either be temporary or permanent, and the change can also be either extremely subtle or drastic. In any case, once such a modification has taken place, the modified material starts responding differently to the laser beam.

This interaction between the laser beam and the modified materials is typically overlooked or even actively prevented because it’s viewed as an undesired artifact. However, researchers at Bilkent University and Middle East Technical University (both in Ankara, Turkey) have taken advantage of these interactions to create structures within silicon that enable photonic devices

In research described in the journal Nature Photonics, the Turkey-based researchers have developed a 3D laser fabrication technique that deliberately creates the conditions for exploiting these interactions, known as nonlinear feedback mechanisms. The researchers have dubbed the resulting structures inside of the silicon “in-chip” devices that they believe can serve in a host of applications. Among the predicted uses: silicon-based photonics components for near- and mid-IR photonics.

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PFM phase image of a 400 nm thick BiFeO3 (BFO) ferroelectric film, which looks like a light orange square inside of a red rectangle.

New AFM Technique Reveals Piezoelectricity in Ferroelectric Materials

A team of researchers at the University Autonoma of Barcelona has created a new atomic force microscopy (AFM) technique that exploits the direct piezoelectric effect to take a measurement of the piezoelectric effect in ferroelectric materials.

The technique, dubbed direct piezoelectric force microscopy, should enable a better understanding of piezoelectric and ferroelectric materials that form the basis of a number of today’s technologies, such as ultrasound generators for echography scanners, or, in the future, CMOS replacement switches.

The piezoelectric effect, in which compressing or stretching a material produces a voltage, or where a voltage can cause a material to expand or contract, has been well characterized since the Curie brothers first measured it in 1880. However, ferroelectric materials—which can have their electric field polarization changed via an electric field—have only recently become better understood, in large part thanks to advances in AFM, in particular piezoresponse force microscopy (PFM).

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Artist's illustration of complementary DNA molecules binding to PNA molecules that are noncovalently anchored on the graphene surface.

New Graphene Sensor is Less Noisy, More Sensitive

Graphene’s ability to detect a variety of chemical and biological molecules would seem to make it a perfect match for sensors. But because graphene is a conductor and lacks an inherent band gap, it’s hard to fashion the material into a transistor that can be turned on and off.

In order to make a sensor out of graphene, you need to use multiple layers of the material, which leads to high levels of electronic noise and reduces its effectiveness.

Now, an international team of researchers has proposed a graphene-based semiconductor device that reduces electronic noise when its electric charge is neutral (referred to as its neutrality point). The group achieved this neutrality point without the need for bulky magnetic equipment that had previously prevented these approaches from being used in portable sensor applications.

In a proof-of-concept device, the researchers used their new sensing scheme to detect HIV-related DNA hybridization at picomolar concentrations.

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SEM image of the repeater array

Carbon Nanotubes Make Big Push in Plasmonic Circuits

The field of plasmonics—which exploits the waves of electrons generated when photons strike a metal structure in order to carry out optoelectronic processes—has been building momentum in the research community over the past half decade. This interest is well placed. Plasmonics has made all sorts of interesting things possible, such as confining wavelengths of light to design smaller photonic devices. During that time, a range of two-dimensional materials, including black phosphorus and graphene, has enabled this growing interest. But the granddaddy of nanomaterials—the single-walled carbon nanotube—may still have a role to play in this exploding field.

Researchers at Peking University in China have gone back to single-walled carbon nanotubes (SWNTs) and used them as the active channel materials in the construction of surface plasmon polariton (SPP)-based plasmonic interconnect circuits. (Just as a bit of a primer, the waves of electrons that are generated when photons hit a metal structure are called either surface plasmons when referring to the oscillations in charge alone, or surface plasmon polaritons when referring to both the charge oscillations and the electromagnetic wave.)

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Microscopic image of the device

Integrating Silicon With Two-Dimensional Material Could Shake Up Photonics

Researchers at MIT have developed a fabrication method for integrating silicon photonics with layered two-dimensional material molybdenum ditelluride (MoTe2) to create a single device that acts as both a light-emitting diode and a photodetector.

This work could have a dramatic impact on the field of silicon photonics, which has become a leading architecture in chip-integrated optical interconnects. This popularity stems, in part, from the promise that many components, such as waveguides, couplers, interferometers and modulators, could someday be directly integrated on silicon-based processors.

This latest MIT research could smooth the path to this level of integration because it represents the first time that an electrically powered light source enabled by a 2D material has been integrated on a passive silicon photonic crystal waveguide, according to the researchers.

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Three researchers in white lab coats look at a tiny photodetector that one holds up under an array of colored lights.

Quantum Mechanical Process Could Double Efficiency of Photodetectors

Researchers at the University of California (UC) Riverside have discovered that the combination of two inorganic two-dimensional (2D) materials produces a quantum mechanical process that could significantly increase the efficiency of photodetectors, leading to revolutionary new ways of collecting solar energy.

In research described in the journal Nature Nanotechnology, the UC Riverside researchers have used the transition metal dichalcogenides  tungsten diselenide and molybdenum diselenide to achieve the effect known as electron-hole multiplication. Electron multiplication involves making multiple electron-hole pairs for each incoming photon. This can dramatically increase the efficiency of a photovoltaic cell in converting light into electricity.

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A close-up image of a tiny memristor that resembles a gray storm cloud with its funnel shape.

Memristor-Driven Analog Compute Engine Would Use Chaos to Compute Efficiently

When you’re really harried, you probably feel like your head is brimful of chaos. You’re pretty close. Neuroscientists say your brain operates in a regime termed the “edge of chaos,” and it’s actually a good thing. It’s a state that allows for fast, efficient analog computation of the kind that can solve problems that grow vastly more difficult as they become bigger in size.

The trouble is, if you’re trying to replicate that kind of chaotic computation with electronics, you need an element that both acts chaotically—how and when you want it to—and could scale up to form a big system.

“No one had been able to show chaotic dynamics in a single scalable electronic device,” says Suhas Kumar, a researcher at Hewlett Packard Labs, in Palo Alto, Calif. Until now, that is.

He, John Paul Strachan, and R. Stanley Williams recently reported in the journal Nature that a particular configuration of a certain type of memristor contains that seed of controlled chaos. What’s more, when they simulated wiring these up into a type of circuit called a Hopfield neural network, the circuit was capable of solving a ridiculously difficult problem—1,000 instances of the traveling salesman problem—at a rate of 10 trillion operations per second per watt.

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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
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