A new way to make laser-like beams using 1,000x less power

With precarious particles called polaritons that straddle the worlds of light and matter, University of Michigan researchers have demonstrated a new, practical and potentially more efficient way to make a coherent laser-like beam.

They have made what's believed to be the first polariton laser that is fueled by electrical current as opposed to light, and also works at room temperature, rather than way below zero.

Those attributes make the device the most real-world ready of the handful of polariton lasers ever developed. It represents a milestone like none the field has seen since the invention of the most common type of laser – the semiconductor diode – in the early 1960s, the researchers say. While the first lasers were made in the 1950s, it wasn't until the semiconductor version, fueled by electricity rather than light, that the technology took off.

This work could advance efforts to put lasers on computer circuits to replace wire connections, leading to smaller and more powerful electronics. It may also have applications in medical devices and treatments and more.

The researchers didn't develop it with a specific use in mind. They point out that when conventional lasers were introduced, no one envisioned how ubiquitous they would become. Today they're used in the fiber-optic communication that makes the Internet and cable television possible. They are also in DVD players, eye surgery tools, robotics sensors and defense technologies, for example.

A polariton is part light and part matter. Polariton lasers harness these particles to emit light. They are predicted to be more energy efficient than traditional lasers. The new prototype requires 1,000 times less electricity to operate than its conventional counterpart made of the same material.

"This is big," said Pallab Bhattacharya, the Charles M. Vest Distinguished University Professor of Electrical Engineering and Computer Science and the James R. Mellor Professor of Engineering at U-M. "For the past 50 years, we have relied on lasers to make coherent light and now we have something else based on a totally new principle."

Bhattacharya's system isn't technically a laser. The term was initially an acronym for Light Amplification by Stimulated Emission of Radiation. Polariton lasers don't stimulate radiation emission. They stimulate scattering of polaritons.

In a typical laser, light—or more often electrical current— is pumped into a material called a gain medium that's designed to amplify the signal. Before the pumping begins, most of the electrons in the gain medium are in their least energetic state, also known as the ground state. Once the light or current hits them, the electrons absorb that energy and move to a higher-energy state. At some point, more electrons are high-energy than are low-energy and the device is said to have achieved a "population inversion." Now any light or current that goes in has the opposite effect on the excited electrons. It kicks them down to the ground state and releases pent-up light in the process.

Polariton lasers don't rely on these population inversions, so they don't need a lot of start-up energy to excite electrons and then knock them back down. "The threshold current can be very small, which is an extremely attractive feature," Bhattacharya said.

He and his team paired the right material – the hard, transparent semiconductor gallium nitride – with a unique design to maintain the controlled circumstances that encourage polaritons to form and then emit light.

How it works

A polariton is a combination of a photon or light particle and an exciton – an electron-hole pair. The electron is negatively charged and the hole is technically the absence of an electron, but it behaves as if it were positively charged. Excitons will only fuse with light particles under just the right conditions. Too much light or electrical current will cause the excitons to break down too early. But with just enough, polaritons will form and then bounce around the system until they come to rest at their lowest energy level in what Bhattacharya describes as a coherent pool. There, the polaritons decay and in the process, release a beam of single-colored light.

The beam they demonstrated was ultraviolet and very low power – less than a millionth of a watt. For context, the laser in a CD player is about one-thousandth of a watt.

"We're thrilled," said Thomas Frost, a doctoral student in electrical and computer engineering. "This is the first really practical polariton laser that could be used on chip for real applications."

The design the team used helped them achieve the beam with an electrical rather than light input signal. Getting the electrical current into the system requires electrodes sandwiching the gallium nitride and several layers of mirrors to render the electrical signal useable. Other groups' approaches put the electrodes outside the mirrors. Bhattacharya said it was tough to get the signal strong enough under those circumstances. So he deconstructed the sandwich. He put the mirrors on the sides of the gallium nitride and left the electrodes on the top and bottom.

Read more at: http://phys.org/news/2014-06-laser-like-1000x-power.html#jCp

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Scientists find stronger 3-D material that behaves like graphene

Scientists at Oxford, SLAC, Stanford and Berkeley Lab have discovered
 that a sturdy 3-D material, cadmium arsenide, mimics the 
electronic behavior of 2-D graphene. This illustration depicts fast-moving,
massless electrons inside the material. The discovery could lead to new 
and faster types of electronic devices. Credit: Greg Stewart/SLAC


Scientists have discovered a material that has the same extraordinary electronic properties as 2-D graphene, but in a sturdy 3-D form that should be much easier to shape into electronic devices such as very fast transistors, sensors and transparent electrodes.

The material, cadmium arsenide, is being explored independently by three groups, one of which includes researchers at the University of Oxford, SLAC, Stanford and Lawrence Berkeley National Laboratory who described their results in a paper published May 25 in Nature Materials.

"Now more and more people realize the potential in the science and technology of this particular material. This growing interest will promote rapid progress in the field – including the exploration of its use in functional devices and the search for similar materials," said Yulin Chen of the University of Oxford, who led the research.

The group's work builds on its earlier studies of a sodium bismuth compound that also mimics graphene but turns to powder when exposed to air. Both compounds had been predicted by co-authors Zhong Fang and Xi Dai, theoretical physicists from the Chinese Academy of Sciences, who suggested that cadmium arsenide, which is used in detectors and sensors, would provide the same properties in a much more stable form.

Their prediction proved correct, said Zhongkai Liu, the paper's first author and a graduate student at SIMES, the Stanford Institute for Materials and Energy Sciences at SLAC. "The environmental stability of cadmium arsenide allows us to explore it very systematically, and makes it easier to study," he said.

Graphene is a one-atom-thick sheet of carbon atoms peeled from a piece of graphite, which is familiar as the lead in pencils. One of its hallmarks is the weird behavior of its electrons: When confined to this thin layer of regularly spaced atoms, these lightweight particles act as if they have no mass at all. This allows them to zip through the material much faster than usual. The scientists who first isolated graphene in 2004 were awarded the Nobel Prize in Physics; and researchers have been racing to explore its properties and find practical uses for it ever since.

One such quest has been to find graphene-like materials that are three-dimensional, and thus much easier to craft into practical devices. Two other international collaborations based at Princeton University and in Dresden, Germany, have also been pursuing cadmium arsenide as a possibility. One published a paper on its results in the May 7 issue of Nature Communications, and the other has posted an unpublished paper on the preprint server arXiv.

Chen's group made samples of cadmium arsenide at Oxford and tested them at the Diamond Light Source in the United Kingdom and at Berkeley Lab's Advanced Light Source.

"We think this family of materials can be a good candidate for everyday use," Chen said, "and we're working with theorists to see if there are even better materials out there. In addition, we can use them as a platform to create and explore even more exotic states of matter; when you open a door, you find there are many other doors behind it."

The research team included Zhi-Xun Shen, a professor at SLAC and Stanford and SLAC's advisor for science and technology; Zahid Hussain, senior staff scientist at Berkeley Lab; and other researchers from SIMES, Berkeley Lab, Oxford University, Fudan University in Shanghai, the Chinese Academy of Sciences and Diamond Light Source. The work was partially funded by the U.S. Department of Energy Office of Science and the Defense Advanced Research Projects Agency (DARPA) Mesodynamic Architectures program.

http://phys.org/

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Structural supercapacitors take a load on

Stress and squeeze: the new structural supercapacitor design

A solid-state supercapacitor that works under great stresses and vibrations has been developed by researchers from the US. Unlike traditional supercapacitors, the new design does not delaminate under stress and could lead to a variety of practical applications, from more-efficient devices to renewable-energy storage.

Unlike batteries – which work through chemical reactions – supercapacitors store energy in the form of electrically charged ions, which are assembled on the surfaces of porous electrodes. Supercapacitors also have numerous benefits: they can charge and discharge in minutes – unlike batteries, which take hours – and have a much longer lifespan, lasting for millions of cycles rather than thousands. Their drawback, however, comes in their reduced storage capacity – to hold a given charge, most supercapacitors need to be much larger and heavier than an equivalent lithium-ion battery.

Heavy-weight storage

One idea to overcome this limitation lies in creating supercapacitors that act as both energy storage and structural support. By doubling up the otherwise "dead weight" of structural materials, the rapid-charging, long-lasting nature of supercapacitors could be utilized without needing an internally distinct power source. This concept of a "structural supercapacitor" could have many potential applications – for example, a laptop where the case acts as a battery, renewable energy stored within the walls of a house, or even a rapidly charging electric car that stores power in its own chassis. To be used as such a structural device, the supercapacitor would need to work under considerable stresses and vibrations. Traditional supercapacitors are ill-suited to this task – being layered, their electrodes and electrolytes are prone to separate when such forces are applied.

Strong bonds

Load bearing

To overcome this problem, a team of researchers, led by Cary Pintfrom Vanderbilt University in Tennessee, has engineered a supercapacitor with better-integrated layers. The design features electrodes made from silicon wafers, the inner sides of which are electrochemically etched to create a surface covered in nano-sized pores. These are then coated with a protective, ultra-thin layer of carbon, before being vacuum-squeezed together around an ion-conducting polymer. This electrolyte seeps into the silicon's nanopores, setting into a strong mechanical bond, which does not come apart easily.

When tested, the researchers found that the design offers an energy density of up to 10 W h/kg and operates perfectly, even under stresses of 44 psi and vibrational accelerations over 80 g – the latter of which is greater than the forces acting within working jet engines. While designs for structural supercapacitors have been reported previously (mostly based around the use of carbon fibres) the team's design is capable of 3 to 4 orders of magnitude more charge storage – comparable, in fact, with (regular) commercial supercapacitors. Pint points out that the design showed that its "performance is not compromised while we operate the device under compression, shearing, tensile stretching, high-amplitude vibrations, and impact forces". He adds that the manufacturing process is simpler than in traditional supercapacitors and involves minimal cost, with both the component materials and the production process being relatively cheap. Furthermore, the constituent parts are both bio-friendly and non-flammable, thereby removing some of the safety concerns traditionally associated with lithium-ion batteries.

"The study is also a nice contribution to the larger area of research on mechanically robust electrodes for electrochemical energy-storage systems," says Vadym Mochalin, a nanomaterial expert at Drexel University in Philadelphia, who was not involved in the research. "[This] will likely inspire similar designs for lithium-ion batteries, micro-supercapacitors on chip, sensors, and other devices."

The research is described in Nano Letters.

About the author

Ian Randall is a science writer based in New Zealand

http://physicsworld.com/

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105-bit optical memory built on a chip

Two fully functional optical memories on single chips have been fabricated by researchers in Japan. The devices use bistable optical cavities to store the bits, and allow multiple bits to be controlled simultaneously by the same waveguide. The researchers hope that, in future, such a memory could be used for optical logic operations to increase the speed of computation.

Today, optical fibres are the material of choice for transmitting data, thanks to their lower signal attenuation compared with copper wires and their much higher bandwidth. Currently, however, optical signals have to be converted into electronic ones for processing, and then once more, to convert the output back to an optical signal. Such conversions consume energy and time, and fail to utilize the biggest advantage of optical transmission – that photons do not interfere with each other, meaning that several signals with different frequencies can travel down one fibre simultaneously in a process known as "multiplexing". Photonic signals have to be "demultiplexed" before an electronic processor can deal with them, and so optical processors are of interest to many researchers.

Random memories

A key element in any processing unit is the random access memory (RAM), in which data are stored temporarily while the computer runs a program. A modern electronic RAM usually stores each bit of memory as the charge on a capacitor, and various optical equivalents have been proposed. In 2012 Masaya Notomi and colleagues at NTT Laboratories in Kanagawa, Japan, designed a four-bit RAM made from a photonic crystal – a periodic optical nanostructure comprising a network of holes that allows some wavelengths of light to propagate while blocking others. Inside the photonic crystal were four identical cavities that had two possible refractive indices – a pulse of light at the cavity's resonant frequency would allow a switch between the two indices, while light at a different frequency would reveal the cavity's state without disturbing it. By designating the two states as 0 and 1, the researchers created a readable and rewritable memory. However, each of the cavities had to be controlled by a separate waveguide.

Now, the same researchers have made the cavities much smaller and non-identical, allowing them to introduce multiplexing. They created two different types of optical RAM – one made from silicon and the other from indium phosphide and indium gallium arsenide phosphide. In each RAM, multiple cavities were arranged lengthways, with a single waveguide passing all of them. The researchers used computer modelling to work out exactly how to move specific holes in the photonic crystal such that each cavity had a slightly different resonant frequency. They were then able to send a "write" pulse down the waveguide containing the frequencies of whichever bits they wanted to switch and only those cavities would respond.

Stable lifetimes?

The silicon RAM contained 105 working cavities, with all the resonant wavelengths falling between 1540 nm and 1570 nm, at an average spacing of just 0.23 nm, all of which was fabricated on a silicon crystal just 1 mm long. Unfortunately, the cavity states were stable for less than 10 ns – too short for a viable optical memory. However, the lifetime of the bits in the indium-phosphide-based RAM was, in principle, infinite. Because indium phosphides are less well established in industry than silicon, the technology for manufacturing indium-phosphide components is less precise, and so Notomi and colleagues could only produce a 28-bit memory. However, they believe this provides a better blueprint for future research. "Our final goal is to produce better indium-phosphide systems by improving the fabrication accuracy," says Notomi.

Martin Hill of the University of Western Australia in Crawley describes the paper as "a nice piece of work on a difficult area of photonics". But he also points out that, at present, the switching speed of the optical cavities is lower than the switching speed of electrical transistors, and says that before the device becomes useful as a product, the researchers need a way of making the switching frequencies more predictable and reproducible.

The research is published in Nature Photonics.

About the author

Tim Wogan is a science writer based in the UK

http://physicsworld.com/

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