Tag Archives: Jesse Emspak

World’s thinnest light bulb created from graphene

graphene-light-bulb

When a current was run through strips of graphene that were placed across a trench of silicon, the result was light emission. (Young Duck Kim/Columbia Engineering)

Graphene, a form of carbon famous for being stronger than steel and more conductive than copper, can add another wonder to the list: making light.

Researchers have developed a light-emitting graphene transistor that works in the same way as the filament in a light bulb.

“We’ve created what is essentially the world’s thinnest light bulb,” study co-author James Hone, a mechanical engineer at Columbia University in New York, said in a statement.

Scientists have long wanted to create a teensy “light bulb” to place on a chip, enabling what is called photonic circuits, which run on light rather than electric current. The problem has been one of size and temperature — incandescent filaments must get extremely hot before they can produce visible light. This new graphene device, however, is so efficient and tiny, the resulting technology could offer new ways to make displays or study high-temperature phenomena at small scales, the researchers said.

Making light

When electric current is passed through an incandescent light bulb’s filament — usually made of tungsten — the filament heats up and glows. Electrons moving through the material knock against electrons in the filament’s atoms, giving them energy. Those electrons return to their former energy levels and emit photons (light) in the process. Crank up the current and voltage enough and the filament in the light bulb hits temperatures of about 5,400 degrees Fahrenheit for an incandescent. This is one reason light bulbs either have no air in them or are filled with an inert gas like argon: At those temperatures tungsten would react with the oxygen in air and simply burn.

In the new study, the scientists used strips of graphene a few microns across and from 6.5 to 14 microns in length, each spanning a trench of silicon like a bridge. (A micron is one-millionth of a meter, where a hair is about 90 microns thick.) An electrode was attached to the ends of each graphene strip. Just like tungsten, run a current through graphene and the material will light up. But there is an added twist, as graphene conducts heat less efficiently as temperature increases, which means the heat stays in a spot in the center, rather than being relatively evenly distributed as in a tungsten filament.

Myung-Ho Bae, one of the study’s authors, told Live Science trapping the heat in one region makes the lighting more efficient. “The temperature of hot electrons at the center of the graphene is about 3,000 K [4,940 F], while the graphene lattice temperature is still about 2,000 K [3,140 F],” he said. “It results in a hotspot at the center and the light emission region is focused at the center of the graphene, which also makes for better efficiency.” It’s also the reason the electrodes at either end of the graphene don’t melt.

As for why this is the first time light has been made from graphene, study co-leader Yun Daniel Park, a professor of physics at Seoul National University, noted that graphene is usually embedded in or in contact with a substrate.

“Physically suspending graphene essentially eliminates pathways in which heat can escape,” Park said. “If the graphene is on a substrate, much of the heat will be dissipated to the substrate. Before us, other groups had only reported inefficient radiation emission in the infrared from graphene.”

The light emitted from the graphene also reflected off the silicon that each piece was suspended in front of. The reflected light interferes with the emitted light, producing a pattern of emission with peaks at different wavelengths. That opened up another possibility: tuning the light by varying the distance to the silicon.

The principle of the graphene is simple, Park said, but it took a long time to discover.

“It took us nearly five years to figure out the exact mechanism but everything (all the physics) fit. And, the project has turned out to be some kind of a Columbus’ Egg,” he said, referring to a legend in which Christopher Columbus challenged a group of men to make an egg stand on its end; they all failed and Columbus solved the problem by just cracking the shell at one end so that it had a flat bottom.

The research is detailed in the June 15 issue of Nature Nantechnology.

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Physicists see 27th dimension of photons

Physicists see 27th dimension of photons

By Jesse Emspak

Published January 29, 2014

  • photon-eye

    Scientists find a way to directly measure quantum states, such as momentum, of photons. (MPQ, Quantum Dynamics Division.)

Quantum computers and communications promise more powerful machines and unbreakable codes. But to make them work, it’s necessary to measure the quantum state of particles such as photons or atoms. Quantum states are numbers that describe particle characteristics such as momentum or energy.

But measuring quantum states is difficult and time-consuming, because the very act of doing so changes them, and because the mathematics can be complex. Now, an international team says they found a more efficient way to do it, which could make it simpler to build quantum-mechanical technologies.

In a study detailed in the Jan. 20 issue of the journal Nature Communications, researchers from the University of Rochester and the University of Glasgow took a direct measurement of a photon’s 27-dimensional quantum state. These dimensions are mathematical, not dimensions in space, and each one is a number that stores information.

To understand a 27-dimensional quantum state, think about a line described in 2 dimensions. A line would have a direction in the X and Y coordinates 3 inches left and 4 inches up, for instance. The quantum state has 27 such coordinates. [Quantum Physics: The Coolest Little Particles in Nature]

“We chose 27, kind of to make a point about 26 letters in the alphabet and throwing in one more,” said Mehul Malik, now a postdoctoral researcher at the University of Vienna. That means each quantum bit, or “qubit,” could store a letter instead of a simple 1 or 0.

Seeing a photon The group, led by Malik and Robert Boyd, a professor of optics and physics at the University of Rochester, was able to see a photon’s states directly. They measured the photon’s orbital angular momentum, which is how much the particles of light “twist” as they travel through space.

Ordinarily, finding the quantum state of a photon requires a two-step process. First, scientists have to measure some property of the photon, such as its polarization or momentum. The measurements are performed on many copies of the quantum state of a photon. But that process sometimes introduces errors. To get rid of the errors, the scientists have to look at what results they got that are “disallowed” states ones that don’t follow the laws of physics. But the only way to find them is to search through all the results and discard the ones that are impossible. That eats up a lot of computing time and effort. This process is called quantum tomography. [The 9 Biggest Unsolved Mysteries in Physics]

A light wave is a combination of an electric and magnetic field, each of which oscillates and makes a wave. Each wave moves in time with the other, and they are perpendicular to each other. A beam of light is made up of lots of these waves.

Light can have what is called orbital angular momentum. In a beam with no orbital angular momentum, the peaks of the waves the electric ones, for example are lined up. A plane connecting these peaks will be flat. If the beam has orbital angular momentum, a plane connecting these peaks will make a spiral, helical pattern, because the light waves are offset from one another slightly as you go around the beam. To measure the state of the photons, scientists must “unravel” this helical shape of the waves in the beam.

Measuring a photon’s quantum state The team first fired a laser through a piece of transparent polymer that refracted the light, “unraveling” the helix formed by the waves. The light then passed through special lenses and into a grating that makes many copies of the beam. After passing through the grating, the light is spread out to form a wider beam.

After the beam is widened, it hits a device called a spatial light modulator. The modulator carries out the first measurement. The beam then reflects back in the same direction it came from and passes through a beam splitter. At that point, part of thebeam moves toward a slit, which makes a second measurement. [Twisted Physics: 7 Mind-Blowing Experiments]

One of the two measurements is called “weak” and the other “strong.” By measuring two properties, the quantum state of the photons can be reconstructed without the lengthy error-correction calculations tomography requires.

In quantum computers, the quantum state of the particle is what stores the qubit. For instance, a qubit can be stored in the photon’s polarization or its orbital-angular momentum, or both. Atoms can also store qubits, in their momenta or spins.

Current quantum computers have only a few bits in them. Malik noted that the record is 14 qubits, using ions. Most of the time, ions or photons will only have acouple of bits they can store, as the states will be two-dimensional. Physicists use two-dimensional systems because that is what they can manipulate it would be very difficult to manipulate more than two dimensions, he said.

Direct measurement, as opposed to tomography, should make it easier to measure the states of particles (photons, in this case). That would mean it is simpler to add more dimensions three, four or even as in this experiment, 27 and store more information.

Mark Hillery, a professor of physics at Hunter College in New York, was skeptical that direct measurement would prove necessarily better than current techniques. “There is a controversy about weak measurements in particular, whether they really are useful or not,” Hillery wrote in an email to LiveScience. “To me, the main issue here is whether the technique they are using is better (more efficient) than quantum-state tomography for reconstructing the quantum state, and in the conclusion, they say they don’t really know.”

Jeff Savail, a master’s candidate researcher at Canada’s Simon Fraser University, worked on a similar direct measurement problem in Boyd’s lab, and his work was cited in Malik’s study. In an email he said one of the more exciting implications is the “measurement problem.” That is, in quantum mechanical systems the question of why some measurements spoil quantum states while others don’t is a deeper philosophical question than it is about the quantum technologies themselves.

“The direct measurement technique gives us a way to see right into the heart of the quantum state we’re dealing with,” he said. That doesn’t mean it’s not useful far from it. “There may also be applications in imaging, as knowing the wave function of the image, rather than the square, can be quite useful.”

Malik agreed that more experiments are needed, but he still thinks the advantages might be in the relative speed direct measurement offers. “Tomography reduces errors, but the post-processing [calculations] can take hours,” he said.

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