lasers (4)

Getting Back Mojo...

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Artist's representation of the circular phonons. (Courtesy: Nadja Haji and Peter Baum, University Konstanz)

Topics: Applied Physics, Lasers, Magnetism, Materials Science, Phonons

When a magnetic material is bombarded with short pulses of laser light, it loses its magnetism within femtoseconds (10–15 seconds). The spin, or angular momentum, of the electrons in the material, thus disappears almost instantly. Yet all that angular momentum cannot simply be lost. It must be conserved – somewhere.

Thanks to new ultrafast electron diffraction experiments, researchers at the University of Konstanz in Germany have now found that this “lost” angular momentum is in fact transferred from the electrons to vibrations of the material’s crystal lattice within a few hundred femtoseconds. The finding could have important implications for magnetic data storage and for developments in spintronics, a technology that exploits electron spins to process information without using much power.

In a ferromagnetic material, magnetism occurs because the magnetic moments of the material’s constituent atoms align parallel to each other. The atoms and their electrons then act as elementary electromagnets, and the magnetic fields are produced mainly by the spin of the electrons.

Because an ultrashort laser pulse can rapidly destroy this alignment, some scientists have proposed using such pulses as an off switch for magnetization, thereby enabling ultra-rapid data processing at frequencies approaching those of light. Understanding this ultrafast demagnetization process is thus crucial for developing such applications as well as for better understanding the foundations of magnetism.

Researchers find ‘lost’ angular momentum, Isabelle Dumé, Physics World

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Cooling Teleportation...

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Image source: CERN - accelerating science

Topics: CERN, Condensed Matter Physics, Entanglement, Lasers, Quantum Mechanics

Much of modern experimental physics relies on a counterintuitive principle: Under the right circumstances, zapping matter with a laser doesn’t inject energy into the system; rather, it sucks the energy out. By cooling the system to a fraction of a degree above absolute zero, one can observe quantum effects that are otherwise invisible.

Laser cooling works like a charm, but only when a system’s ladder of quantum states is just right. Atoms of alkali metals and a few other elements are ideal. Molecules, with their multitudes of energy levels, pose a much greater challenge. And fundamental particles such as protons, which lack internal states altogether, can’t be laser-cooled at all.

Nevertheless, there’s a lot of interest in experimenting on protons at low temperature—in particular, precisely testing how their mass, magnetic moment, and other properties compare with those of antiprotons. Toward that end, the Baryon Antibaryon Symmetry Experiment (BASE) collaboration has now demonstrated a method for using a cloud of laser-cooled beryllium ions to sympathetically cool a single proton, even when the proton and ions are too distant to directly interact.

A superconducting circuit is a cooling teleporter, Johanna L. Miller, Physics Today

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Nano Laser...

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In their experiments, the researchers used ultrathin crystals consisting of a single layer of atoms. These sheets were sandwiched between two layers of mirror-like materials. The whole structure acts as a cage for light and is called a microcavity.

Topics: Applied Physics, Bose-Einstein Condensate, Lasers, Nanotechnology, Optics

Physicists have taken a step towards realizing the smallest-ever solid-state laser by generating an exotic quantum state known as a Bose-Einstein condensate (BEC) in quasiparticles consisting of both matter and light. Although the effect has so far only been observed at ultracold temperatures in atomically thin crystals of molybdenum diselenide (MoSe2), it might also be produced at room temperature in other materials.

When particles are cooled down to temperatures just above absolute zero, they form a BEC – a state of matter in which all the particles occupy the same quantum state and act in unison, like a superfluid. A BEC made up of tens of thousands of particles behaves as if it were just one giant quantum particle.

An international team of researchers led by Carlos Anton-Solanas and Christian Schneider from the University of Oldenburg, GermanySven Höfling of the University of Würzburg, GermanySefaattin Tongay at Arizona State University, US; and Alexey Kavokin of Westlake University in China, has now generated a BEC from quasiparticles known as exciton-polaritons in atomically thin crystals. These quasiparticles form when excited electrons in solids couple strongly with photons.

“Devices that can control these novel light-matter states hold the promise of a technological leap in comparison with current electronic circuits,” explains Anton-Solanas, who is in the quantum materials group at Oldenburg’s Institute of Physics. “Such optoelectronic circuits, which operate using light instead of electric current, could be better and faster at processing information than today’s processors.”

Anton-Solanas, Schneider, and colleagues studied crystals of MoSe2 that were just a single atomic layer thick. MoSe2belongs to a family of materials known as transition-metal dichalcogenides (TMDCs). In their bulk form, these materials act as indirect band-gap semiconductors. Still, when scaled down to a monolayer thickness, they behave as direct band-gap semiconductors, capable of efficiently absorbing and emitting light.

In their experiments, the researchers assembled sheets of MoSe2 less than a nanometer thick and sandwiched them between alternating layers of silicon dioxide and titanium dioxide (SiO2/TiO2), which reflect light like a mirror. The resulting structure is known as a microcavity and acts as a cage for light. “It’s like trapping the light-emitting material in a room filled with mirrors and mirrors only,” Tongay tells Physics World. “The light gets reflected these mirrors and is absorbed by the material back and forth.”

Exotic quantum state could make smallest-ever laser, Isabelle Dumé, Physics World

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Plasma Guides and Lasers...

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Lasers are used to create an indestructible optical fiber out of plasma.

Credit: Intense Laser-Matter Interactions Lab, University of Maryland

Topics: Lasers, Optics, Plasma, Research, Star Trek, Star Wars

In science fiction, firing powerful lasers looks easy — the Death Star can just send destructive power hurtling through space as a tight beam. But in reality, once a powerful laser has been fired, care must be taken to ensure it doesn’t get spread too thin.

If you’ve ever pointed a flashlight at a wall, you’ve observed an example of the diffusion of light. The farther you are from the wall, the more the beam spreads, resulting in a larger and dimmer spot of light. Lasers generally expand much more slowly than the beams from flashlights, but the effect of diffusion is important when the laser travels a long way or must maintain a high intensity.

Whether your goal is to achieve galactic domination or, more realistically, to accelerate electrons to incredible speeds for physics research, you’ll want as tight and powerful a beam as possible to maximize the intensity.

In their experiments, researchers can use devices called waveguides, like the optical fibers that might be carrying the internet throughout your neighborhood, to transport lasers while keeping them contained to narrow beams.

Plasma guides maintain focus of lasers, National Science Foundation Public Affairs

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