Tuesday, April 29, 2008

World's shortest single photon pulse created

The world’s shortest light pulse containing just one photon has been produced by Oxford University scientists.

The Oxford team can create individual photons that are 65 femtoseconds in duration: that’s approximately fifty times shorter than any single photon previously produced.

And every photon this source produces is identical to the previous one. Such photons could be a major breakthrough in quantum computing: the harnessing of quantum effects to perform calculations that would take conventional computers thousands of years to resolve.

‘Creating single photons even under controlled conditions is extremely challenging,’ said Peter Mosley of Oxford’s Department of Physics. ‘Even the purest laser light beam consists of many photons all bunched together. Our approach enables us to generate individual photon replicas, identical packets of light of very short duration that are ideal for quantum computing.’

Peter Mosley, a member of Oxford’s Ultrafast Group, is a co-author of a report of the research in Physical Review Letters.

Monday, April 14, 2008

Laser triggered lightning

Reported from Photonics.com: Scientists have used ultrashort laser pulses to trigger electrical activity in thunderclouds, a first step toward creating man-made lightning.

In a modern-day take on Benjamin Franklin's experiment during a storm more than 200 years ago with a kite, a key and a silk ribbon to prove electricity exists in the atmosphere, the French, Swiss and German scientists aimed high-power pulses of laser light into two passing thunderstorms at the top of South Baldy Peak in New Mexico. The laser pulses created plasma filaments that could conduct electricity. No air-to-ground lightning was triggered because the plasma filaments were too short-lived, but the laser pulses generated discharges in the thunderclouds themselves, the scientists said.

Triggering lightning strikes is an important tool for basic and applied research because it enables researchers to study the mechanisms underlying lightning strikes. Triggered lightning strikes will also allow engineers to evaluate and test the lightning sensitivity of airplanes and critical infrastructure such as power lines.

The idea of using lasers to trigger lightning strikes was first suggested more than 30 years ago, but until recently lasers were not powerful enough to generate the long plasma channels needed. The current generation of more powerful pulsed lasers, like the one developed by Kasparian's team, may change that because they can form a large number of plasma filaments -- ionized channels of molecules in the air that act like conducting wires extending into the thundercloud.

Kasparian and his colleagues involved in the Teramobile project, an international program initiated by the National Center for Scientific Research (CNRS) in France and the German Research Foundation (DFG), built a powerful mobile femtosecond-terawatt laser capable of generating long plasma channels by firing ultrashort laser pulses. They chose to test their laser at the Langmuir Laboratory in New Mexico, which is equipped to measure atmospheric electrical discharges. Sitting at the top of 10,500-ft South Baldy Peak, this laboratory is in an ideal location because its altitude places it close to the high thunderclouds.

During the tests, the research team quantified the electrical activity in the clouds after discharging laser pulses. Statistical analysis showed that their laser pulses indeed enhanced the electrical activity in the thundercloud where it was aimed—in effect they generated small local discharges located at the position of the plasma channels.

The limitation of the experiment, though, was that they could not generate plasma channels that lived long enough to conduct lightning all the way to the ground. The plasma channels dissipated before the lightning could travel more than a few meters along them. The team is currently looking to increase the power of the laser pulses by a factor of 10 and use bursts of pulses to generate the plasmas much more efficiently.

The paper, "Electric Events Synchronized With Laser Filaments in Thunderclouds," appears in the April 14 issue of Optics Express, the Optical Society of America's (OSA) open-access journal.

Wednesday, April 09, 2008

Petawatt Power Peak Reached

The Texas Petawatt laser produced a petawatt of peak power on March 31, making it the highest powered laser in the world, said Todd Ditmire, a physicist at the University of Texas at Austin.

There has only been one petawatt laser in the US history, the Nova laser at Lawrence Livermore Laboratory (LLNL, operated by the University of California for the energy department). Nova, which took up a football field in space, is now defunct. In the past eight or so years, there has been a worldwide push to achieve petawatts (10 to the 15th power). Terawatts (10 to the 12th power) were produced by short pulse lasers in the late 1980s using chirped pulse amplification, the method Ditmire is using.

Other US petawatt projects include the OmegaEP laser at the University of Rochester, The Ohio State University petawatt, and the Z-Beamlet project at the Sandia National Labs Z-Petawatt Laser Facility. Projects are also underway in the UK, France, Germany, Japan, China, and other countries.

The challenge for researchers is to produce a lot of energy in a little time, and a petwatt can be the result if enough energy can be produced in a short enough pulse. The Hercules laser at the University of Michigan, for example, is only 0.3 petawatts, but it focuses to an incredibly tiny spot. For sheer power -- energy divided by pulse duration -- the Texas petawatt laser now leads the way in the US.

The laser produces a very short duration, very low-energy pulse, and this pulse is stretched in time to a very long pulse, is amplified to huge energy, then finally is compressed to a high-energy, super-short-duration pulse. One of the critical aspects of the system is the diffraction gratings used to compress the pulse; these were made by Jerry Britten's group at LLNL, and they are some of the most difficult-to-manufacture optics in the world.

Related Link: Texas High Intensity Laser

via: photonics.com