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2Physics Quote:
"Can photons in vacuum interact? The answer is not, since the vacuum is a linear medium where electromagnetic excitations and waves simply sum up, crossing themselves with no interaction. There exist a plenty of nonlinear media where the propagation features depend on the concentration of the waves or particles themselves. For example travelling photons in a nonlinear optical medium modify their structures during the propagation, attracting or repelling each other depending on the focusing or defocusing properties of the medium, and giving rise to self-sustained preserving profiles such as space and time solitons or rapidly rising fronts such as shock waves." -- Lorenzo Dominici, Mikhail Petrov, Michal Matuszewski, Dario Ballarini, Milena De Giorgi, David Colas, Emiliano Cancellieri, Blanca Silva Fernández, Alberto Bramati, Giuseppe Gigli, Alexei Kavokin, Fabrice Laussy, Daniele Sanvitto. (Read Full Article: "The Real-Space Collapse of a Two Dimensional Polariton Gas" )

Monday, August 07, 2006

Reflection of Atoms

Graduate Student Edward Vliegen with his 'atom-reflecting' set-up

So far, physicists could succeed in developing lenses and prisms to manipulate beams of atoms and molecules as though they were beams of light. Such techniques were used to manipulate atoms in studies of atomic properties, in research on ultra-cold quantum states of atoms, and in devices like atom-based gyroscopes and atomic clocks.

But as far as the reflection is concerned there had only been some limited success. In the past researchers have built mirrors for ground state molecules that have electric dipole moments as an outcome of asymmetries in the locations of positive and negative charges in their molecular structures. Some researchers tried to reflect atoms and molecules excited into very high, so-called Rydberg states, which also have large dipole moments. But they failed to bounce those atoms straight back from a head-on collision. Atoms could only graze off at shallow angles.

Now, Edward Vliegen and Frederic Merkt of the Swiss Federal Institute of Technology (ETH) in Zurich have reported that they were able to stop and then reflect these so-called "Rydberg atoms" using a system of electric fields. Rydberg atoms are unusual in that they contain an electron that has been excited to such a high energy level that it orbits a very long way from the nucleus. Since the outer electron is so loosely bound, Rydberg atoms are highly sensitive to external perturbations, such as electric fields. In the present study, for example, hydrogen atoms were used in which the electron had been excited by a laser beam so that its principal quantum number, n, was 27. The electron in one of these atoms can be as far as 37 nm from the nucleus.

Vliegen and Merkt began by using a laser to split up ammonia (NH3) in a quartz capillary tube. As the gas left the tube, it underwent a supersonic expansion so that the hydrogen atoms were traveling at a speed of 720 meter per second. The atoms then entered a gap between four metallic electrodes, where there is a rapidly changing electric field. As they did so, the atoms were excited by two ultraviolet laser beams to create Rydberg states. By applying a sequence of voltages to the four electrodes, Vliegen and Merkt found that they could stop the Rydberg atoms in a time of 4.8 ms just 1.9 mm away from the position where the atoms were excited by the laser beams. They were then able to reflect the atoms back from the middle of the plates to their original positions with accelerations of 2 x 108 ms-2. And since the atoms are focused about six microseconds after being reflected, Vliegen claims that their mirror also works as a cylindrical lens.

There could be some interesting applications of this work. According to Vliegen, the new mirror could be used to perform interferometry experiments with Rydberg atoms. He even thinks the mirror could help prevent antihydrogen Rydberg atoms generated at the CERN “antimatter factory“ from colliding with the walls of the experiment chamber and annihilating there.

"Normal-Incidence Electrostatic Rydberg Atom Mirror", E. Vliegen and F. Merkt,
Phys. Rev. Lett. 97, 033002 (issue of 21 July 2006)

To know more about Rydberg atom, visit: Wikipedia page on Rydberg atom

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