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Physics News Update
Number 825, May 23, 2007 by Phil Schewe and Ben Stein

Son et Lumiere

Son et lumiere which in French means “sound and light,” is the name for popular nighttime outdoor shows in which pictures are projected on the walls of famous buildings (in France and other countries) accompanied by stories played out over loudspeakers. Now scientists hope to make a miniature sound-and-light show in fibers with the intention of producing not entertainment but ultrasensitive optical switches or the means of transporting bits in future all-optical computers.

The new scheme being developed by scientists at Ben-Gurion University and Tel Aviv University uses sound waves to help slow light nearly to a halt under conditions (ordinary materials at room temperature) more practicable than for most other slow-light experiments. Richard Tasgal (tasgal@bgu.ac.il) and his colleagues use as their medium a so-called Bragg grating fiber; the UV-sensitive core of a fiber is exposed through a mask to ultraviolet light.

This treatment changes the germanium-doped silica fiber core in periodic way along its length so that the index of refraction varies periodically. Light sent into such a fiber, and encountering a regularly changing index of refraction, will reflect multiply, not just at the ends but all along the fiber. A fiber with this condition is sometimes referred to as a distributed mirror.

If, furthermore, the light beam is intense and the fiber material possesses a nonlinear response to light, the net effect of light wavelets propagating in the forward and backward direction can be a light pulse traveling at speeds much less than the speed of light in vacuum. Here’s where the sound part comes in. Very intense light will cause a slight bunching in the density of the fiber and this can create sound waves.

This process is enhanced when the light pulse is traveling close to the speed of sound (around 5 km/s) in the material, and recent work has shown this could be achieved. But the enhancement can work both ways. A passing sound wave alters very slightly the material’s index of refraction and this in turn can result in a shortening and slowing of a passing light pulse-in this case referred to as opto-acoustic solitons.

Tasgal says that he and his colleagues are the first to recognize the potential of sound waves in slowing and processing light pulses in this way. The first great difficulty in implementing the whole scheme is to getting light pulses to enter and stay in a Bragg fiber in the first place since the fiber looks at first just like one long mirror. This might be achieved by gradually increasing the strength of the grating along the fiber. (Tasgal, Band, Malomed, Physical Review Letters, upcoming article)

Catalyzing Primordial Nuclear Chemistry

One of the principal predictions of the standard big bang model is the creation of light nuclei-heavy hydrogen (deuterium), helium-3 and -4 and lithium-6 and -7, in the minutes and hours after the big bang itself. Understanding big bang nucleosynthesis (BBN) is important since it corresponds to the earliest epoch in the early universe for which observation and theory can be tested against each other (the creation of the first stable compound nuclei comes long before the first creation of stable atoms).

Agreement between observations and predictions has been pretty good so far, with the predictions being sharpened in recent years by high-precision maps of the cosmic microwave background. Furthermore, the measurements of elemental abundances can be used to look for phenomena beyond known physics. Indeed, some cosmic abundance studies have already set limits on the number of additional light particles and most recently on the nature of hypothetical extra spatial dimensions.

But a remaining puzzle is the amount of primordial lithium; both Li-6 and Li-7 are unexpectedly abundant in metal-poor stars (those with very few heavier elements). For example, a much higher than expected level of Li-6 might be pointing to a primordial origin (that is, not made later in stellar cores or in supernovas), in which case the BBN model would need to be amended. Maxim Pospelov (pospelov@uvic.ca) of the Perimeter Institute for Theoretical Physics in Waterloo, Ontario, and University of Victoria, British Columbia suggests that the anomaly can be explained if early nucleosynthesis was aided---catalyzed---by the presence of charged heavy particles, which are common in many models of particle physics.

In fact such particles are suggested in supersymmetry (SUSY for short) theories. In the SUSY scenario every known fermi (possessing a half-integral spin) particle such as quarks would have a boson counterpart, and every known boson (integral-spin) such as photons would have fermi counterparts. Pospelov argues that charged SUSY particles long-lived enough (more than a thousand seconds) to survive into the BBN era would bind with light nuclei and enhance (by a factor as large as 10^8) the buildup to heavier nuclei before the SUSY particle itself would decay. Pospelov says that his theory should be testable at the Large Hadron Collider (LHC) now under construction in Geneva. (Physical Review Letters, upcoming article

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