baudrunner's space: More fun with light
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Tuesday, January 22, 2008

More fun with light

An opaque medium only passes a small percentage of the light that strikes its surface. How much light is passed through it depends on its opacity. Light can be shone through an eggshell, for example, but the light that leaves the other side is scattered and uneven. Even laser light shone through an opaque substance becomes scattered and loses coherence. If the characteristics of the resulting scattered light waves can be reverse-programmed into a laser beam before it passes through the opaque substance than the beam that exits should be coherent.

That is the reasoning behind an experiment conducted by researchers I.M. Vellekoop and A.P. Mosk of the University of Twente in the Netherlands. After first decoding the scattering characteristics of the emerging light, they placed a programmed phase modulator in the laser beam before the light passed through the medium. The waves exiting the phase modulator perfectly matched the scattering sample such that all the phase irregularities created by the opaque medium were effectively cancelled out.

The opaque substance was then found to act as a lens, focusing the beam into a narrow region. This phenomenon is probably attributable to Kerr-induced self-focusing. The beam still contains non-linear components of the refractive index of the material and where this component is positive, as it is in most materials, the refractive index becomes larger in the areas where the intensity is higher, usually at the center of the beam. This causes the beam to collapse in on itself. Self-focusing occurs where the radiating power is greater than the critical power, a constant of the medium and, as was demonstrated, the angle of focus is indeed a function of the material.

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