Applied Physics

Extended Darkness

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Science  19 Oct 2012:
Vol. 338, Issue 6105, pp. 305-307
DOI: 10.1126/science.338.6105.305-c

Titanium sapphire laser sources rely on a phenomenon termed Kerr lensing to generate ultrashort light pulses. Essentially, the refractive index of the crystal in the laser cavity rises with the intensity of the light passing through it, which induces focusing toward the center where selective amplification sets the mode structure. Kasala and Saravanamuttu have created an environment in which a propagating light beam is instead pushed away from the center. Specifically, they diminish the intensity of a ∼0.1-mm central spot in a ∼5-mm beam—derived from an incoherent source of incandescent white light—and then direct the beam through a medium containing a photoinitiator and a polymer precursor. Because the polymerization rate is intensity dependent, and the refractive index of the polymer being formed is high, light migrates away from the center, but not too far away—the confining effect of the polymer keeps the dark core region narrow. As a result, a channel forms, impenetrable by light that can be observed relative to the bright background by optical microscopy. Experiments with smaller ratios of the background beam to the central depression bolstered the authors' posited mechanism.

J. Am. Chem. Soc. 134, 14195 (2012).

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