Evanescent waves order 2-D particle arrays

Although a light beam incident on the surface of an optically less-dense medium is totally internally reflected above the critical angle, an evanescent wave arises that decays exponentially within the medium and that can exert attractive forces on small particles.

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Although a light beam incident on the surface of an optically less-dense medium is totally internally reflected above the critical angle, an evanescent wave arises that decays exponentially within the medium and that can exert attractive forces on small particles. Taking advantage of this force, researchers at the University of Oxford (Oxford, England) have used counterpropagating laser beams to self-assemble submicron particles into 2-D arrays with a rectangular or pseudo-hexagonal packing structure.

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To build the arrays, light from a 1064-nm diode-pumped solid-state laser was focused into a silica prism such that the focal spot of the reflected beam overlapped the incident beam, creating an evanescent wave with a decay length on the order of 800 nm in water. When 500-nm-diameter particles suspended in solution were placed on top of the prism, 2-D ordering of the particles was observed. Unlike optical-tweezer particle arrays formed by spatially varying electric fields, the researchers discovered that ordering arises from scattering of the evanescent laser field by the particles themselves, resulting in a new method for building matter with light and for real-time exploration of the processes that occur during crystal formation. Contact Colin D. Bain at [email protected]

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