Trapping Monolayer of Colloidal Particles in a Laser Lattice

Tag : High Power Laser

Tiles for kitchens and bathrooms are usually square or rectangular.There is a good reason for this: anyone trying to tile a bathroomwith five-sided tiles will not be able to cover the wall withoutany gaps. This is only possible with three-, four-, or six-sidedtiles. For a long time it seemed that Nature also adhered to thisprinciple. In 1984, however, the Israeli physicist Dan Shechtmanreported the first crystals whose surfaces are indeed described bytiles having pentagonal and other shapes and are as imaginative asIslamic decoration.
Now physicists at the University of Stuttgart and the Max PlanckInstitute of Metals Research have discovered structures thatcombine crystalline and quasicrystalline structural elements. Theycreated a light lattice with a quasicrystalline structure byoverlapping five laser beams. In the optical potential wells ofthis lattice they trapped a single layer of 3 micrometer plasticspheres floating in water that could easily be observed through amicroscope. At high laser intensities and deep wells, the lightlattice forced the spheres into a quasicrystalline arrangement withpentagonal-, star-, and diamond-shaped basic elements. At lowintensities, however, the negatively charged particles were barelyinfluenced by the light lattice. Under these conditions, theyposition periodically with each particle surrounded by sixneighbours at equal distance, behaving as the scientists hadexpected.
"What was actually intriguing is the structure we observed atintermediate intensities," says Clemens Bechinger, head of the2nd Physical Institute at the University of Stuttgart and fellow atthe Max Planck Institute for Metals Research. "In this case,the plastic spheres arrange strictly periodically in one direction,as in a crystal, however, perpendicular to this direction, theparticles order not like in a crystal, but in a quasicrystal,"explains Jules Mikhael, a doctoral student of Lebanese decentworking on the project. Evidently, the competition between themutual particle´s interaction and that with the light fieldresults in an intermediate structure which exhibits bothcrystalline and quasicrystalline properties. There are clearlyrecognizable bands of squares that are separated by randomlyarranged single and double rows of equilateral triangles in anaperiodic rhythm.
This structure is similar to a specific Archimedean tiling, asfirst mentioned by Archimedes and fully characterized by JohannesKepler in 1619. Archimedean tiles meet two conditions: first, theirsides are all of the same length, irrespective of whether they havethree, four or more angles. Second, the points where tiles meetmust be identical. Using these structural principles, one canconstruct exactly eleven different tiling patterns that can fullycover a surface. In one of them, rows of squares alternate withrows of equilateral triangles. "At short distances, theintermediate pattern we found is identical to this tiling pattern.At larger scales, however, we observe characteristic disruptions,as the strictly periodic Archimedean pattern would not fit into thequasiperiodic structure of the light lattice," explainsClemens Bechinger.
Because crystals and quasicrystals comprise different materialclasses with differing physical and chemical properties, theobserved intermediate structure is striking. "The combinationof crystalline and quasicrystalline structural elements will likelylead to novel material properties", says Clemens Bechinger.Because colloids - in contrast to atoms - can be directly observedwith optical techniques and their pair interactions can be tailoredover a large range, the knowledge gained by the Stuttgartphysicists from their experiments with colloids will help toexplore the conditions where similar structures form in atomicsystems. In this respect, colloidal systems can be regarded as amodel system as they reveal a great deal about the conditions underwhich particles arrange on quasicrystalline surfaces in the mannerof Archimedean tilings.