Quasicrystals differ from conventional crystals in that they have rotational but not translational symmetry. One- and two-dimensional quasicrystals are known to have valuable optical properties, such as tuneable photonic bandgaps. Now, in Optics Express, Yael Roichman and David G. Grier1 show that holographic assembly techniques can be used to construct equivalent three-dimensional structures.

Holographic assembly involves the use of a large number of ‘optical tweezers’, which trap particles at the focus of a highly controlled laser beam. Using this method, objects as small as a few nanometres in diameter can be manipulated with unparalleled accuracy.

Roichman and Grier used almost two hundred colloidal silica microspheres dispersed in aqueous solution to create a quasicrystalline icosohedral heterostructure. By first arranging the particles into a two-dimensional projection of the required structure, the authors were then able to manipulate them along the axis of the laser beam, to produce the required three-dimensional configuration. After reducing the separation of the spheres to create a more optically dense material, the aqueous solution was ‘gelled’ using ultraviolet light. Then the structure of the quasicrystal could be investigated using diffraction, the results confirming the symmetries expected within an icosohedral structure.

The significance of this technique lies in the optical properties of the resulting quasicrystalline heterostructures — in particular, in the possibility of creating materials with definable photonic bandgaps. Such ‘tuneability’ is possible because, owing to the lack of constraints imposed by the fabrication process, the configuration of the particles in the quasicrystal is essentially unrestricted.

Constructing quasicrystals from microscopic particles is one of the simplest applications of this three-dimensional holographic assembly process. Roichman and Grier demonstrate the production of particle-free channels that can act as narrow-band waveguides and frequency-selective filters for electromagnetic waves. They also suggest that the technique could be used to make optical switches and to form domains within a material that have distinct, definable chemical properties.