A quasicrystal is a solid in which atoms are arranged in a seemingly regular pattern that never quite repeats. First observed in 1982, quasicrystals have fuelled the debate on exactly what it means to be a crystal. Even though they are not technically periodic, they scatter radiation to cast sharp diffraction spots — behaviour usually only associated with perfect crystals. To explore such phenomena, Weining Man and colleagues1 have constructed their own three-dimensional quasicrystals from scratch, and find that despite such structural irregularity, the quasicrystals possess surprisingly regular optical properties. These results, reported in Nature this week, add to the growing interest in the photonic properties of quasicrystals and their potential for the manipulation of light in applications such as telecommunications.

The appeal of quasicrystals stems not only from their peculiar structure but also from the nature of the photonic bandgaps (frequency ranges in which the propagation of light is forbidden) that they are expected to possess. Their greater spherical symmetry means that in contrast to regular crystals, quasicrystals are more likely to form complete photonic bandgaps — that is, bandgaps of similar width and frequency along all the different crystal axes — essential for many applications.

Until recently, studies of quasicrystal behaviour have focused on one- and two-dimensional structures for which theoretical calculations are possible. In three dimensions, however, numerical modelling of the optical properties is a tough challenge. Using stereolithographic techniques, Man et al. circumvent this problem by constructing their own three-dimensional quasicrystal from plastic (Fig. 1) — the world’s largest specimen — for which they measure the bandgap character experimentally.

Figure 1: Photograph of the authors' 3D quasicrystal
figure 1

The rods forming the crystal are each 1 cm long.

Full size image

The structure of the authors’ quasicrystal consists of 694 plastic rods, each 1 cm long, joined together in an irregular icosahedral lattice. For comparison, a crystalline counterpart based upon the diamond structure (expected to have the most optimal three-dimensional bandgap characteristics) was also produced. Owing to the large size and spacing of these rods, photonic bandgaps emerge not at visible or near-infrared wavelengths as in many conventional photonic crystals, but in the microwave regime.

By measuring the transmission of microwaves through a variety of different models, the team discovered that despite their inherent complexity, quasicrystals possess strikingly simple bandgap behaviour. As in a crystal, the quasicrystals prevent the transmission of microwaves at certain angles and wavelengths consistent with conventional Bragg scattering — suggesting that much of the wisdom acquired in the study of regular crystals is also applicable to quasicrystals. More significantly, the authors find that the more spherical symmetry of the quasicrystals does indeed result in a bandgap structure that is more isotropic than that of the crystalline diamond, and as such they could offer exciting possibilities for the future.