Researchers from the University of Tokyo, in collaboration with NTT Basic Research Laboratories have discovered that an ‘amorphous diamond’ structure would make a perfect photonic crystal.

Just like atomic crystals, conventional photonic crystals are periodically structured materials. The periodic features of the photonic crystal strongly influence the propagation of light—with wavelengths similar to the dimensions of the host photonic crystal—through the structure. For certain wavelengths and photonic crystal designs, it may not be possible for light of matching wavelengths to exist in such a structure; such photonic crystal structures would be perfect reflectors of light.

Fig. 1: Photonic crystals with diamond-structure. a, The original photonic crystal structure based on the diamond. b, The amorphous diamond photonic crystal.Reproduced with permission. © 2008 American Physical Society.

However, while it is relatively straightforward to design structures that reflect light approaching the crystal from only one direction, there are only a few known three-dimensional photonic crystal structures that are capable of perfectly reflecting light regardless of the direction of incidence. All of these novel structures have crystal structures related to the atomic lattice of diamond (Fig. 1a).

Now, Keiichi Edagawa and co-workers propose a photonic crystal structure consisting of a ‘non-periodic amorphous’ design that completely reflects light of a wide range of wavelengths.1 The structure studied by the researchers is based on the tetrahedral base unit of the diamond structure, although in their case, the tetrahedral base units of the diamond structure are now assembled in a disordered fashion (Fig. 1b).

The group’s theoretical calculations showed that the reflecting properties of this amorphous diamond structure were almost the same as that of the periodic diamond structure.

Importantly, just like diamond photonic crystals, their theoretical structure reflected light regardless of its angle of incidence. Although perfectly reflecting amorphous structures are known in two dimensions, the present findings are the first report of a three-dimensional amorphous structure exhibiting perfect reflection for all incident angles.

The structure still shows remnant features of the periodic diamond structure, and it will be important to clarify the mechanisms behind this novel reflecting behaviour. In particular, it seems vital to fabricate an actual photonic crystals based on this group’s design and to study the interaction of light with the amorphous structure in more detail.