Source: School of Physics and Engineering
Written by: School of Physics and Engineering
Edited by: Wang Dongmei

Dirac cone, one kind of conical dispersion in photonic crystals, has many profound implications in physics world. Photonic conical dispersion has become an important topic. Recently, a class of periodic photonic crystals with a conical dispersion at the Brillouin zone center has been demonstrated to behave as an effective zero-refractive index. But it is not obvious that such effective medium description could be applied to noncrystalline systems in the finite frequency such as a Dirac point. Perhaps more importantly, whether or not a conical dispersion can exist in a noncrystalline system without periodicity is still an open question. Help has arrived. 

Scientist with the School of Physics and Engineering (SPE) and State Key Laboratory of Optoelectronic Materials and Technologies (OEMT) in Sun Yat-sen University (SYSU) have shown, both conical dispersion and zero refractive index can be found in certain photonic quasicrystalline system. 

"The key question is to ask whether Dirac point can exist in aperiodic structures and if they exist, is it associated to a zero refractive index material?" says Jianwen Dong, professor of SPE who led this study. “This paper shows that conical dispersions and effective zero-refractive index can go beyond periodicity. The essence of the physics is that conical dispersions require high symmetry to ensure linear bands crossing each other without level repulsion, and quasicrystals provide high enough rotation symmetry to support the cones.”

Dong, who is elected in New Century Excellent Talents in University and Guangdong Distinguished Young Scientists in 2013, is the first author of a paper describing this physics research in the journal Physical Review Letters [Phys. Rev. Lett. 114, 163901 (2015)]. The paper is titled “Conical Dispersion and Effective Zero Refractive Index in Photonic Quasicrystals”. The other authors are Mingli Chang (Master, SYSU), Zhichao Zhong (undergraduate, SYSU), and a team of Professor Che Ting Chan from Hong Kong University of Science and Technology, etc.

Conical dispersion is the necessary condition to qualify as a zero refractive-index medium. The states in the conical dispersions are extended and have a nearly constant phase. Microwave experimental characterizations of finite-sized samples show evidence on asymmetry propagation, cloak, and Snell’s law. 

"These concepts could be extended to optical frequencies," says Mingli Chang, second author of the PRL paper and a PhD candidate of Professor Chan’s research group. “Many semiconductor materials are of high permittivity and transparency in optical frequencies. The behaviors should be observed with the same procedures discussed in the paper.”

Many Dirac-cone based behaviors, such as zero refractive index, topological photonics, one-way transport, and phase tailoring, are of interest to broad science and engineering communities. Last December, Dong and his group already realized a photonic topological insulator made by photonic metacrystal. They relaxed the material requirements for matching ε and μ, which was in advances to measure the ‘spin-polarized’ edge transport and its robustness at around 3 GHz. The paper has been published in the journal Nature Communications. [Nature Communs.?5, 5782 (2014)].

This research was supported by the Grants of NSFC, 973 project, OEMT, and IRT. 

Additional Information
Jianwen Dong can be reached for more about the research and comment at dongjwen@mail.sysu.edu.cn.